/A

Background: Insulin resistance is a major characteristic of non-insulin-dependent diabetes mellitus and obesity. Many studies have indicated that increased intake of fat are associated with obesity and insulin resistance. On the other hand, chronic exercise is known to improve insulin sensitivity. However, the mechanisms by which high fat diet induces insulin resistance and exercise trainmg improves insulin sensitivity are not established. This study was undertaken to examine the mechanisms by which high fat diet and exercise training affect the insulin sensitivity in the whole body and in skeletal muscles. Methods: Male Sprague-Dawley rats were divided into three groups: high fat sedentary group, high fat exercise group, and control(low fat sedentary) group. High fat diet consists of 66.5% fat and 12.5% carbohydrate, and control(low fat) diet consists of 12 5% fat and 66.5% carbohydrate. Exercise training was performed by swimming three hours per day. After 3 weeks, animals underwent hyperinsulinemic euglycemic clamp study to measure whole body glucose metabolic fluxes. Glycogen synthase activity and glucose-6-phosphate (G-6-P) levels were measured in skeletal muscle at the end of the clamp study. Results: In the high fat diet group, whole body glycolysis and glycogen synthesis were decreased. Exercise training reversed the insulin resistance induced by high fat diet by increasing both glycolysis and glycogen synthesis. Glycogen synthase activity in skeletal muscle was reduced in high fat diet group, and it was partially reversed by exercise training. G-6-P level in skeletal muscle was increased in high fat diet group, and it was further increased by exercise training. Conclusion: These results suggested that the insulin resistance in high fat diet-fed rats is due to the impairment in glucose metabolism at sites distal to G-6-P, i.e. glycolysis and glycogen synthesis. In contrast, the improvement in insulin sensitivity by exercise training in high fat-fed rats is primarily due to the increased glucose metabolic flux proximal to G-6-P, i.e. glucose transport and phosphorylation.