Two studies investigated the effects of peripheral (IP) administration of the dietary indispensible amino acid tryptophan, on food intake and macronutrient selection in rats adapted to a 12 hr nocturnal feeding period and a choice of 10% and 60% casein diets. In a dose-response study (35, 55, 75, 95, 115 mg/kg), the threshold dose of 75 mg/kg produced a significant reduction in total food intake (3.6 to 2.3 g, p<0.05) during the first hour of feeding. The reduction in carbohydrate intake (2.1 vs. 1.2 g, p<0.05) was greater than that for protein intake (1.6 vs. 1.1 g, p<0.05). Twelve hr total food intake was also decreased (20.9 to 19.5 g, p<0.05) and this was attributable to decreased carbohydrate intake (13.2 to 11.8 g, p<0.05). In a second study designed to determine if tryptophan's effects were mediated by the central nervous system, brain tryptophan uptake was blocked by co-injecting valine with tryptophan. The significant reduction in first hour total food intake by tryptophan was not prevented by co-injection of an equal quantity of valine (3.5 to 1.8 g, p<0.05). Again the suppression of carbohydrate intake (2.0 to 0.9 g p<0.05) was greater than that for protein intake (1.5 to 0.9 g, p<0.05). This dose of valine significantly reduced brain tryptophan uptake by 16% (21.3 to 17.8 μg/g, p<0.05) and when administered alone did not affect first hour total food intake (3.1 vs 3.2 g). A higher dose of valine (300 mg/kg) produced a larger decrease in brain tryptophan (15.9 to 11.7 μg/g, p<0.05), but when administered alone also significantly decreased food intake (2.9 to 0.7 g, p<0.05). Thus the site and mechanism of action for tryptophan's effects could not be elucidated. It was concluded that intraperitoneal administration of tryptophan suppresses total food intake, with a small but significantly greater effect on carbohydrate intake compared to protein intake.
- Carbohydrate intake
- Protein intake