The work reviewed clearly establishes that the nutritive value of proteins is determined by their yields of the eighteen or more amino-acids which are formed on digestion. The more nearly these proportions correspond to the content of amino-acids in the tissues of the growing animal, the more effectively can food proteins be transformed into body proteins. There are surprising differences in the biological values of the proteins in certain of our more important foods. These will be discussed more fully later.

There is no convincing evidence that the processes of maintenance, or the repair of tissue waste, are of a lower order than those of growth, and that the needs of the former are met by a list of amino-acids which would be lacking in certain ones which are essential for the latter. Neither has it been possible to demonstrate that through the agency of the mammary gland, amino-acids (e. g., lysin) can be synthesized, for the nutrition of the young for the preservation of the species, whereas this cannot be effected for the preservation of the individual. The evidence all points to the conclusion that the transformation of food protein into milk protein is governed by the same laws as apply to growth.