Some relationships between the amino acid contents of proteins and their nutritive values for the rat.

It is obvious that the nutritive value of a protein or mixture of proteins for any biological function or combination of functions is limited by the relative proportions of the essential amino acids contained in it; i.e., those amino acids that cannot be synthesized by the animal at a sufficiently rapid rate from any substances present in the usual diets. But it is not so clear that the amino acid make-up of a protein is the only considerabble factor limiting its utilization within the animal body. The experiments of Sherman and Woods (1) on the determination of cystine in proteins by means of feeding experiments with growing rats afford an illustration of a close relationship between amino acid content and the growth-promoting value of protein, though perhaps not sufficiently close to justify using a rat growth technique to check the accuracy of a chemical assay method for an amino acid, as Grau and Almquist (2) have done in their study of the methionine content of various feed proteins. With much the same conviction, Munks, Robinson, Beach, and Williams (3) have assessed the amino acid requirements of the laying hen for the production of one egg as being equal to the amino acids contained in one egg. Undoubtedly, this is a good basis on which to assess the net amino acid requirements, but it may afford quite uncertain information of the amino acid intakes from different protein combinations needed to cover Obese requirements.