Behavioral and endocrine traits of obesity-prone and obesity-resistant rats on macronutrient diets

Patterns of eating behavior, body weight gain, and hormone changes were examined in normal-weight albino Sprague-Dawley rats on macronutrient diets. These diets consisted of either three separate jars with pure macronutrients, fat, carbohydrate and protein, from which to choose, or a single diet with different concentrations of fat and carbohydrate. Similar patterns on the choice-diet and single-diet paradigms were observed. During the first 7-10 days on these diets but not subsequently, the rats consuming a fat-rich diet exhibit significant hyperphagia, an increase in both total and fat intake that produces higher body weight gain. Compared with a 10% fat diet, a 30% fat diet is associated with a decline in insulin and corticosterone (CORT) levels, whereas a 60% fat diet produces an increase in circulating glucose. Levels of glucose are positively correlated with fat intake, and together these measures are consistently related to body fat. These relationships are most strongly expressed in rats that consume a fat-rich diet with >30% fat. Whereas insulin levels are also positively related to body fat, CORT is inversely related in these normal-weight subjects. In animals consuming a high-fat diet, a clear separation can be seen between "obesity-prone" (OP) rats with 100% greater body fat than "obesity-resistant" (OR) rats. The OP rats, which consume 15% more total calories, have significantly higher insulin and glucose levels. In animals that consume a diet with >30% fat, it is the OP but not the OR rats that exhibit a positive relation between fat intake, glucose levels, and body fat and reveal an additional association between carbohydrate intake, insulin, and body fat. Thus these rats on macronutrient diets exhibit distinct traits that relate behavior to hormone disturbances and adiposity and distinguish subjects that are prone vs. resistant to obesity.