Short-term and Repetitive Administration of Oral Tryptophan in Normal Men

Tryptophan loading produced lethargy and drowsiness within 30 minutes of ingestion under all loading conditions, and subjects with the slowest kynurenine response to tryptophan were most behaviorally affected.

Abstract

Single and repetitive tryptophan loads were consumed by normal, adult male volunteers, and blood concentrations of tryptophan, serotonin, and kynurenine, their time courses, and their distributions within blood were measured. Repeated measures of basal and tryptophan-induced changes in tryptophan and serotonin blood concentrations were characteristic for individual subjects. Tryptophan dose-responsive increases in measured substances returned to basal levels within 24 hours after single tryptophan loads. However, cumulative increases in serotonin concentration in early-morning, predose blood samples were seen following repetitive daily tryptophan administration. Extra-platelet serotonin could be detected in blood samples taken after tryptophan loading and after repetitive daily tryptophan consumption but not in baseline samples taken before short-term loading. Neither platelet number nor size was altered by the loading procedures. Tryptophan loading produced lethargy and drowsiness within 30 minutes of ingestion under all loading conditions. Subjects with the slowest kynurenine response to tryptophan were most behaviorally affected.