Effects of aspartame ingestion on the carbohydrate-induced rise in tryptophan hydroxylation rate in rat brain. 
Effects of aspartame (aspartyl-phenylalanine-methylester) on increases in brain-tryptophan level and hydroxylation rate following a high-carbohydrate, protein-free meal were tested.  After an overnight fast, rats consumed a protein-free meal containing one of several levels of aspartame.  Blood and brain amino acid levels and the in vivo rate of tryptophan hydroxylation in brain were estimated at intervals thereafter.  Ingestion of the meal alone increased brain-tryptophan level and hydroxylation rate.  Aspartame did not modify these effects, except at doses of 530 mg/kg body weight or more.  Results suggest a threshold dose of aspartame can be identified for the rat in single-meal studies above which suppression of carbohydrate-induced increases in brain-tryptophan level and serotonin synthesis occurs.  This dose, however, is large and, when corrected for species differences in metabolic rate, is unlikely to be ingested by a human subject as a single load.