Effects of mild hyperinsulinemia on the metabolic response to exercise

To assess the effects of mild hyperinsulinemia on the metabolic adaptations to exercise, five normal male subjects were studied during 2 cycles of 30 min rest followed by 60 min mild exercise. During one cycle, insulin was infused at a rate of 0.33 mU/kg/min. During both cycles, plasma glucose concentration was kept constant by a glucose-controlled glucose infusion system. Studies with and without insulin were performed in random order, with 30 min between studies. During the insulin infusion, plasma non-esterified fatty acids fell during rest and failed to rise with exercise, indicating a limited availability of this substrate to working muscle. Insulin infusion also inhibited the expected rise in glycerol and 3-hydroxybutyrate normally observed during exercise. Plasma lactate concentrations at the completion of exercise with insulin infusion were higher than after exercise without insulin infusion. Greater metabolic dependence on carbohydrate metabolism is suggested by an increased respiratory quotient during insulin infusion. Insulin infusion had no significant effect on the amount of glucose which needed to be infused for maintaining a constant plasma glucose during rest, but there was a large and significant difference in the need for infused glucose during exercise with and without insulin infusion. The results indicate that even mild hyperinsulinism interferes with normal metabolic responses to exercise, and suggest that fall in insulin concentration seen with exercise is an important regulatory process, not merely a secondary consequences of a declining plasma glucose level.