Folin and Denis (12), Van Slyke and Meyer (13), and Davis and Whipple (14) have shown beyond a doubt that protein is readily synthesized from amino-acids, for in the intestine, the protein is digested to the amino-acid stage, and the identity of the food proteins is lost. In the process of growth, in the regeneration of liver tissue destroyed as the result of chloroform narcosis, or in the replacement of the complexes in the tissues lost through the disintegration of certain structures in normal metabolism of protoplasm the amino-acids derived from food protein are used for the construction of new proteins of structures different from those of the food.

Mitchell (15) conducted experiments with mice fed a diet modelled after one which was extensively employed by Osborne and Mendel, and which will be discussed later. In this diet he replaced the protein by a definite mixture of amino-acids. The diet contained a product known as "protein-free milk," which furnished approximately half a maintenance ration of protein, but this fact was not known at the time. The efforts to keep mice alive on this diet containing from eight to fifteen amino-acids were unsuccessful. This was in part due to the failure of the animals to eat a sufficient amount of the food to meet their energy requirements. It was found necessary to feed the mice on alternate days on the portion of the food mixture minus the amino-acids, to induce them to take a greater amount of food. He found the omission of tryptophan of greater significance than that of other amino-acids.