Biological evaluation of proteins: a new aspect

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Abstract
The growth method of comparing the nutritive value of proteins, namely the estimation of protein efficiency ratio (P.E.R.) has long been popular because of its simplicity despite its well-known drawbacks (Mitchell, 1944). Among the more serious disadvantages of this method are: (a) no allowance is made for maintenance requirements of the test animal, (6) the result varies with food intake, and (c) the assumption that the gain in bodyweight is indicative of the protein tissue laid down is not always valid. A modification is described whereby a control group of animals fed on a protein-free diet is included in the experiment, and the difference between the weights of this group and the test group is used in the calculation instead of merely weight gain. This procedure allows for maintenance requirements and also permits the evaluation of poor proteins which do not promote growth (and of which, consequently, P.E.R. cannot be measured). The results obtained are independent of food intake. T h e third criticism mentioned above is shown to be of minor significance under our experimental conditions. EXPERIMENTAL Animals and diets. The rats and diets and amino-acid mixtures were as described previously (Bender, 1956). Body water. The whole carcasses, without evisceration or cleaning of the gut, were dried to constant weight in a hot air oven at 1 0 5 ~. Fat. The dried carcasses were crushed in a mortar and extracted with diethyl ether in a Soxhlet apparatus for 24 h. After evaporation of the solvent the extracted fat was dried at 1 0 5 '. Nitrogen. In the earlier experiments the whole defatted carcass was digested in a large Kjeldhal flask. I n later experiments the defatted carcass was dissolved in 400 ml. 20% (w/v) HC1 in a Kilner jar by autoclaving for IQ h at 10 lb. pressure; the suspension was diluted to 500 ml. and portions were taken for nitrogen determinations. The digestion was catalysed with selenium. Netprotein utilization (N.P.u.). It was determined by the method of Bender & Miller (1953) and Miller & Bender (1955), the modified calculation of Bender & Doell (1957) being used. The nitrogen content of the carcasses was calculated from the water content and often verified by direct estimation. Net protein ratio (N.P.R.). Two groups, each of four animals, balanced as regards litter-mates and combined bodyweight , were used in each test. One group was fed on