Food intake and selection after peripheral tryptophan

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.