Protein intake regulation and nitrogen retention in young obese and lean mice

Young female obese (ob/ob) and lean mice were fed a single diet containing 10 or 20% casein or were allowed to self-select from two diets containing 10 and 50, 20 and 60, or 30 and 70% casein for 3 weeks. Obese and lean mice offered a choice of two diets varying in protein-consumed 36% and 32%, respectively, of energy from protein. Although both obese and lean mice consumed more protein when allowed to self-select, each group maintained the same energy intake as observed when a single diet was fed. Because obese mice consumed more energy than lean mice, their self-selected intake of protein was 55% greater than observed in lean mice. The increased protein intake in self-selected obese mice was associated with a decreased tryptophan:large neutral amino acid ratio in their plasma. Average nitrogen retention was only slightly less in obese mice than in lean mice, but the sites of nitrogen deposition differed considerably. Obese mice retained only 35% of their nitrogen in the carcass (skeletal muscle and skeleton) while lean mice retained 58% of their nitrogen in the carcass. In summary, young obese mice allowed to self-select from two diets varying in protein and carbohydrate consumed more protein and more energy, but deposited less nitrogen in their carcasses, than lean mice. Hyperphagia in young obese mice is not directly linked to an increased demand for dietary protein.