Employing the method described above, in which an effort is made to determine the extent to which any protein or mixture of proteins falls short of the best quality yet observed in proteins, McCollum, Simmonds and Parsons made an elaborate study of a long list of simple combinations of two natural foods, supplemented with respect to all factors other than protein. In all cases the protein constituted 9 per cent of the diet since this plane of intake is a critical one even when the protein is of good quality. The life histories of experimental rats confined to such diets show clearly whether the protein content of any diet made on the plan described should be classed as poor, good or excellent. The details of the extent to which the animals are able to grow, perform the functions of reproduction, and retain the characteristics of youth, enable one to differentiate between shades of quality in this moiety of the diet, to a degree of refinement not equalled by any other method of study.

Employing this method, McCollum, Simmonds and Parsons studied the relative values of the proteins of kidney, liver and muscle of the ox as the sole source of protein. Of these kidney is easily shown to be superior to the other two tissues. As sources of amino-acids liver is second, and muscle third in value.

In Chart 4 are presented curves showing the extent to which young rats are able to grow when confined to diets which were comparable in all respects and the protein came from a single source. With the exception of the diet in which milk proteins were supplied as the sole source of nitrogen, the diets described in this chart derived their protein from a single seed, either a cereal grain or a legume seed.