Nutrient composition: effects on appetite in monkeys with oral factors held constant.

The effects of macronutrients on appetite and total caloric intake in monkeys (Macaca mulatta) was studied using a new feeding and infusion system which yoked intragastric infusion of various nutrients to oral ad lib intake and removed the confounding factor of palatability from the assessment of nutrient effects on feeding behavior. A suction-activated liquid diet feeding system provided free access to a nutritionally complete diet, with 1 ml of diet delivered orally by pump with each discrete suck by the monkey. A second pump was yoked to the oral feeding pump and delivered various nutrients directly into the stomach via an implanted intragastric cannula. Thus, while oral diet composition remained constant, the net diet reaching the stomach varied over ranges of 28 to 77% carbohydrate, 16 to 65% fat and 7 to 36% protein. No significant differences in total caloric intake were observed between intakes of diets with net composition of high carbohydrate or high fat. When protein was increased to 36%, total caloric intake was generally reduced, and this effect was sustained for at least 3 weeks. Therefore, protein appears to have an increased specific satiating effect beyond the caloric content, when compared to carbohydrate or fat.