Biological evaluation of proteins; a new aspect.

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 body-weight 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. The third criticism mentioned above is shown to be of minor significance under our experimental conditions.