Hypothalamic obesity in the weanling rat: dietary self-selection, actual macro-nutrient intake, caloric regulation and response to subsequent low palatability diet.
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Weanling male Sprague-Dawley rats received bilateral electrolytic lesions in the ventromedial hypothalamic nuclei (VMNL rats); sham-operated rats served as controls. For 28 post-operative d all animals were fed three equicaloric [16.9 kJ/g (4.03 kcal)] diets with different amounts of macronutrients in each (HCD = high-carbohydrate diet, HFD = high-fat diet and HPD = high-protein diet). VMNL rats selected more than controls from HCD (P less than 0.05) and HFD (P less than 0.01) but similar amounts from the HPD. The total intake from all three diets was greater (P less than 0.001) than that of the controls, but in percent of total diet intakes, VMNL selected proportions comparable to the controls. In terms of macronutient intake, VMNL rats ingested more than controls from each (carbohydrate, P less than 0.001; fat, P less than 0.05; protein, P less than 0.01). Again, in percent of total macronutrient intake, they ingested proportions comparable to the controls. Lee Index (P less than 0.001) was greater and body weight gain/kJ (kcal) smaller (P less than 0.001) in VMNL rats than in controls. However, body weight gains were normal. For the following 14 d, one group of VMNL rats and one control group continued to self-select from the three diets while another VMNL and control group received lab chow [14.2 kJ/g (3.39 kcal)]. Analysis of variance showed a lesion effect for the Lee Index (P less than 0.001) and Lee Index gain /kJ(kcal) (P less than 0.01) but body weight gains and caloric intake were normal among the groups, ie the VMNL rats switched to chow decreased their caloric intake to control levels. On sacrifice, white and brown fat percent protein (P less than 0.001 for both), carcass lipid (P less than 0.001) and protein (P less than 0.01) and plasma insulin (P less than 0.001) showed lesion effects, but there were no differences among the groups in plasma glucose, glycerol, total protein and free-fatty acids. Availability of palatable diets immediately following VMN lesion placement in weanling rats will result in hyperphagia that after one month recedes to normophagia, whether the rats are fed palatable or less palatable diets. Availability of a less palatable diet (chow) following presentation of palatable diets will not result in diminished caloric intake, body weight, obesity and hyperinsulinemia.
