Almost simultaneously Grafe (21) announced that nitrogen equilibrium could be attained on a diet of carbohydrate and ammonium acetate. Abderhalden (22) repeated this work and found a partial utilization of ammonium salts for replacing the nitrogen lost through tissue waste, but denied the possibility of attaining nitrogen equilibrium. Underhill and Goldschmidt (23) confirmed the view that there is under these conditions a sparing action of body protein by ammonium citrate, but all the tissue loss could not be replaced from nitrogen in the form of ammonium salts. In all these studies we now know that the diets employed were deficient in the three vitamins, fat-soluble A, water-soluble B, and water-soluble C, so the interpretation of the results are not now as satisfactory as they once appeared to be.

It has been abundantly demonstrated by several workers, that the simplest of the amino-acids, glycocoll, is readily synthesized by the body tissues (24). Casein contains no glycocoll, yet it is a complete protein, and can meet all the requirements of an animal for nitrogen in the form of amino-acids. From these results it is safe to conclude that the tissues of the higher animals are capable of synthesizing certain amino-acids, and that these are made use of in some way to conserve the body proteins, even though the list which can be so synthesized is incomplete. It appears certain that tyrosin, tryptophan, and probably all the other cyclic amino-acids and cystin cannot be synthesized by the mammal (25). Abderhalden suggested that the sparing action of ammonium salts is due to prevention of destruction of some of the amino-acids derived from the tissues, and that these may be used over again.