Influence of Variable Quantities of Lysine, Tryptophan and Isoleucine on Nitrogen Retention of Adult Human Subjects

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Abstract
Experiments were conducted in which healthy adult human subjects consumed a basal diet containing 200 gm. of white wheat flour, a small amount of supplementary lysine and a few foods low in nitrogen. Variable amounts of lysine, tryptophan and/or isoleucine were added to the basal diet. The effects of these diets were compared with each other and with the control diet which contained amounts of essential amino acids comparable to those in 20 gm. of egg protein. In experiment i, results obtained with the basal diet modified to provide 1,000 mg. of lysine, 300 mg. of tryptophan and either 720 or 1,000 mg. of isoleucine did not differ from those obtained with the control diet based on 20 gm. of egg protein. In experiment ii, no significant difference in nitrogen retention was observed whether 1,000, 1,500 or 2,000 mg. of lysine was provided, together with 300 mg. of tryptophan and 1,000 mg. of isoleucine, and all experimental diets were as satisfactory as the control diet. In experiment iii, retention was improved by increasing isoleucine from 720 to 1,170 or 1,600 mg. when 1,000 mg. of lysine and 300 mg. of tryptophan were present, but was more satisfactory when 220 mg. rather than 300 mg. of tryptophan was administered with only 720 mg. of isoleucine. In experiment iv, retention was less when either 220 mg. of tryptophan with 720 mg. of isoleucine or 500 mg. of tryptophan with 1,000 mg. of isoleucine were fed than when other proportions of tryptophan and isoleucine were administered. In experiment v, in which crystalline amino acids were substituted for wheat flour, quantities of all amino acids were constant except that three levels of tryptophan, 220, 300 and 500 mg., were administered in combination with 720 and 1,000 mg. of isoleucine. The nitrogen balances obtained with the unsupplemented basal diet were slightly negative. The addition of either tryptophan or isoleucine improved retention but the effect was not additive. The implications of these findings in respect to the relative proportions of essential amino acids consumed by adult human subjects are discussed.