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We evaluated insulin secretion and insulin action with the hyperglycemic (125 mg/dl above basal) and euglycemic insulin (40 mU.m-2.min-1) clamps in seven moderately obese subjects before and after a 6-wk exercise training program. Thirty-nine normal-weight, age-matched subjects served as controls. Both fasting plasma insulin concentration and glucose-stimulated (hyperglycemic clamp) insulin secretion were significantly (P less than .001) increased in the obese subjects. After the training program fasting insulin levels decreased by 26% (P less than .01). Insulin secretion in response to hyperglycemia decreased by a similar percentage (P less than .01). Nonetheless, total-body glucose metabolism increased significantly (P less than .05) during the hyperglycemic clamp. With the euglycemic insulin clamp, obese subjects were shown to be significantly (P less than .001) insulin resistant compared with controls. The decrease in total-body glucose uptake resulted from defects in both peripheral glucose disposal and suppression of hepatic glucose production. After the 6-wk training program, insulin-mediated total-body glucose metabolism increased due to significant improvements in peripheral glucose uptake (P less than .01) and more effective suppression of hepatic glucose production (P less than .05). These results indicate that a moderate-intensity physical conditioning program is capable of ameliorating the insulin resistance and reducing glucose-stimulated hyperinsulinemia observed in obese subjects with normal glucose tolerance.

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