Protein intake regulation in adult obese (ob/ob) and lean mice: effects of nonprotein energy source and of supplemental tryptophan.

Adult obese (ob/ob) and lean mice were allowed to self-select between diets varying in level of protein and carbohydrate, or protein and fat. Obese and lean mice self-selected 29% and 26% of their energy, respectively, from protein when fed diets varying in protein and carbohydrate. Intake of protein decreased 45% in obese mice and 32% in lean mice when diets varying in level of protein and fat were fed. These reductions in protein intake were not associated with an elevation in brain serotonin (5-HT), a putative regulator of protein intake. When the tryptophan content of diets varying in protein and carbohydrate was quadrupled in an attempt to elevate brain 5-HT and thereby reduce protein intake in obese and lean mice, neither the level of brain 5-HT nor protein intake were affected. Under the conditions of these experiments we found no evidence that protein intake was inversely related to brain 5-HT levels in obese and lean mice, nor were we able to manipulate protein intake with dietary tryptophan supplementation. The reduced protein intake in obese and lean mice fed diets varying in level of protein and fat, rather than protein and carbohydrate, may result from their greater preference for a high-fat diet than for a high-carbohydrate diet.