Dietary self-selection and the Zucker rat.

Impaired protein deposition has been suggested to be a critical factor promoting the hyperphagia of the obese Zucker rat. The selection patterns of adult, male, genetically obese (fa/fa) and lean (Fa/-) Zucker rats were studied to determine if obese rats would select a diet that was higher in protein than the diet selected by lean littermates. Rats were provided ad libitum access to three macronutrient sources and were allowed to compose their own diets for 9 days. The three dietary items were: a vitamin + mineral--supplemented carbohydrate source (cornstarch), a vitamin + mineral--supplemented protein source (casein) and commercially available corn oil. Obese rats ate 43% more calories than lean littermates. Further, obese rats selected a diet that provided 12% of their total caloric intake as protein, 24% as carbohydrate and 64% as fat. Lean rats selected a diet that provided 33% of their total caloric intake as protein, 37% as carbohydrate and 30% as fat. These selection data are not consistent with the hypothesized importance of the role of dietary protein and its incorporation into lean body mass as a stimulus promoting the hyperphagia demonstrated by the genetically obese Zucker rats.