Dietary tryptophan enhances platelet aggregation in rats.

The effect of diets enriched with tryptophan (4 and 10 g/kg) on ADP induced platelet aggregation and on plasma lipids was studied with growing rats, following feeding periods of up to 3 weeks. Dietary tryptophan was found to enhance markedly ADP induced platelet aggregation. It appears that this effect was not related to plasma lipid levels. In vitro studies aimed to clarify the mechanism by which tryptophan exerts its action showed that the latter, at concentrations up to 100 micrograms/ml, had no effect on ADP induced platelet aggregation. On the other hand, serotonin, the metabolic product of tryptophan, increased the ADP induced platelet aggregation in a dose dependent pattern. The possible involvement of serotonin in the observed enhancement of platelet aggregation was further substantiated by the observed high levels of 5-hydroxyindole acetic acid, which is a catabolite of serotonin, in the urine of the experimental animals. It is conceivable that tryptophan enhances platelet aggregation in rats via its metabolite, serotonin, and this may in turn contribute to increased atheroslerotic risk.