Table IV shows the composition of a number of proteins of animal and vegetable origin with respect to glutamic acid. This amino-acid is a never-failing component of all vegetable proteins and of all animal proteins except the protamins, which have been found in the spermatozoa of fish. It will be seen that this hydrolytic product is frequently much more abundant in vegetable proteins than in animal tissues or in egg or milk proteins.

With these data available, it is easy to understand why food proteins show great variations in nutritive value, for this depends upon the efficiency with which they can be transformed during growth, or in the course of the repair of tissue waste into tissue proteins. They also reveal the logic of taking a mixture of proteins in the food, since by this means there is a probability that the deficiencies of one protein may be in some measure corrected by another which may yield liberal amounts of those amino-acids it lacks wholly or in part. Since in practice both in human nutrition and animal nutrition several proteins are always taken together, either when vegetable or animal tissues are used as food or when the by-products of manufacture are employed as supplements to naturally occurring foods, it is of great economic importance to discover what foods contain proteins which possess high supplementary values. Protein is the most expensive component of foods or feeding-stuffs, and agricultural economics can be greatly advanced by scientific discovery in this field. In human nutrition the element of economy, in ordinary times, enters less as a factor than physiological well-being, but it would appear that it is no less important here that a protein supply of high biological value be provided in the diet. It seems logical to prefer to serve the needs of the body by providing it with the optimal amount of such proteins as can be utilized very effectively rather than to require it to digest and assimilate an excessive amount of proteins of low value. In this connection it must be remembered that the moiety of amino-acids which cannot be properly matched with each other so as to form the necessary structures have to be promptly destroyed and their degradation products eliminated as waste. This would impose an unnecessary burden on the organs concerned with the metabolism of the nitrogenous components of food.