Objective: In obese patients, brain serotonergic stimulation via orally administered 5-hydroxy-tryptophan (5-HTP), the precursor of serotonin, causes decreased carbohydrate intake and weight loss. Since diabetes mellitus is associated with depressed brain serotonin, hyperphagia and carbohydrate craving, we hypothesized that in diabetic patients, orally administered 5-HTP stimulates brain serotonergic activity and thus normalizes eating behaviour. To test this hypothesis, we investigated whether in diabetic patients: 1) predicted brain serotonin concentrations are depressed as a result of decreased availability of the precursor, tryptophan; and 2) oral 5-HTP is effective in reducing energy and carbohydrate intake.
Subjects and methods: 25 overweight non-insulin dependent diabetic outpatients were enrolled in a double-blind, placebo-controlled study, and randomized to receive either 5-HTP (750 mg/d) or placebo for two consecutive weeks, during which no dietary restriction was prescribed. Energy intake and eating behaviour, as expressed by macronutrient selection, were evaluated using a daily diet diary. Plasma amino acid concentrations and body weight, as well as serum glucose, insulin and glycosylated haemoglobin were assessed.
Results: 20 patients (nine from the 5-HTP group and 11 from the Placebo group) completed the study. Brain tryptophan availability in diabetic patients was significantly reduced when compared to a group of healthy controls. Patients receiving 5-HTP significantly decreased their daily energy intake, by reducing carbohydrate and fat intake, and reduced their body weight.
Conclusions: These data confirm the role of the serotonergic system in reducing energy intake, by predominantly inhibiting carbohydrate intake, and suggest that 5-HTP may be safely utilized to improve the compliance to dietary prescriptions in non-insulin dependent diabetes mellitus.