Comparison of the Nutritive Values of L-, DL- and D-Tryptophan in the Rat and Chick

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Abstract
Experiments were designed to compare the nutritive values of L-, DL- and D-tryptophan in rats and chicks. Growing rats and chicks were fed for 19 and 21 days, respectively, diets containing amino acid mixtures with graded levels of either L-, DL- or D-tryptophan so that the regression of weight gain or protein retention on tryptophan intake could be established. After the end of the experiments, plasma free L-tryptophan was estimated by a microbiological method. The nutritive values of DL- and D-tryptophan relative to the L-isomer were 47 and 21%, respectively in chicks and close to 100% in rats. In chicks, plasma free L-tryptophan concentration increased with the increase of L- and DL-tryptophan levels in the diet, but remained at a low level regardless of the D-tryptophan level in the diet. In rats, however, a good correlation was observed between plasma free L-tryptophan and tryptophan level in the diet.