Plasma Tryptophan Response Curve and Its Relation to Tryptophan Requirements in Young Adult Men

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Abstract
A total of 17 adult human subjects was studied to determine the relationships between the concentration of free tryptophan in fasting and postprandial blood plasma, dietary tryptophan intake, and the daily requirement for this amino acid. The diet, containing an L-amino acid mixture, which was patterned as in egg protein, furnished nitrogen equivalent to 0.5 g egg protein per kilogram body weight/day. In two separate experiments dietary tryptophan intake was varied over a range of 1.0 to 8.7 mg/kilogram body weight. Plasma tryptophan concentration was constant above and below tryptophan intakes of 5 and 3 mg/kilogram body weight, respectively, and changed linearly with altered tryptophan intake within this range. The level of dietary tryptophan above which a linear increase in plasma tryptophan occurred was taken to be the requirement level. Nitrogen balance measurements indicated a tryptophan requirement of 2 to 2.6 mg/kilogram body weight whereas the plasma tryptophan data suggested a requirement of about 3 mg/kilogram body weight. The present nitrogen balance data, which did not include integumental and sweat nitrogen losses, probably underestimated tryptophan needs.

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