A Method of Determining the Biological Value of Protein

Probably the most direct method of measuring the degree to which the animal body utilizes dietary protein for those purposes for which this class of nutrients alone can serve is by means of a study of the nitrogenous metabolism under certain definite conditions. These conditions are: first, that in so far as possible the diet contains only that protein or mixture of proteins which it is desired to investigate; second, that in so far as possible the diet contains no non-protein nitrogenous substances except those present in the food under investigation; and, third, that the food intake be so adjusted as to composition and amount that dietary protein will not be used as a source of energy except in so far as such disposition is conditioned by incomplete utilization for the maintenance of the nitrogenous integrity of the tissues, for their growth, or for the elaboration of milk. It would seem to be legitimate to investigate the value of a food in covering the body’s requirements for nitrogen, even though a considerable fraction of the nitrogen content of the food is in non-protein forms. The biological value obtained in such a case should, of course, refer to the total nitrogen of the food, not to its total protein. However, even when these conditions are satisfactorily met, the interpretation of the data of the nitrogenous metabolism relative to the purpose for which they have been obtained is not simple, being complicated by the fact that both urinary and fecal excretions contain nitrogen resulting from tissue catabolism or unavoidable tissue wastage, as well as the nitrogen resulting from the incomplete digestibility or the subsequent incomplete utilization of the dietary protein. Since it is the purpose of the experiment to measure the wastage of dietary nitrogen in digea-