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The relationship of the amino acid constitution of a protein, or of the protein component of a food product, to its nutritive value for the growing rat can be best revealed, in the absence of accurate values for the amino acid requirements, by computing for each protein, or protein mojety, the percentage deviations of the contents of each essential eamino acid, expressed per 16 gm. of nitrogen, from the corresponding contents of a protein mixture, such as that of whole egg. thart is almost completely digestible by the rat and utilizable in adolescent metabolism. This has been done for a series of twenty-eight proteins and protein mixtures for which satisfactory analyses have been secured for all of the essential amino acids. 2. From such computations, the essential amino acid limiting the nutritive efficiency of the protein will be revealed as that one whose percentage deficit from that of the standard protein (whole egg) is the greatest, due consideration being given to the limiting amino acids thus indicated agree with those determined in feeding experiments with only one or two exceptions. 3. The proteins of foods may be ranked in the order of decreasing nutritive efficiency on the basis of increasing percentage deficits (as above defined) in their respective limiting essential amino acids. These percentage deficits are highly correlated (r = -0.86) with the corresponding biological values determined by the nitrogen metabolism method. Little or no correlation exists between the chemical ratings of th proteins and their digestibility by the growing rat. 4. The biological value of a protein (y) may be roughly estimated from its maximum percentage deficit in an essential amino acid (x) by the equation: = 102 - 0.634x. 5. However, there are known instances in which the biological value of a protein, or protein mixture, and its chemical rating do not agree, for various reasons discussed in the text. 6. In particular, the nutritive value of cereal proteins may be greatly impaired bt the application of heat with no demonstrated alteration in their content of the essential amino acids. 7. A basis for prediciting the effect of heat on the biological value of a food protein is suggested.
Mitchell et al. (Sat,) studied this question.