The effects of high-fat diet on exercise-induced changes in metabolic parameters in Zucker fa/fa rats

The objectives of this study were to document the effects of moderate aerobic exercise on insulin secretion and other metabolic indices in fa/fa rats and to determine if a high-fat (HF) diet altered these effects. Six-week-old fa/fa and lean Zucker rats were either sedentary or exercised by daily swimming for 4 weeks. Half of the exercised and sedentary rats were fed a diet with 16% fat and 44% carbohydrate, while the control groups were fed a diet with 4.5% fat and 49% carbohydrate. At the end of 4 weeks, caloric intake, weight gain, plasma hormone and nutrient levels, and oral glucose tolerance were measured. The pancreatic islet beta-cell function was assessed by measuring glucose-stimulated insulin secretion, glucose phosphorylating activity, and free fatty acid (FFA) oxidation in cultured islets. In fa/fa rats fed the control diet, exercise reduced weight gain, caloric intake, and fasting plasma triglyceride (TG) concentrations without affecting fasting glucose and insulin concentrations. HF diet blocked the effects of exercise on weight gain and food intake and worsened insulin resistance of fa/fa rats. In vitro, neither exercise nor HF diet alone affected islet beta-cell function. However, in combination, exercise and high dietary fat reduced glucokinase sensitivity to glucose and increased islet cell response to mannoheptulose inhibitory actions. We conclude that beneficial effects of moderate exercise on metabolism are not mediated by effects on pancreatic beta cells. Diets elevated in fat decrease the beneficial effects of exercise on metabolic indices in vivo.