The effects of a 10-day low-calorie diet (LCD; n = 8) or exercise training (ET; n = 8) on insulin secretion and action were compared in obese men (n = 9) and women (n = 7), aged 53 +/- 1 yr, with abnormal glucose tolerance by using a hyperglycemic clamp with superimposed arginine infusion and a high-fat drink. Body mass (LCD, 115 +/- 5 versus 110 +/- 5 kg; ET, 111 +/- 7 versus 109 +/- 7 kg; p < 0.01) and fasting plasma glucose (LCD, 115 +/- 10 versus 99 +/- 4 mg/dl; ET, 112 +/- 4 versus 101 +/- 5 mg/dl, p < 0.01) and insulin (LCD, 23.9 +/- 5.6 versus 15.2 +/- 3.9 microU/ml; ET, 17.6 +/- 1.9 versus 13.9 +/- 2.4 microU/ml; p < 0.05) decreased in both groups. There was a 40% reduction in plasma insulin during hyperglycemia (0 to 45 min) after LCD (peak: 118 +/- 18 versus 71 +/- 14 microU/ml; p < 0.05) and ET (69 +/- 14 versus 41 +/- 7 microU/ml; p < 0.05) and trends for reductions during arginine infusion and a high-fat drink. The 56% increase in glucose uptake after ET (4.95 +/- 0.90 versus 7.74 +/- 0.82 mg/min/kg fat-free mass; p < 0.01) was significantly (p < 0.01) greater than the 19% increase (5.72 +/- 1.12 versus 6.80 +/- 0.94 mg/min/kg fat-free mass; p = not significant) that occurred after LCD. The marked increase in glucose disposal after ET, despite lower insulin levels, suggests that short-term exercise is more effective than diet in enhancing insulin action in individuals with abnormal glucose tolerance.

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