Dietary lipids and blood cholesterol: quantitative meta-analysis of metabolic ward studies

Abstract

Objective: To determine the quantitative importance of dietary fatty acids and dietary cholesterol to blood concentrations of total, low density lipoprotein, and high density lipoprotein cholesterol. Design: Meta-analysis of metabolic ward studies of solid food diets in healthy volunteers. Subjects: 395 dietary experiments (median duration 1 month) among 129 groups of individuals. Results: Isocaloric replacement of saturated fats by complex carbohydrates for 10% of dietary calories resulted in blood total cholesterol falling by 0.52 (SE 0.03) mmol/l and low density lipoprotein cholesterol falling by 0.36 (0.05) mmol/l. Isocaloric replacement of complex carbohydrates by polyunsaturated fats for 5% of dietary calories resulted in total cholesterol falling by a further 0.13 (0.02) mmol/l and low density lipoprotein cholesterol falling by 0.11 (0.02) mmol/l. Similar replacement of carbohydrates by monounsaturated fats produced no significant effect on total or low density lipoprotein cholesterol. Avoiding 200 mg/day dietary cholesterol further decreased blood total cholesterol by 0.13 (0.02) mmol/l and low density lipoprotein cholesterol by 0.10 (0.02) mmol/l. Conclusions: In typical British diets replacing 60% of saturated fats by other fats and avoiding 60% of dietary cholesterol would reduce blood total cholesterol by about 0.8 mmol/l (that is, by 10-15%), with four fifths of this reduction being in low density lipoprotein cholesterol. Key messages: The quantitative importance of diet to blood cholesterol remains uncertain because non-experimental dietary studies in community subjects are unreliable and experimental (“metabolic ward”) studies have been too small to be separately reliable We conducted a meta-analysis of 395 published, metabolic ward experiments of the effects of various dietary lipids on blood cholesterol Isocaloric increases in saturated fat intake were associated with increases in total and low density lipoprotein cholesterol and smaller increases in high density lipoprotein cholesterol; increased polyunsaturated fat intake decreased total and low density lipoprotein cholesterol and increased high density lipoprotein cholesterol; monounsaturated fat had no significant effect on total and low density lipoprotein cholesterol but increased high density lipoprotein cholesterol In the average British diet replacement of 60% of the saturated fat by other dietary fats and avoidance of 60% of dietary cholesterol would reduce blood cholesterol by about 0.8 mmol/l (that is, by 10-15%), with four fifths of this reduction being in low density lipoprotein cholesterol The effect on vascular disease of a prolonged difference of 0.8 mmol/l in blood cholesterol concentration depends on the relative importance at different ages of the benefits of reducing low density lipoprotein cholesterol and the hazards of reducing high density lipoprotein cholesterol, which require further study