The Determination of the Net Utilization of Proteins by a Shortened Method

Abstract

The various methods of measuring the nutritive value of proteins have been frequently reviewed (Chick, 1942;Mitchell, 1944;Allison, 1949).One of the more valuable of these is the measurement of the biological value by the balance-sheet procedure of Thomas (1909) and Mitchell (1923-4), although a considerable drawback to its use is the time and labour required.For example, with the technique described by Henry, Kon & Watson (1937) the examination of three proteins with twelve rats extends over 7 weeks and involves 120 nitrogen estimations on urine and faeces.A method is described here, whereby seven proteins can be tested simultaneously in 10 days, using thirty-two rats and involving no N estimations other than on the food.The method is based on the formula already discussed (Bender & Miller, 1953 a).In principle the values obtained are the same as those obtained by the Thomas-Mitchell procedure, being a measure of that proportion of the food N eaten retained by the test animals. EXPERIMENTAL Animals.Rats of our hooded strain were used in all the experiments.T h e animals were weaned at 21 days at a weight of about 35 g, and fed for I week on stock diet so that they weighed 50-60 g at the beginning of the experiment. Chemical analyses.Body water was determined after killing the rats with chloroform.Incisions were made into the skull, thoracic and body cavities, and the carcasses were dried to a constant weight at 105' (48 h).Intestinal contents were included.N was determined by the Kjeldahl method.Small rats, of less than 15 g dry weight, were completely digested with 10 ml.conc.H,SO,/g dry matter and 20 g catalyst (2 g SeO,, 2 g CuSO,.5H,O, 16 g K,SO,).T h e dried carcasses of the larger rats were pounded in a mortar and 10-15 g portions digested.The large size of the portions compensated for any lack of homogeneity as shown by the agreement between duplicate digestions.Samples of this size, especially those rich in fat, frequently froth violently during digestion, but this frothing was avoided by maintaining the digestion mixture at 120' for 2 h before strong heating.T h e digested material was finally diluted for the micro-estimation of ammonia (Markham, 1942).Diets.T h e basal non-protein diet was composed of 15% margarine fat, 10% potato starch, 15 yo glucose, 5 yo vitaminized carbohydrate mixture (see p. 383), 5 yo salt mixture (Hawk, Oser & Summerson, 1947) and 50% rice starch.The protein