A further illustration of the fact that no generalization can be made on the basis of available data, concerning the greater importance of lysin as a practical problem in nutrition, than certain other amino-acids, is afforded by the history of lactalbumin studies. These have already been described (p. 69). Although this protein is one of the richest known in lysin, it is an incomplete protein and not capable of supporting growth unless it is supplemented with a source of a certain missing complex or complexes, the identity of which has not been established.

The proteins of the pea or bean appear from chemical data available, to be more closely similar in their constitution to the principal animal-tissue proteins than are those of the cereal grains, and they are certainly more abundant sources of lysin. Their dietary values, when unsupplemented, fall far short, indeed, of those of the cereals.

Studies like those that have been described, in which the aim was to determine the possibility of supplying the needs of a growing animal for protein-building material with mixtures of amino-acids lacking in one or more of those found among the digestion products of most proteins are extremely valuable. So are studies directed toward determining the nature of the simple additions of amino-acids which make complete certain proteins themselves inadequate as a source of nutriment for tissue building. It is these types of investigations which bring to light the synthetic capacity of the cells and contribute to an understanding of the chemical processes normal to the living body (54). The problems concerned with making the best possible use of our available food-stuffs demand, among other things, an accurate understanding of the biological value for the purposes of maintenance and growth, of the protein mixtures contained in our foods. These problems also demand a knowledge of the extent to which the various possible combinations of foods furnish protein mixtures superior to those of the individual foods when the latter constitute the sole source of protein in the diet (55).