The anorectic effect of increasing doses of L-tryptophan in obese patients.

Serotonin (5-HT) synthesis in neurons is initiated by hydroxylation of the essential amino acid tryptophan. Treatments that raise the level of tryptophan in the brain can rapidly alter the rate at which it is converted to 5-HT. This paper compares the effect of 1, 2 and 3 g L-tryptophan administered 1 h before a plated meal on total food intake and carbohydrate and protein consumption in 10 obese subjects (group I) versus a lactose placebo in another 10 obese subjects (group II). The mean +/- SD total calorie intakes were: group I, 1188 +/- 94 (1 g); 1031 +/- 93 (2 g: p < 0.05) and 1016 +/- 85 (3g: p < 0.05); group II: 1294 +/- 62. There was a progressive decrease in carbohydrate consumption (expressed as available monosaccharide) in function of the tryptophan dose: placebo 131 +/- 8 g; one g tryptophan 123 +/- 9; two g 114 +/- 13; three g 107 +/- 10. Protein consumption was less affected. These results provide further support for the view that serotoninergic mechanisms play a role in the regulation of human food intake. They are also consistent with the hypothesis that nutrients which increase 5-HT availability selectively alter carbohydrate consumption. Further studies with modified molecules of naturally occurring tryptophan (hydroxytryptophan hydrochloride or diethylpropionate) may offer a potential field for the treatment of pathological ingestive behavior.