The biological value of proteins

Abstract

DETERMINATION OF ENDOGENOUS NITROGEN EXCRETION.IN attempts to compare the nutritive value of different proteins by their relative power to maintain nitrogenous equilibrium, a trustworthy estimate of the daily loss of endogenous nitrogen, for the particular experimental animal employed, is a primary requirement.The usual method is to determine the nitrogen excreted in the urine and faeces, when the diet consumed is as far as possible free of nitrogen and consists only of carbohydrate and fat, with the addition of the necessary minerals and vitamins.In the course of a long series of experiments, in which the biological value of different proteins has been studied by the "balance sheet method", the variability in the endogenous nitrogen excretion shown by the same animal at different times has been a disturbing factor, and attempts to achieve a greater degree of constancy have not been altogether successful.The method used, employing the adult male rat as experimental animal and the formula adopted for calculating the biological value of the protein under examination have been previously described [Chick and Roscoe, 1930; Boas-Fixsen, 1930; Boas-Fixsen and Jackson, 1932].The formula used is derived from the original expression of Thomas [1909; 1910] as follows:The relative biological value of a protein X is expressed as: 100 x Body nitrogen saved Food nitrogen absorbed =100x 11 (N excreted in experiment with X) -(N excreted on N-free diet)l = 100~(N True nitrogen intake in experiment withX X =100where Ix, UX and FX are the daily N intake, urinary N excretion and faecal N excretion, respec- tively, in the experiment with X, and Ue and Fe are the daily (endogenous) N excreted in urine and faeces, respectively, as determined in experiments on a nitrogen-free diet.Fx, -F. will represent the nitrogen in the faeces derived from undigested food and I,, -(Fx -Fe) the true N intake, in the experiment with protein X.Any irregularity in the values obtained for Ue will affect the value of the above expression (1) more seriously when U, and I, are relatively small, i.e. when the protein investigated is present at a low level in the diet.In our experience a greater divergence has been found in the results of experiments with diets containing 3 % protein, than when 10 % was present.