Failure of L-tryptophan to stimulate prolactin secretion in man

The effect of oral L-tryptophan (90 mg/kg BW) on prolactin secretion in eight normal women studied either in the early or late follicular phase of the menstrual cycle was compared to the effect of a placebo alone. Plasma prolactin levels were measured before and at frequent intervals over a period of 240 min after the administration of the amino acid or placebo. Plasma free L-tryptophan concentrations were determined prior to and 60 min after ingestion of the amino acid in two of the women. Despite a 25-fold increase in plasma free L-tryptophan levels following the administration of this amino acid, the prolactin response to L-tryptophan was not significantly different from a placebo.