Glycaemic index methodology

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§ Glycaemic index methodology > Introduction

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§ Glycaemic index methodology > Introduction

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§ Glycaemic index methodology > Introduction

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§ Glycaemic index methodology > Test number

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§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

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§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

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§ Glycaemic index methodology > Meal volume, composition and consumption time

The cited sources that could be checked couldn’t confirm or dispute this claim.

§ Glycaemic index methodology > Meal volume, composition and consumption time

Every source the paper cites for this claim was checked and supports it.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

The cited sources support only part of this claim — read each source’s verdict below.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

The cited sources support only part of this claim — read each source’s verdict below.

§ Glycaemic index methodology > Subject preparation

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§ Glycaemic index methodology > Subject preparation

The cited sources that could be checked couldn’t confirm or dispute this claim.

§ Glycaemic index methodology > Re-utilisation of control test results

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§ Glycaemic index methodology > Re-utilisation of control test results

Every source the paper cites for this claim was checked and supports it.

§ Glycaemic index methodology > Re-utilisation of control test results

Every source the paper cites for this claim was checked and supports it.

§ Glycaemic index methodology > Re-utilisation of control test results

The cited sources that could be checked couldn’t confirm or dispute this claim.

§ Glycaemic index methodology > Re-utilisation of control test results

Every source the paper cites for this claim was checked and supports it.

§ Glycaemic index methodology > Scientific background.

The cited sources support only part of this claim — read each source’s verdict below.

§ Glycaemic index methodology > Scientific background.

A claim is checked only against the cited paper’s full text, never its abstract, and none is available for its sources.

§ Glycaemic index methodology > Meal volume, composition and consumption time

The paper states this without a source or data behind it here, so there is nothing to check it against. The quote shows it is in the paper.

§ Glycaemic index methodology > Meal volume, composition and consumption time

§ Glycaemic index methodology > Subject status

A result the paper reports from its own data — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Test number

§ Glycaemic index methodology > Subject status

§ Glycaemic index methodology > Test number

§ Glycaemic index methodology > Subject number

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§ Glycaemic index methodology > Test number

§ Glycaemic index methodology > Meal volume, composition and consumption time

None of the sources cited for this claim have been checked yet.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

A result the paper reports from its own data — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

None of the sources cited for this claim have been checked yet.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

A result the paper reports from its own data — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Washout period between two consecutive tests

A result the paper reports from its own data — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Blood-sampling time schedule What should be the optimal blood-sampling schedule?Scientific background.Usually blood is sampled in the fasting state and at 15, 30, 45, 60, 90 and 120 min after starting to eat the test meal in individuals without diabetes.And in subjects with diabetes, usually blood is obtained in the fasting state (0) and at 30, 60, 90, 120, 150 and 180 min after starting to eat.Taking blood samples less frequently (sampling blood every 30 min instead of every 15 min) or for less than 2 h in normal subjects significantly influences the mean and variation of the resulting AUC (Wolever, 2004b).The mean and variation of the resulting GI values also tend to increase as the frequency and duration of blood sampling decreases.'In addition, reduced frequency and duration of blood sampling resulted in the development of statistically significant correlations between the mean AUC of the reference food and the mean GI of the test foods in the different subjects, a correlation which is not significant' (Wolever, 2004b).The existence of a correlation between AUC and GI indicates that GI is not adequately controlling for differences in glycaemic response between subjects, and hence that the GI varies in different subjects.The GI is known to be a property of the food independent of subject characteristics.Therefore, methods that result in GI values that do not control for differences in AUC between individuals cannot be considered to be valid.Gannon & Nuttal (1987) analysed the effect on RGR of altering the length of time over which blood samples were taken from 1 to 5 h.RGR is defined as the net incremental glucose AUC (see Fig. 11) after a test meal expressed as a percentage of the net area after 50 g glucose.The results are not necessarily able to apply to the GI because RGR is based on calculating AUC using a different method than used for determining GI (incremental AUC).For legumes, the RGR markedly increased as the length of time of blood sampling increased.This is presumably because, after consuming foods such as legumes, which are slowly digested and absorbed, blood glucose concentrations tend to remain above the fasting level for several hours, in contrast to the consumption of glucose after which the blood glucose concentrations undershoot the baseline.Thus, as the time over which net AUC is calculated is extended, positive area accumulates for the legumes (i.e.net AUC becomes larger), while negative area accumulates for glucose (i.e.net AUC becomes smaller).After sucrose, milk and fructose, the RGR markedly decreased as the length of time over which blood was sampled increased, presumably because blood glucose concentrations after these test meals undershot more than after glucose.For starchy foods such as bread, oats and potato, there was only a small effect of time of blood sampling on RGR.Although an analysis to see if extending the length of blood sampling beyond 2 h affects the GI value has not been performed, it seems probable that there may be effects, which may vary for different types of foods. Thus, adhering to a standardised schedule of blood sampling with respect to both frequency and length of time is likely to be important for obtaining valid and consistent GI values.Analysing more than one baseline sample may theoretically reduce the variability (more sampling) or increase variability (baseline may fall with time).In practice, analysing data from nineteen subjects showed this to have only a very small effect overall on the incremental AUC.

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§ Glycaemic index methodology > Interlaboratory values and variation How comparable are GI values measured by different laboratories?Scientific background.Mean GI values measured in different laboratories vary.Fig. 13 shows the distribution of mean GI values reported for several foods (Foster-Powell et al. 2002).By just looking at these data, it is difficult to draw conclusions about whether the variation represents methodological differences, true differences between foods of similar description, or random variation.It is also difficult to determine the magnitude of true random variation of mean GI values between different laboratories.For some foods, such as apple, lentils and brown bread, the interlaboratory SD (the SD of the mean GI calculated within each laboratory) of the GI values is , 10, whereas for others, such as potato, the interlaboratory SD of the mean values is .20.An interlaboratory SD of , 10 would likely be acceptable because this would mean that the 95 % CI for any GI value would be , ^20.Such a value is larger than ideal, but does allow one to know with confidence that the GI values of many low-GI foods (for example, GI of lentils ¼ 28) will almost always have a lower GI than many high-GI foods (for example, GI of bread ¼ 71) when determined by different laboratories using standardised and correct methods.However, if the interlaboratory SD was much over 10, then the CI formed by combining the mean GI from each laboratory into an overall mean would become so large that it would be difficult to distinguish between foods, and the GI would lose practical utility. Recently an interlaboratory study was performed to estimate the between-laboratory variation of mean GI values (Wolever et al. 2003).It included five laboratories, which used capillary blood sampling to determine the GI values (glucose ¼ 100) of four centrally provided foods and locally obtained white bread (Fig. 14).The pooled interlaboratory SD of the GI values for the five foods was approximately 9. From this the probability of two laboratories obtaining GI values for the same food within limits from 0 to 20 have been calculated as 12 2N, where N equals the area under the normal distribution curve between 2 C and minus infinity, where C ¼ D=ðsd £ p 2Þ, where D is the GI difference and SD is the interlaboratory SD (Fig. 15).With an interlaboratory SD of 9, the chance of two laboratories obtaining a GI value for the same food which differs by less than 15 is about 76 % (i.e. a 24 % chance that the difference is greater than 15).If the interlaboratory SD could be decreased to 8, 7, 6 or 5, the chance of obtaining a GI difference of , 15 increases to 82, 87, 92 and 97 %, respectively.With an interlaboratory SD of 9, there is a 95 % chance that the difference in GI value obtained for the same food in two different laboratories is , 25.If the interlaboratory SD were decreased to 8, 7, 6, or 5, the respective difference would fall to 23, 20, 17 and 14.It should be noted that these calculations are based on exactly two laboratories.The probability of such differences between any two of a group of laboratories will be higher.

The paper states this without a source or data behind it here, so there is nothing to check it against. The quote shows it is in the paper.

§ Glycaemic index methodology > Washout period between two consecutive tests

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Subject status

§ Glycaemic index methodology > Introduction

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§ Glycaemic index methodology > Meal volume, composition and consumption time

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Meal volume, composition and consumption time

§ Glycaemic index methodology > Introduction

The paper states this without a source or data behind it here, so there is nothing to check it against. The quote shows it is in the paper.

§ Glycaemic index methodology > Meal volume, composition and consumption time

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

None of the sources cited for this claim have been checked yet.

§ Glycaemic index methodology > Meal volume, composition and consumption time

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

None of the sources cited for this claim have been checked yet.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Meal volume, composition and consumption time

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Meal volume, composition and consumption time

None of the sources cited for this claim have been checked yet.

§ Glycaemic index methodology > Meal volume, composition and consumption time

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

The paper states this without a source or data behind it here, so there is nothing to check it against. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Meal volume, composition and consumption time

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Subject preparation

None of the sources cited for this claim have been checked yet.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

None of the sources cited for this claim have been checked yet.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

None of the sources cited for this claim have been checked yet.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Subject preparation

§ Glycaemic index methodology > Subject preparation

§ Glycaemic index methodology > Subject preparation

§ Glycaemic index methodology > Subject preparation

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Carbohydrate basis for glycaemic index determination Should the test serving be based on available (glycaemic) CHO? Scientific background.In the classical GI concept only CHO sources that are assumed to be fully digestible, absorbable and glycaemic are included in the calculation of the 50 g CHO portion size.However, this approach has been criticised for several reasons (Monro, 2003).One is the suggestion that 'available' CHO is difficult to measure accurately.Accordingly, the usual way of determining 'available CHO', by subtracting 'dietary fibre' from total CHO, is not really a true reflection of in vivo available CHO content and may lead to overestimation in the case of products containing indigestible CHO, which are not recovered as dietary fibre.Thus, it could be argued that 'available' CHO is only a concept rather than a definable quantity.The truly available CHO fraction is difficult to determine by in vitro models, since these do not account for any influences within the gastrointestinal tract, for example the fact that some in vitro digestible CHO may escape small-intestinal digestion and absorption, thus ending unabsorbed in the colon.The analytical aspect is particularly important in the case of starchy foods.Until recently it was difficult to measure the amount of type 3 RS in a food.The former AOAC analytical dietary fibre method did not measure type 3 RS appropriately, leading to some underestimation of the total RS content of many foods in the past.However, in 2002 AOAC recognised an additional new method (McCleary & Monaghan, 2002) that measures type 3-retrograded RS specifically and accurately. This analytical progress seems of relevance since an unknown and significant fraction of native starch, once it has been prepared for consumption, may be resistant to digestion.This fraction is usually referred to as intrinsic RS.The amount of RS in a food is further influenced by the food processing and preparation conditions applied as well as by adding RS as a food ingredient.Accordingly, starch may be resistant due to the native (raw and indigestible) botanical structure or starch crystallinity being formed after cooking, during cooling down, by a process called retrogradation (formation of indigestible crystalline structures).Alternatively, RS can be added as an ingredient during food processing.The RS content may also change depending on the degree of ripening of a fruit or vegetable.For example, a green banana has a very high content of RS, which is lowered during ripening to almost zero in a fully ripe banana.As a consequence, the amount of RS in a starch-based food may range from a few percent to as much as 40 % depending on the native source and/or the way of meal preparation (Champ et al. 2003).Analysis of total starch less RS obtained by approved methods makes possible estimation of the available starch fraction.There are also several enzymic procedures available simulating the digestion in the gastrointestinal tract and making it possible to analyse the available and RS content of foods (Englyst et al. 1996;1998).Therefore, we can conclude that measuring available CHO is no longer a problem for most common foods. Since existing GI values are mostly based on available CHO being defined as total CHO minus dietary fibre, it is possible that existing GI values are based on an overestimate of the amount of available CHO in foods, and, thus, portion sizes which contain less than 50 g available CHO.This, in turn, raises the possibility that the reduced glycaemic responses of low-GI foods may be due entirely to CHO malabsorption.However, this does not appear to be the case.The extent to which CHO malabsorption accounts for reduced glycaemic responses will be discussed in detail below. A second problem with the classical GI approach has been suggested to be the fact that it does not actually indicate the blood glucose response elicited by a food, which depends also upon the amount of CHO it contains (Monro, 2003).For example, when a food or meal is formulated to lead to a net decrease in glycaemic CHO by increasing protein and dietary fibre content, for instance, the reduced glycaemic potency that will be achieved cannot be accurately reflected in GI.This may disadvantage consumers.Thus it also depends on the amounts and types of fat, protein, fibre and other compounds and their interactions.Accordingly it has been suggested that the classical use of GI is difficult to understand, and that it would be easier to understand and apply on a whole-food basis, as a food effect, including all its constituents.(Monro, 2003).In principle this would also allow glycaemia to be reduced by replacing starch and sugars with indigestible CHO such as RS and sugar alcohols.Thus the concept of a food-based GI differs from the classical concept of GI based on available CHO.The classical GI concept implies that the diet is high in available CHO, and aims to provide a means of controlling glycaemic responses without reducing available CHO intake.By contrast, the concept of a food-based glycaemic response implies that it is useful to control the magnitude of glycaemic responses by any means.All mechanisms by which glycaemia may be reduced (such as adding fat, or protein, or replacing available CHO with sugar alcohols, RS, fat or protein) are assumed to have an equivalent impact on health.Yet it is difficult to prove that these concepts differ because there are no long-term studies comparing the effects of high-available CHO low-GI diets with diets in which available CHO was replaced with unavailable CHO.However, it has been shown that short-term metabolic effects of reducing available CHO intake are different from those of slowing the rate of CHO absorption.This will be discussed later (p.153) under the heading of second-meal effects. In order to address the issue of whether a GI value should be based explicitly on available CHO, we need to ask also to what extent the low glycaemic responses elicited by low-GI foods (based on available CHO) are due to CHO malabsorption.If low glycaemic responses occur due to CHO malabsorption, then there would be no value in retaining the concept that the GI should be based on available CHO.However, this seems not to be the case.Although low-GI foods tend to be associated with a decreased amount of CHO absorbed (i.e. the amount of available CHO they contain has been overestimated), this only accounts for maximally 15 % of the blood glucoselowering effect.In addition, the effects of a low-GI food, lentils, on the second-meal glycaemic response is the opposite to that seen with a reduced amount of CHO absorption, but the same as that seen by slowing CHO absorption (Jenkins et al. 1990) (see also second-meal effects later; p. 153). Amount of carbohydrate absorbed.Wolever et al. (1986a) compared the amount of CHO malabsorbed from wholewheat bread and lentils using a breath hydrogen method and comparing the obtained data with those of directly measured contents in ileal effluent from individuals with an ileostomy.Both methods indicated that about 10 % of the 'available' CHO in bread and about 18 % in lentils was malabsorbed.Thus, portions of bread and lentils containing 50 g 'available' CHO (defined as total CHO minus dietary fibre) actually contained about 45 g and 41 g, respectively, of the truly measured absorbed CHO.This difference in absorbed CHO would account for about a 9 % difference in glycaemic response (100 £ (45 2 41)/45 ¼ 8•9 %).In fact, however, the glycaemic response elicited by lentils was 75 % less than that of bread (Wolever et al. 1986a).Thus, malabsorption only accounted for about 12 % of the reduction in glycaemic response (100 £ 9/75 ¼ 12 %).Fig. 6 shows the relationship between GI and percentage CHO malabsorbed for twenty foods as directly measured in a subject with an ileostomy.The dotted line indicates the magnitude of reduction in glycaemic response which would be expected based on the reduced amount of CHO absorbed.Even at high levels of CHO malabsorption seen with legumes, the reduction in CHO malabsorption accounts for , 20 % of the reduced glycaemic response.It is therefore believed that the major determinant of the low GI values of starchy foods in this study is a reduced rate of CHO absorption.Of course other mechanisms are possible, but it is true that slowing the rate of absorption of CHO, for example by sipping glucose slowly over 3 h, reduces glucose and insulin responses (Jenkins et al. 1990). If, as suggested by Fig. 6, not all of the 'available' CHO in foods is actually absorbed, the question is raised as to how much GI values are underestimated because the portion sizes used contained less than 50 g available CHO.This question has been addressed by determining the glycaemic responses of starchy foods containing high amounts of intrinsic RS (15 or 35 % on total starch basis) using two different portion sizes, one matching for total starch (or available CHO defined as total CHO minus dietary fibre), and a larger portion in which the RS was excluded from available CHO.In the case of high-amylose maize bread, the glycaemic response elicited by the 45 g total starch portion, 49 mmol £ min/l, was identical to the response after a 55 % larger portion size to account for the 35 % RS, 49 mmol £ min/l (Granfeldt et al. 1995a).In the case of barley, an 18 % increase in portion size resulted in a non-significant 13 % increase in GI from 39 to 44 (glucose ¼ 100; Wolever et al. 2003).The reason for this is probably that the food factors responsible for the intrinsic RS formation also reduce the in vivo availability of the bulk of starch.In any case it is advised to estimate available and indigestible CHO using validated methods and correct for the indigestible part. Glycaemic response as a function of the digestibility of available carbohydrate.In several comparative studies, the glycaemic impact of CHO foods has been related to the rate of digestion of available CHO. Englyst et al. (2003) correlated twenty-three cereal-based starchy foods for their GI in vivo and their rate of CHO digestion in vitro (as assessed by the content in rapidly available glucose).Foods were ingested on a 50 ^2 g available CHO basis.According to the results of analytical measurement of NSP and RS contents the twenty-three foods had very similar content in non-available CHO.However, the GI was highly variable between the different foods studied but was significantly related to the in vitro rate of digestion (rapidly available glucose content).Accordingly, GI was more explained by the rate of digestion of the available CHO fraction than by the content in non-available CHO (dietary fibre and RS). Second-meal effects.To see if the acute metabolic effects of a low-GI food, lentils, was due to a reduced rate or a reduced amount of available CHO absorption, Jenkins et al. (1982) studied the effects of different breakfast test meal on the glycaemic response after a standard lunch.Subjects consumed four different breakfast meals, followed 4 h later by a standard lunch.The two basic breakfast meals were matched for available CHO (127 -128 g), dietary fibre (26 -29 g), protein (57 g) and fat (13 -18 g) and consisted of lentils, butter and tomato, or wholemeal bread, cheese and tomatoes.The breakfasts were consumed over a period of 15 -20 min.Two further breakfasts were taken; one, representing reduced CHO intake, contained only onequarter of the wholemeal bread but was identical in all other respects.The other, representing slow CHO absorption, contained the full amount of bread, cheese and tomato but eaten in small divided portions over 4 h.The glycaemic response elicited by lentils and the continuous and the quarter bread meals were significantly less than that of the control bread breakfast (Fig. 7).However, the pattern of blood glucose response after lentils (Fig. 8) was followed much more closely by the continuous meal than by the quarter bread meal (Fig. 9).The second-meal response refers to the glycaemic response elicited by the standard lunch meal consumed 4 h after breakfast.The second-meal response after lentils was 38 % less than that after the control bread breakfast (Fig. 7).The second-meal response tended to be less than control after the continuous bread breakfast but was significantly increased after the quarter bread breakfast (Fig. 7).These results suggest that the acute metabolic effects of slowing CHO absorption differ from those of reducing the amount of CHO absorbed, and that the metabolic effect of lentils is not due to CHO malabsorption.Similar conclusions were drawn from second-meal experiments done by Liljeberg et al. (1999). These data suggest that the concept of GI based on 50 g available CHO differs from the concept of the GL or GGE based on glycaemic response as a food effect and it is therefore essential to distinguish between these measures.

None of the sources cited for this claim have been checked yet.

§ Glycaemic index methodology > Subject preparation

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Re-utilisation of control test results

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Re-utilisation of control test results

• AssumptionRests on an assumption: GI trials are cross-over studies in which all subjects undergo all treatments.
• AssumptionRests on an assumption: It is usually impossible to blind either subjects or investigators in GI trials.

The paper states this without a source or data behind it here, so there is nothing to check it against. The quote shows it is in the paper.

§ Glycaemic index methodology > Re-utilisation of control test results

§ Glycaemic index methodology > Re-utilisation of control test results

§ Glycaemic index methodology > Blood-sampling time schedule What should be the optimal blood-sampling schedule?Scientific background.Usually blood is sampled in the fasting state and at 15, 30, 45, 60, 90 and 120 min after starting to eat the test meal in individuals without diabetes.And in subjects with diabetes, usually blood is obtained in the fasting state (0) and at 30, 60, 90, 120, 150 and 180 min after starting to eat.Taking blood samples less frequently (sampling blood every 30 min instead of every 15 min) or for less than 2 h in normal subjects significantly influences the mean and variation of the resulting AUC (Wolever, 2004b).The mean and variation of the resulting GI values also tend to increase as the frequency and duration of blood sampling decreases.'In addition, reduced frequency and duration of blood sampling resulted in the development of statistically significant correlations between the mean AUC of the reference food and the mean GI of the test foods in the different subjects, a correlation which is not significant' (Wolever, 2004b).The existence of a correlation between AUC and GI indicates that GI is not adequately controlling for differences in glycaemic response between subjects, and hence that the GI varies in different subjects.The GI is known to be a property of the food independent of subject characteristics.Therefore, methods that result in GI values that do not control for differences in AUC between individuals cannot be considered to be valid.Gannon & Nuttal (1987) analysed the effect on RGR of altering the length of time over which blood samples were taken from 1 to 5 h.RGR is defined as the net incremental glucose AUC (see Fig. 11) after a test meal expressed as a percentage of the net area after 50 g glucose.The results are not necessarily able to apply to the GI because RGR is based on calculating AUC using a different method than used for determining GI (incremental AUC).For legumes, the RGR markedly increased as the length of time of blood sampling increased.This is presumably because, after consuming foods such as legumes, which are slowly digested and absorbed, blood glucose concentrations tend to remain above the fasting level for several hours, in contrast to the consumption of glucose after which the blood glucose concentrations undershoot the baseline.Thus, as the time over which net AUC is calculated is extended, positive area accumulates for the legumes (i.e.net AUC becomes larger), while negative area accumulates for glucose (i.e.net AUC becomes smaller).After sucrose, milk and fructose, the RGR markedly decreased as the length of time over which blood was sampled increased, presumably because blood glucose concentrations after these test meals undershot more than after glucose.For starchy foods such as bread, oats and potato, there was only a small effect of time of blood sampling on RGR.Although an analysis to see if extending the length of blood sampling beyond 2 h affects the GI value has not been performed, it seems probable that there may be effects, which may vary for different types of foods. Thus, adhering to a standardised schedule of blood sampling with respect to both frequency and length of time is likely to be important for obtaining valid and consistent GI values.Analysing more than one baseline sample may theoretically reduce the variability (more sampling) or increase variability (baseline may fall with time).In practice, analysing data from nineteen subjects showed this to have only a very small effect overall on the incremental AUC.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Washout period between two consecutive tests

• AssumptionRests on an assumption: The impact of the food tested on metabolism 2 days later is negligible.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Blood-sampling time schedule What should be the optimal blood-sampling schedule?Scientific background.Usually blood is sampled in the fasting state and at 15, 30, 45, 60, 90 and 120 min after starting to eat the test meal in individuals without diabetes.And in subjects with diabetes, usually blood is obtained in the fasting state (0) and at 30, 60, 90, 120, 150 and 180 min after starting to eat.Taking blood samples less frequently (sampling blood every 30 min instead of every 15 min) or for less than 2 h in normal subjects significantly influences the mean and variation of the resulting AUC (Wolever, 2004b).The mean and variation of the resulting GI values also tend to increase as the frequency and duration of blood sampling decreases.'In addition, reduced frequency and duration of blood sampling resulted in the development of statistically significant correlations between the mean AUC of the reference food and the mean GI of the test foods in the different subjects, a correlation which is not significant' (Wolever, 2004b).The existence of a correlation between AUC and GI indicates that GI is not adequately controlling for differences in glycaemic response between subjects, and hence that the GI varies in different subjects.The GI is known to be a property of the food independent of subject characteristics.Therefore, methods that result in GI values that do not control for differences in AUC between individuals cannot be considered to be valid.Gannon & Nuttal (1987) analysed the effect on RGR of altering the length of time over which blood samples were taken from 1 to 5 h.RGR is defined as the net incremental glucose AUC (see Fig. 11) after a test meal expressed as a percentage of the net area after 50 g glucose.The results are not necessarily able to apply to the GI because RGR is based on calculating AUC using a different method than used for determining GI (incremental AUC).For legumes, the RGR markedly increased as the length of time of blood sampling increased.This is presumably because, after consuming foods such as legumes, which are slowly digested and absorbed, blood glucose concentrations tend to remain above the fasting level for several hours, in contrast to the consumption of glucose after which the blood glucose concentrations undershoot the baseline.Thus, as the time over which net AUC is calculated is extended, positive area accumulates for the legumes (i.e.net AUC becomes larger), while negative area accumulates for glucose (i.e.net AUC becomes smaller).After sucrose, milk and fructose, the RGR markedly decreased as the length of time over which blood was sampled increased, presumably because blood glucose concentrations after these test meals undershot more than after glucose.For starchy foods such as bread, oats and potato, there was only a small effect of time of blood sampling on RGR.Although an analysis to see if extending the length of blood sampling beyond 2 h affects the GI value has not been performed, it seems probable that there may be effects, which may vary for different types of foods. Thus, adhering to a standardised schedule of blood sampling with respect to both frequency and length of time is likely to be important for obtaining valid and consistent GI values.Analysing more than one baseline sample may theoretically reduce the variability (more sampling) or increase variability (baseline may fall with time).In practice, analysing data from nineteen subjects showed this to have only a very small effect overall on the incremental AUC.

• AssumptionRests on an assumption: GI is a property of the food independent of subject characteristics.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Re-utilisation of control test results

§ Glycaemic index methodology > Blood-sampling time schedule What should be the optimal blood-sampling schedule?Scientific background.Usually blood is sampled in the fasting state and at 15, 30, 45, 60, 90 and 120 min after starting to eat the test meal in individuals without diabetes.And in subjects with diabetes, usually blood is obtained in the fasting state (0) and at 30, 60, 90, 120, 150 and 180 min after starting to eat.Taking blood samples less frequently (sampling blood every 30 min instead of every 15 min) or for less than 2 h in normal subjects significantly influences the mean and variation of the resulting AUC (Wolever, 2004b).The mean and variation of the resulting GI values also tend to increase as the frequency and duration of blood sampling decreases.'In addition, reduced frequency and duration of blood sampling resulted in the development of statistically significant correlations between the mean AUC of the reference food and the mean GI of the test foods in the different subjects, a correlation which is not significant' (Wolever, 2004b).The existence of a correlation between AUC and GI indicates that GI is not adequately controlling for differences in glycaemic response between subjects, and hence that the GI varies in different subjects.The GI is known to be a property of the food independent of subject characteristics.Therefore, methods that result in GI values that do not control for differences in AUC between individuals cannot be considered to be valid.Gannon & Nuttal (1987) analysed the effect on RGR of altering the length of time over which blood samples were taken from 1 to 5 h.RGR is defined as the net incremental glucose AUC (see Fig. 11) after a test meal expressed as a percentage of the net area after 50 g glucose.The results are not necessarily able to apply to the GI because RGR is based on calculating AUC using a different method than used for determining GI (incremental AUC).For legumes, the RGR markedly increased as the length of time of blood sampling increased.This is presumably because, after consuming foods such as legumes, which are slowly digested and absorbed, blood glucose concentrations tend to remain above the fasting level for several hours, in contrast to the consumption of glucose after which the blood glucose concentrations undershoot the baseline.Thus, as the time over which net AUC is calculated is extended, positive area accumulates for the legumes (i.e.net AUC becomes larger), while negative area accumulates for glucose (i.e.net AUC becomes smaller).After sucrose, milk and fructose, the RGR markedly decreased as the length of time over which blood was sampled increased, presumably because blood glucose concentrations after these test meals undershot more than after glucose.For starchy foods such as bread, oats and potato, there was only a small effect of time of blood sampling on RGR.Although an analysis to see if extending the length of blood sampling beyond 2 h affects the GI value has not been performed, it seems probable that there may be effects, which may vary for different types of foods. Thus, adhering to a standardised schedule of blood sampling with respect to both frequency and length of time is likely to be important for obtaining valid and consistent GI values.Analysing more than one baseline sample may theoretically reduce the variability (more sampling) or increase variability (baseline may fall with time).In practice, analysing data from nineteen subjects showed this to have only a very small effect overall on the incremental AUC.

§ Glycaemic index methodology > Blood-sampling time schedule What should be the optimal blood-sampling schedule?Scientific background.Usually blood is sampled in the fasting state and at 15, 30, 45, 60, 90 and 120 min after starting to eat the test meal in individuals without diabetes.And in subjects with diabetes, usually blood is obtained in the fasting state (0) and at 30, 60, 90, 120, 150 and 180 min after starting to eat.Taking blood samples less frequently (sampling blood every 30 min instead of every 15 min) or for less than 2 h in normal subjects significantly influences the mean and variation of the resulting AUC (Wolever, 2004b).The mean and variation of the resulting GI values also tend to increase as the frequency and duration of blood sampling decreases.'In addition, reduced frequency and duration of blood sampling resulted in the development of statistically significant correlations between the mean AUC of the reference food and the mean GI of the test foods in the different subjects, a correlation which is not significant' (Wolever, 2004b).The existence of a correlation between AUC and GI indicates that GI is not adequately controlling for differences in glycaemic response between subjects, and hence that the GI varies in different subjects.The GI is known to be a property of the food independent of subject characteristics.Therefore, methods that result in GI values that do not control for differences in AUC between individuals cannot be considered to be valid.Gannon & Nuttal (1987) analysed the effect on RGR of altering the length of time over which blood samples were taken from 1 to 5 h.RGR is defined as the net incremental glucose AUC (see Fig. 11) after a test meal expressed as a percentage of the net area after 50 g glucose.The results are not necessarily able to apply to the GI because RGR is based on calculating AUC using a different method than used for determining GI (incremental AUC).For legumes, the RGR markedly increased as the length of time of blood sampling increased.This is presumably because, after consuming foods such as legumes, which are slowly digested and absorbed, blood glucose concentrations tend to remain above the fasting level for several hours, in contrast to the consumption of glucose after which the blood glucose concentrations undershoot the baseline.Thus, as the time over which net AUC is calculated is extended, positive area accumulates for the legumes (i.e.net AUC becomes larger), while negative area accumulates for glucose (i.e.net AUC becomes smaller).After sucrose, milk and fructose, the RGR markedly decreased as the length of time over which blood was sampled increased, presumably because blood glucose concentrations after these test meals undershot more than after glucose.For starchy foods such as bread, oats and potato, there was only a small effect of time of blood sampling on RGR.Although an analysis to see if extending the length of blood sampling beyond 2 h affects the GI value has not been performed, it seems probable that there may be effects, which may vary for different types of foods. Thus, adhering to a standardised schedule of blood sampling with respect to both frequency and length of time is likely to be important for obtaining valid and consistent GI values.Analysing more than one baseline sample may theoretically reduce the variability (more sampling) or increase variability (baseline may fall with time).In practice, analysing data from nineteen subjects showed this to have only a very small effect overall on the incremental AUC.

None of the sources cited for this claim have been checked yet.

§ Glycaemic index methodology > Re-utilisation of control test results

§ Glycaemic index methodology > Blood-sampling time schedule What should be the optimal blood-sampling schedule?Scientific background.Usually blood is sampled in the fasting state and at 15, 30, 45, 60, 90 and 120 min after starting to eat the test meal in individuals without diabetes.And in subjects with diabetes, usually blood is obtained in the fasting state (0) and at 30, 60, 90, 120, 150 and 180 min after starting to eat.Taking blood samples less frequently (sampling blood every 30 min instead of every 15 min) or for less than 2 h in normal subjects significantly influences the mean and variation of the resulting AUC (Wolever, 2004b).The mean and variation of the resulting GI values also tend to increase as the frequency and duration of blood sampling decreases.'In addition, reduced frequency and duration of blood sampling resulted in the development of statistically significant correlations between the mean AUC of the reference food and the mean GI of the test foods in the different subjects, a correlation which is not significant' (Wolever, 2004b).The existence of a correlation between AUC and GI indicates that GI is not adequately controlling for differences in glycaemic response between subjects, and hence that the GI varies in different subjects.The GI is known to be a property of the food independent of subject characteristics.Therefore, methods that result in GI values that do not control for differences in AUC between individuals cannot be considered to be valid.Gannon & Nuttal (1987) analysed the effect on RGR of altering the length of time over which blood samples were taken from 1 to 5 h.RGR is defined as the net incremental glucose AUC (see Fig. 11) after a test meal expressed as a percentage of the net area after 50 g glucose.The results are not necessarily able to apply to the GI because RGR is based on calculating AUC using a different method than used for determining GI (incremental AUC).For legumes, the RGR markedly increased as the length of time of blood sampling increased.This is presumably because, after consuming foods such as legumes, which are slowly digested and absorbed, blood glucose concentrations tend to remain above the fasting level for several hours, in contrast to the consumption of glucose after which the blood glucose concentrations undershoot the baseline.Thus, as the time over which net AUC is calculated is extended, positive area accumulates for the legumes (i.e.net AUC becomes larger), while negative area accumulates for glucose (i.e.net AUC becomes smaller).After sucrose, milk and fructose, the RGR markedly decreased as the length of time over which blood was sampled increased, presumably because blood glucose concentrations after these test meals undershot more than after glucose.For starchy foods such as bread, oats and potato, there was only a small effect of time of blood sampling on RGR.Although an analysis to see if extending the length of blood sampling beyond 2 h affects the GI value has not been performed, it seems probable that there may be effects, which may vary for different types of foods. Thus, adhering to a standardised schedule of blood sampling with respect to both frequency and length of time is likely to be important for obtaining valid and consistent GI values.Analysing more than one baseline sample may theoretically reduce the variability (more sampling) or increase variability (baseline may fall with time).In practice, analysing data from nineteen subjects showed this to have only a very small effect overall on the incremental AUC.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Washout period between two consecutive tests

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Re-utilisation of control test results

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Blood-sampling time schedule What should be the optimal blood-sampling schedule?Scientific background.Usually blood is sampled in the fasting state and at 15, 30, 45, 60, 90 and 120 min after starting to eat the test meal in individuals without diabetes.And in subjects with diabetes, usually blood is obtained in the fasting state (0) and at 30, 60, 90, 120, 150 and 180 min after starting to eat.Taking blood samples less frequently (sampling blood every 30 min instead of every 15 min) or for less than 2 h in normal subjects significantly influences the mean and variation of the resulting AUC (Wolever, 2004b).The mean and variation of the resulting GI values also tend to increase as the frequency and duration of blood sampling decreases.'In addition, reduced frequency and duration of blood sampling resulted in the development of statistically significant correlations between the mean AUC of the reference food and the mean GI of the test foods in the different subjects, a correlation which is not significant' (Wolever, 2004b).The existence of a correlation between AUC and GI indicates that GI is not adequately controlling for differences in glycaemic response between subjects, and hence that the GI varies in different subjects.The GI is known to be a property of the food independent of subject characteristics.Therefore, methods that result in GI values that do not control for differences in AUC between individuals cannot be considered to be valid.Gannon & Nuttal (1987) analysed the effect on RGR of altering the length of time over which blood samples were taken from 1 to 5 h.RGR is defined as the net incremental glucose AUC (see Fig. 11) after a test meal expressed as a percentage of the net area after 50 g glucose.The results are not necessarily able to apply to the GI because RGR is based on calculating AUC using a different method than used for determining GI (incremental AUC).For legumes, the RGR markedly increased as the length of time of blood sampling increased.This is presumably because, after consuming foods such as legumes, which are slowly digested and absorbed, blood glucose concentrations tend to remain above the fasting level for several hours, in contrast to the consumption of glucose after which the blood glucose concentrations undershoot the baseline.Thus, as the time over which net AUC is calculated is extended, positive area accumulates for the legumes (i.e.net AUC becomes larger), while negative area accumulates for glucose (i.e.net AUC becomes smaller).After sucrose, milk and fructose, the RGR markedly decreased as the length of time over which blood was sampled increased, presumably because blood glucose concentrations after these test meals undershot more than after glucose.For starchy foods such as bread, oats and potato, there was only a small effect of time of blood sampling on RGR.Although an analysis to see if extending the length of blood sampling beyond 2 h affects the GI value has not been performed, it seems probable that there may be effects, which may vary for different types of foods. Thus, adhering to a standardised schedule of blood sampling with respect to both frequency and length of time is likely to be important for obtaining valid and consistent GI values.Analysing more than one baseline sample may theoretically reduce the variability (more sampling) or increase variability (baseline may fall with time).In practice, analysing data from nineteen subjects showed this to have only a very small effect overall on the incremental AUC.

§ Glycaemic index methodology > Washout period between two consecutive tests

§ Glycaemic index methodology > Re-utilisation of control test results

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Re-utilisation of control test results

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Washout period between two consecutive tests

• AssumptionRests on an assumption: Fasting glucose is independent of the food consumed.
• AssumptionRests on an assumption: The portion of total AUC below fasting glucose is independent of the test meal.

§ Glycaemic index methodology > Re-utilisation of control test results

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Washout period between two consecutive tests

• AssumptionRests on an assumption: Meals between sessions have more effect on the next test than the test food eaten 2 days before.

§ Glycaemic index methodology > Re-utilisation of control test results

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Washout period between two consecutive tests

§ Glycaemic index methodology > Blood-sampling time schedule What should be the optimal blood-sampling schedule?Scientific background.Usually blood is sampled in the fasting state and at 15, 30, 45, 60, 90 and 120 min after starting to eat the test meal in individuals without diabetes.And in subjects with diabetes, usually blood is obtained in the fasting state (0) and at 30, 60, 90, 120, 150 and 180 min after starting to eat.Taking blood samples less frequently (sampling blood every 30 min instead of every 15 min) or for less than 2 h in normal subjects significantly influences the mean and variation of the resulting AUC (Wolever, 2004b).The mean and variation of the resulting GI values also tend to increase as the frequency and duration of blood sampling decreases.'In addition, reduced frequency and duration of blood sampling resulted in the development of statistically significant correlations between the mean AUC of the reference food and the mean GI of the test foods in the different subjects, a correlation which is not significant' (Wolever, 2004b).The existence of a correlation between AUC and GI indicates that GI is not adequately controlling for differences in glycaemic response between subjects, and hence that the GI varies in different subjects.The GI is known to be a property of the food independent of subject characteristics.Therefore, methods that result in GI values that do not control for differences in AUC between individuals cannot be considered to be valid.Gannon & Nuttal (1987) analysed the effect on RGR of altering the length of time over which blood samples were taken from 1 to 5 h.RGR is defined as the net incremental glucose AUC (see Fig. 11) after a test meal expressed as a percentage of the net area after 50 g glucose.The results are not necessarily able to apply to the GI because RGR is based on calculating AUC using a different method than used for determining GI (incremental AUC).For legumes, the RGR markedly increased as the length of time of blood sampling increased.This is presumably because, after consuming foods such as legumes, which are slowly digested and absorbed, blood glucose concentrations tend to remain above the fasting level for several hours, in contrast to the consumption of glucose after which the blood glucose concentrations undershoot the baseline.Thus, as the time over which net AUC is calculated is extended, positive area accumulates for the legumes (i.e.net AUC becomes larger), while negative area accumulates for glucose (i.e.net AUC becomes smaller).After sucrose, milk and fructose, the RGR markedly decreased as the length of time over which blood was sampled increased, presumably because blood glucose concentrations after these test meals undershot more than after glucose.For starchy foods such as bread, oats and potato, there was only a small effect of time of blood sampling on RGR.Although an analysis to see if extending the length of blood sampling beyond 2 h affects the GI value has not been performed, it seems probable that there may be effects, which may vary for different types of foods. Thus, adhering to a standardised schedule of blood sampling with respect to both frequency and length of time is likely to be important for obtaining valid and consistent GI values.Analysing more than one baseline sample may theoretically reduce the variability (more sampling) or increase variability (baseline may fall with time).In practice, analysing data from nineteen subjects showed this to have only a very small effect overall on the incremental AUC.

§ Glycaemic index methodology > Blood-sampling time schedule What should be the optimal blood-sampling schedule?Scientific background.Usually blood is sampled in the fasting state and at 15, 30, 45, 60, 90 and 120 min after starting to eat the test meal in individuals without diabetes.And in subjects with diabetes, usually blood is obtained in the fasting state (0) and at 30, 60, 90, 120, 150 and 180 min after starting to eat.Taking blood samples less frequently (sampling blood every 30 min instead of every 15 min) or for less than 2 h in normal subjects significantly influences the mean and variation of the resulting AUC (Wolever, 2004b).The mean and variation of the resulting GI values also tend to increase as the frequency and duration of blood sampling decreases.'In addition, reduced frequency and duration of blood sampling resulted in the development of statistically significant correlations between the mean AUC of the reference food and the mean GI of the test foods in the different subjects, a correlation which is not significant' (Wolever, 2004b).The existence of a correlation between AUC and GI indicates that GI is not adequately controlling for differences in glycaemic response between subjects, and hence that the GI varies in different subjects.The GI is known to be a property of the food independent of subject characteristics.Therefore, methods that result in GI values that do not control for differences in AUC between individuals cannot be considered to be valid.Gannon & Nuttal (1987) analysed the effect on RGR of altering the length of time over which blood samples were taken from 1 to 5 h.RGR is defined as the net incremental glucose AUC (see Fig. 11) after a test meal expressed as a percentage of the net area after 50 g glucose.The results are not necessarily able to apply to the GI because RGR is based on calculating AUC using a different method than used for determining GI (incremental AUC).For legumes, the RGR markedly increased as the length of time of blood sampling increased.This is presumably because, after consuming foods such as legumes, which are slowly digested and absorbed, blood glucose concentrations tend to remain above the fasting level for several hours, in contrast to the consumption of glucose after which the blood glucose concentrations undershoot the baseline.Thus, as the time over which net AUC is calculated is extended, positive area accumulates for the legumes (i.e.net AUC becomes larger), while negative area accumulates for glucose (i.e.net AUC becomes smaller).After sucrose, milk and fructose, the RGR markedly decreased as the length of time over which blood was sampled increased, presumably because blood glucose concentrations after these test meals undershot more than after glucose.For starchy foods such as bread, oats and potato, there was only a small effect of time of blood sampling on RGR.Although an analysis to see if extending the length of blood sampling beyond 2 h affects the GI value has not been performed, it seems probable that there may be effects, which may vary for different types of foods. Thus, adhering to a standardised schedule of blood sampling with respect to both frequency and length of time is likely to be important for obtaining valid and consistent GI values.Analysing more than one baseline sample may theoretically reduce the variability (more sampling) or increase variability (baseline may fall with time).In practice, analysing data from nineteen subjects showed this to have only a very small effect overall on the incremental AUC.

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§ Glycaemic index methodology > Re-utilisation of control test results

The paper states this without a source or data behind it here, so there is nothing to check it against. The quote shows it is in the paper.

§ Glycaemic index methodology > Scientific background.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Scientific background.

§ Glycaemic index methodology > How to handle outlier values? The advantage in dealing with GI values is that by indexing the individual glycaemic response to a standard, the variation that occurs due to several factors, for example, subject and food-related ones, the between-individual variation is minimised (Wolever, 1990a).Checking the data obtained for the individual subjects may sometimes reveal values that are implausible by comparison with the rest of the data from other subjects.Outliers are particularly important because they can have considerable influence on the results of a statistical analysis (Altman, 1991).Outliers could be due to incorrect subject preparation (for example, non-fasting), analytical error, or errors in data calculation.In the absence of errors, an outlier could be due to an unrepresentative response to the standard or test food or to the subject having a true idiosyncratic response.An unrepresentative response to the standard food is suspected if the subject's GI values for other test foods are also outliers in the same direction, or if there is large variability for the repeated standard tests (Wolever et al. 1991).If outliers are excluded, this fact should be stated, and the results including the outlying point(s) should be given along with the position of the outlying point(s) in relation to the rest of the data, i.e. number of SD from the mean (including the outlier(s)).This will allow others to assess the impact of removing the outlier(s) on the results and conclusions and to form an opinion of the reasonableness of excluding the outlier(s). Recommendation.There is no general advice or rule in statistical analysis to exclude outlying data from the complete data-set.Nevertheless, if an outlying value does not seem to be plausible or if any mistake in determining this data point is evident, this value could be regarded as suspicious and removed from the data-set (Altman, 1991).Therefore, under certain circumstances and for reasons of practicability an outlier could be removed from the data-set without reassessment of that particular GI value.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Conclusion.

• AssumptionRests on an assumption: The five foods and five laboratories in the interlaboratory study are representative enough that the pooled SD of about 9 applies to standardised GI measurements generally.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Interlaboratory values and variation How comparable are GI values measured by different laboratories?Scientific background.Mean GI values measured in different laboratories vary.Fig. 13 shows the distribution of mean GI values reported for several foods (Foster-Powell et al. 2002).By just looking at these data, it is difficult to draw conclusions about whether the variation represents methodological differences, true differences between foods of similar description, or random variation.It is also difficult to determine the magnitude of true random variation of mean GI values between different laboratories.For some foods, such as apple, lentils and brown bread, the interlaboratory SD (the SD of the mean GI calculated within each laboratory) of the GI values is , 10, whereas for others, such as potato, the interlaboratory SD of the mean values is .20.An interlaboratory SD of , 10 would likely be acceptable because this would mean that the 95 % CI for any GI value would be , ^20.Such a value is larger than ideal, but does allow one to know with confidence that the GI values of many low-GI foods (for example, GI of lentils ¼ 28) will almost always have a lower GI than many high-GI foods (for example, GI of bread ¼ 71) when determined by different laboratories using standardised and correct methods.However, if the interlaboratory SD was much over 10, then the CI formed by combining the mean GI from each laboratory into an overall mean would become so large that it would be difficult to distinguish between foods, and the GI would lose practical utility. Recently an interlaboratory study was performed to estimate the between-laboratory variation of mean GI values (Wolever et al. 2003).It included five laboratories, which used capillary blood sampling to determine the GI values (glucose ¼ 100) of four centrally provided foods and locally obtained white bread (Fig. 14).The pooled interlaboratory SD of the GI values for the five foods was approximately 9. From this the probability of two laboratories obtaining GI values for the same food within limits from 0 to 20 have been calculated as 12 2N, where N equals the area under the normal distribution curve between 2 C and minus infinity, where C ¼ D=ðsd £ p 2Þ, where D is the GI difference and SD is the interlaboratory SD (Fig. 15).With an interlaboratory SD of 9, the chance of two laboratories obtaining a GI value for the same food which differs by less than 15 is about 76 % (i.e. a 24 % chance that the difference is greater than 15).If the interlaboratory SD could be decreased to 8, 7, 6 or 5, the chance of obtaining a GI difference of , 15 increases to 82, 87, 92 and 97 %, respectively.With an interlaboratory SD of 9, there is a 95 % chance that the difference in GI value obtained for the same food in two different laboratories is , 25.If the interlaboratory SD were decreased to 8, 7, 6, or 5, the respective difference would fall to 23, 20, 17 and 14.It should be noted that these calculations are based on exactly two laboratories.The probability of such differences between any two of a group of laboratories will be higher.

§ Glycaemic index methodology > Scientific background.

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§ Glycaemic index methodology > Scientific background.

The paper states this without a source or data behind it here, so there is nothing to check it against. The quote shows it is in the paper.

§ Glycaemic index methodology > Scientific background.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Glucose analysis

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Scientific background.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Interlaboratory values and variation How comparable are GI values measured by different laboratories?Scientific background.Mean GI values measured in different laboratories vary.Fig. 13 shows the distribution of mean GI values reported for several foods (Foster-Powell et al. 2002).By just looking at these data, it is difficult to draw conclusions about whether the variation represents methodological differences, true differences between foods of similar description, or random variation.It is also difficult to determine the magnitude of true random variation of mean GI values between different laboratories.For some foods, such as apple, lentils and brown bread, the interlaboratory SD (the SD of the mean GI calculated within each laboratory) of the GI values is , 10, whereas for others, such as potato, the interlaboratory SD of the mean values is .20.An interlaboratory SD of , 10 would likely be acceptable because this would mean that the 95 % CI for any GI value would be , ^20.Such a value is larger than ideal, but does allow one to know with confidence that the GI values of many low-GI foods (for example, GI of lentils ¼ 28) will almost always have a lower GI than many high-GI foods (for example, GI of bread ¼ 71) when determined by different laboratories using standardised and correct methods.However, if the interlaboratory SD was much over 10, then the CI formed by combining the mean GI from each laboratory into an overall mean would become so large that it would be difficult to distinguish between foods, and the GI would lose practical utility. Recently an interlaboratory study was performed to estimate the between-laboratory variation of mean GI values (Wolever et al. 2003).It included five laboratories, which used capillary blood sampling to determine the GI values (glucose ¼ 100) of four centrally provided foods and locally obtained white bread (Fig. 14).The pooled interlaboratory SD of the GI values for the five foods was approximately 9. From this the probability of two laboratories obtaining GI values for the same food within limits from 0 to 20 have been calculated as 12 2N, where N equals the area under the normal distribution curve between 2 C and minus infinity, where C ¼ D=ðsd £ p 2Þ, where D is the GI difference and SD is the interlaboratory SD (Fig. 15).With an interlaboratory SD of 9, the chance of two laboratories obtaining a GI value for the same food which differs by less than 15 is about 76 % (i.e. a 24 % chance that the difference is greater than 15).If the interlaboratory SD could be decreased to 8, 7, 6 or 5, the chance of obtaining a GI difference of , 15 increases to 82, 87, 92 and 97 %, respectively.With an interlaboratory SD of 9, there is a 95 % chance that the difference in GI value obtained for the same food in two different laboratories is , 25.If the interlaboratory SD were decreased to 8, 7, 6, or 5, the respective difference would fall to 23, 20, 17 and 14.It should be noted that these calculations are based on exactly two laboratories.The probability of such differences between any two of a group of laboratories will be higher.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Interlaboratory values and variation How comparable are GI values measured by different laboratories?Scientific background.Mean GI values measured in different laboratories vary.Fig. 13 shows the distribution of mean GI values reported for several foods (Foster-Powell et al. 2002).By just looking at these data, it is difficult to draw conclusions about whether the variation represents methodological differences, true differences between foods of similar description, or random variation.It is also difficult to determine the magnitude of true random variation of mean GI values between different laboratories.For some foods, such as apple, lentils and brown bread, the interlaboratory SD (the SD of the mean GI calculated within each laboratory) of the GI values is , 10, whereas for others, such as potato, the interlaboratory SD of the mean values is .20.An interlaboratory SD of , 10 would likely be acceptable because this would mean that the 95 % CI for any GI value would be , ^20.Such a value is larger than ideal, but does allow one to know with confidence that the GI values of many low-GI foods (for example, GI of lentils ¼ 28) will almost always have a lower GI than many high-GI foods (for example, GI of bread ¼ 71) when determined by different laboratories using standardised and correct methods.However, if the interlaboratory SD was much over 10, then the CI formed by combining the mean GI from each laboratory into an overall mean would become so large that it would be difficult to distinguish between foods, and the GI would lose practical utility. Recently an interlaboratory study was performed to estimate the between-laboratory variation of mean GI values (Wolever et al. 2003).It included five laboratories, which used capillary blood sampling to determine the GI values (glucose ¼ 100) of four centrally provided foods and locally obtained white bread (Fig. 14).The pooled interlaboratory SD of the GI values for the five foods was approximately 9. From this the probability of two laboratories obtaining GI values for the same food within limits from 0 to 20 have been calculated as 12 2N, where N equals the area under the normal distribution curve between 2 C and minus infinity, where C ¼ D=ðsd £ p 2Þ, where D is the GI difference and SD is the interlaboratory SD (Fig. 15).With an interlaboratory SD of 9, the chance of two laboratories obtaining a GI value for the same food which differs by less than 15 is about 76 % (i.e. a 24 % chance that the difference is greater than 15).If the interlaboratory SD could be decreased to 8, 7, 6 or 5, the chance of obtaining a GI difference of , 15 increases to 82, 87, 92 and 97 %, respectively.With an interlaboratory SD of 9, there is a 95 % chance that the difference in GI value obtained for the same food in two different laboratories is , 25.If the interlaboratory SD were decreased to 8, 7, 6, or 5, the respective difference would fall to 23, 20, 17 and 14.It should be noted that these calculations are based on exactly two laboratories.The probability of such differences between any two of a group of laboratories will be higher.

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§ Glycaemic index methodology > Scientific background.

§ Glycaemic index methodology > Scientific background.

§ Glycaemic index methodology > Calculation of glycaemic index

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Interlaboratory values and variation How comparable are GI values measured by different laboratories?Scientific background.Mean GI values measured in different laboratories vary.Fig. 13 shows the distribution of mean GI values reported for several foods (Foster-Powell et al. 2002).By just looking at these data, it is difficult to draw conclusions about whether the variation represents methodological differences, true differences between foods of similar description, or random variation.It is also difficult to determine the magnitude of true random variation of mean GI values between different laboratories.For some foods, such as apple, lentils and brown bread, the interlaboratory SD (the SD of the mean GI calculated within each laboratory) of the GI values is , 10, whereas for others, such as potato, the interlaboratory SD of the mean values is .20.An interlaboratory SD of , 10 would likely be acceptable because this would mean that the 95 % CI for any GI value would be , ^20.Such a value is larger than ideal, but does allow one to know with confidence that the GI values of many low-GI foods (for example, GI of lentils ¼ 28) will almost always have a lower GI than many high-GI foods (for example, GI of bread ¼ 71) when determined by different laboratories using standardised and correct methods.However, if the interlaboratory SD was much over 10, then the CI formed by combining the mean GI from each laboratory into an overall mean would become so large that it would be difficult to distinguish between foods, and the GI would lose practical utility. Recently an interlaboratory study was performed to estimate the between-laboratory variation of mean GI values (Wolever et al. 2003).It included five laboratories, which used capillary blood sampling to determine the GI values (glucose ¼ 100) of four centrally provided foods and locally obtained white bread (Fig. 14).The pooled interlaboratory SD of the GI values for the five foods was approximately 9. From this the probability of two laboratories obtaining GI values for the same food within limits from 0 to 20 have been calculated as 12 2N, where N equals the area under the normal distribution curve between 2 C and minus infinity, where C ¼ D=ðsd £ p 2Þ, where D is the GI difference and SD is the interlaboratory SD (Fig. 15).With an interlaboratory SD of 9, the chance of two laboratories obtaining a GI value for the same food which differs by less than 15 is about 76 % (i.e. a 24 % chance that the difference is greater than 15).If the interlaboratory SD could be decreased to 8, 7, 6 or 5, the chance of obtaining a GI difference of , 15 increases to 82, 87, 92 and 97 %, respectively.With an interlaboratory SD of 9, there is a 95 % chance that the difference in GI value obtained for the same food in two different laboratories is , 25.If the interlaboratory SD were decreased to 8, 7, 6, or 5, the respective difference would fall to 23, 20, 17 and 14.It should be noted that these calculations are based on exactly two laboratories.The probability of such differences between any two of a group of laboratories will be higher.

§ Glycaemic index methodology > How to handle outlier values? The advantage in dealing with GI values is that by indexing the individual glycaemic response to a standard, the variation that occurs due to several factors, for example, subject and food-related ones, the between-individual variation is minimised (Wolever, 1990a).Checking the data obtained for the individual subjects may sometimes reveal values that are implausible by comparison with the rest of the data from other subjects.Outliers are particularly important because they can have considerable influence on the results of a statistical analysis (Altman, 1991).Outliers could be due to incorrect subject preparation (for example, non-fasting), analytical error, or errors in data calculation.In the absence of errors, an outlier could be due to an unrepresentative response to the standard or test food or to the subject having a true idiosyncratic response.An unrepresentative response to the standard food is suspected if the subject's GI values for other test foods are also outliers in the same direction, or if there is large variability for the repeated standard tests (Wolever et al. 1991).If outliers are excluded, this fact should be stated, and the results including the outlying point(s) should be given along with the position of the outlying point(s) in relation to the rest of the data, i.e. number of SD from the mean (including the outlier(s)).This will allow others to assess the impact of removing the outlier(s) on the results and conclusions and to form an opinion of the reasonableness of excluding the outlier(s). Recommendation.There is no general advice or rule in statistical analysis to exclude outlying data from the complete data-set.Nevertheless, if an outlying value does not seem to be plausible or if any mistake in determining this data point is evident, this value could be regarded as suspicious and removed from the data-set (Altman, 1991).Therefore, under certain circumstances and for reasons of practicability an outlier could be removed from the data-set without reassessment of that particular GI value.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Interlaboratory values and variation How comparable are GI values measured by different laboratories?Scientific background.Mean GI values measured in different laboratories vary.Fig. 13 shows the distribution of mean GI values reported for several foods (Foster-Powell et al. 2002).By just looking at these data, it is difficult to draw conclusions about whether the variation represents methodological differences, true differences between foods of similar description, or random variation.It is also difficult to determine the magnitude of true random variation of mean GI values between different laboratories.For some foods, such as apple, lentils and brown bread, the interlaboratory SD (the SD of the mean GI calculated within each laboratory) of the GI values is , 10, whereas for others, such as potato, the interlaboratory SD of the mean values is .20.An interlaboratory SD of , 10 would likely be acceptable because this would mean that the 95 % CI for any GI value would be , ^20.Such a value is larger than ideal, but does allow one to know with confidence that the GI values of many low-GI foods (for example, GI of lentils ¼ 28) will almost always have a lower GI than many high-GI foods (for example, GI of bread ¼ 71) when determined by different laboratories using standardised and correct methods.However, if the interlaboratory SD was much over 10, then the CI formed by combining the mean GI from each laboratory into an overall mean would become so large that it would be difficult to distinguish between foods, and the GI would lose practical utility. Recently an interlaboratory study was performed to estimate the between-laboratory variation of mean GI values (Wolever et al. 2003).It included five laboratories, which used capillary blood sampling to determine the GI values (glucose ¼ 100) of four centrally provided foods and locally obtained white bread (Fig. 14).The pooled interlaboratory SD of the GI values for the five foods was approximately 9. From this the probability of two laboratories obtaining GI values for the same food within limits from 0 to 20 have been calculated as 12 2N, where N equals the area under the normal distribution curve between 2 C and minus infinity, where C ¼ D=ðsd £ p 2Þ, where D is the GI difference and SD is the interlaboratory SD (Fig. 15).With an interlaboratory SD of 9, the chance of two laboratories obtaining a GI value for the same food which differs by less than 15 is about 76 % (i.e. a 24 % chance that the difference is greater than 15).If the interlaboratory SD could be decreased to 8, 7, 6 or 5, the chance of obtaining a GI difference of , 15 increases to 82, 87, 92 and 97 %, respectively.With an interlaboratory SD of 9, there is a 95 % chance that the difference in GI value obtained for the same food in two different laboratories is , 25.If the interlaboratory SD were decreased to 8, 7, 6, or 5, the respective difference would fall to 23, 20, 17 and 14.It should be noted that these calculations are based on exactly two laboratories.The probability of such differences between any two of a group of laboratories will be higher.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Scientific background.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Washout period between two consecutive tests

§ Glycaemic index methodology > Interlaboratory values and variation How comparable are GI values measured by different laboratories?Scientific background.Mean GI values measured in different laboratories vary.Fig. 13 shows the distribution of mean GI values reported for several foods (Foster-Powell et al. 2002).By just looking at these data, it is difficult to draw conclusions about whether the variation represents methodological differences, true differences between foods of similar description, or random variation.It is also difficult to determine the magnitude of true random variation of mean GI values between different laboratories.For some foods, such as apple, lentils and brown bread, the interlaboratory SD (the SD of the mean GI calculated within each laboratory) of the GI values is , 10, whereas for others, such as potato, the interlaboratory SD of the mean values is .20.An interlaboratory SD of , 10 would likely be acceptable because this would mean that the 95 % CI for any GI value would be , ^20.Such a value is larger than ideal, but does allow one to know with confidence that the GI values of many low-GI foods (for example, GI of lentils ¼ 28) will almost always have a lower GI than many high-GI foods (for example, GI of bread ¼ 71) when determined by different laboratories using standardised and correct methods.However, if the interlaboratory SD was much over 10, then the CI formed by combining the mean GI from each laboratory into an overall mean would become so large that it would be difficult to distinguish between foods, and the GI would lose practical utility. Recently an interlaboratory study was performed to estimate the between-laboratory variation of mean GI values (Wolever et al. 2003).It included five laboratories, which used capillary blood sampling to determine the GI values (glucose ¼ 100) of four centrally provided foods and locally obtained white bread (Fig. 14).The pooled interlaboratory SD of the GI values for the five foods was approximately 9. From this the probability of two laboratories obtaining GI values for the same food within limits from 0 to 20 have been calculated as 12 2N, where N equals the area under the normal distribution curve between 2 C and minus infinity, where C ¼ D=ðsd £ p 2Þ, where D is the GI difference and SD is the interlaboratory SD (Fig. 15).With an interlaboratory SD of 9, the chance of two laboratories obtaining a GI value for the same food which differs by less than 15 is about 76 % (i.e. a 24 % chance that the difference is greater than 15).If the interlaboratory SD could be decreased to 8, 7, 6 or 5, the chance of obtaining a GI difference of , 15 increases to 82, 87, 92 and 97 %, respectively.With an interlaboratory SD of 9, there is a 95 % chance that the difference in GI value obtained for the same food in two different laboratories is , 25.If the interlaboratory SD were decreased to 8, 7, 6, or 5, the respective difference would fall to 23, 20, 17 and 14.It should be noted that these calculations are based on exactly two laboratories.The probability of such differences between any two of a group of laboratories will be higher.

• AssumptionRests on an assumption: The five foods and five laboratories in the interlaboratory study are representative enough that the pooled SD of about 9 applies to standardised GI measurements generally.

The paper states this without a source or data behind it here, so there is nothing to check it against. The quote shows it is in the paper.

§ Glycaemic index methodology > Scientific background.

A conclusion the paper draws from its own results — it cites no other source for it, so there is none to check. The quote shows it is in the paper.

§ Glycaemic index methodology > Washout period between two consecutive tests