The Urinary Excretion of Tryptophan by Human Subjects on Controlled Diets Varying in Levels and Sources of Protein

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Abstract
The amounts of “free” and total tryptophan excreted per 24 hours by 12 young adult women maintained on diets varying in levels and sources of protein have been studied. The amounts of the conjugated forms in the urine were derived by difference. On constant intakes of a control diet providing 9.8 gm of protein and 885 mg of tryptophan over periods of from 14 to 21 days, the mean levels of excretion were, 30.6 ± 1.91, 10.8 ± 0.73 and 19.8 ± 1.38 mg per day for total, “free” and “bound” tryptophan, respectively. The same subjects were fed a basal diet with total nitrogen intakes of from 7.10 to 8.42 gm and tryptophan intakes of from 521 to 616 mg per day for 14 to 35 days. On this regime there was a slight decrease in the average amount of the “bound” form excreted (from 19.8 to 15.5 mg per day) while the average amount of “free” tryptophan excreted was the same: namely, 10.8 mg per day on both control and basal dietary regimes. There was no apparent relation between the amounts of total, “free” and “bound” tryptophan excreted and the source or amount of dietary nitrogen. Further, the amount of tryptophan excreted was not related to body weight, surface area, basal calories or urine volume. The data presented indicate a selective reabsorption of this amino acid by the tubules from the glomerular filtrate.