Physical exercise is traditionally recommended to diabetic patients as part of their treatment. Although healthy athletes exhibit enhanced skeletal muscle insulin sensitivity, the metabolic effects of vigorous training in patients with insulin-dependent diabetes mellitus (IDDM) are not known. This study was designed to examine the effects of competitive sports on fuel homeostasis and insulin sensitivity in athletes with IDDM. We studied 11 athletes and 12 matched sedentary men with IDDM. In each subject, we measured glycemic control, insulin-stimulated glucose uptake in the whole body and forearm, rates of glucose and lipid oxidation, and muscle glycogen, glycogen synthase, and glucose transport protein (GLUT4) concentrations. The athletes had higher VO2max (52 ± 1 vs. 42 ± 1−1 · min−1, P < 0.001) and HbA1c levels (8.4 ± 0.4 vs. 7.2 ± 0.2%, P < 0.05) than sedentary patients, but took smaller insulin doses (41 ± 3 vs. 53 ± 3 U/day, P < 0.05). The insulin-stimulated rates of whole-body and forearm glucose uptake and glucose oxidation were similar in the two groups, whereas both energy expenditure and lipid oxidation were increased in the athletes. Lipid oxidation correlated inversely with glycogen synthase activity. The mean glucose arterialized venous blood-deep venous blood (A-V) difference during the insulin infusion (60-240 min) correlated with the whole-body glucose disposal throughout the insulin infusion (after 60 min, r > 0.73, P < 0.001 for all 30-min periods). This association is accounted for by the relationship between glucose A-V difference and nonoxidative glucose disposal. Muscle glycogen and GLUT4 protein contents were not different in the two groups. In conclusion, in athletes with IDDM: 1) competitive exercise performed at variable schedules and intensities leads to a decrease in required insulin dose, impairment of metabolic control, and increase in lipid utilization; 2) insulin sensitivity is not enhanced; and 3) glucose A-V difference, not blood flow, is the major determinant of body sensitivity to insulin. Thus, more intense glucose monitoring and education may be required for the maintenance of good control in patients with IDDM involved in competitive sports.

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