We studied the effect of physical training on in vivo tissue sensitivity to insulin and insulin binding to monocytes in six previously untrained healthy adults. Physical training (one hour of cycle-ergometer exercise four times per week for six weeks) failed to alter body weight but resulted in a 20 per cent increase (P<0.02) in maximal aerobic power (V̇O2 max) and a 30 per cent increase (P<0.01) in insulin-mediated glucose uptake (determined by the insulin clamp technique). The increase in insulin sensitivity correlated directly with the rise in V̇O2 max (P<0.05). Binding of [125I]insulin to monocytes also rose by 35 per cent after physical training (P<0.02), primarily because of an increase in the concentration of insulin receptors. Our data indicate that physical training increases tissue sensitivity to insulin in proportion to the improvement in physical fitness. Physical training may have a role in the management of insulin-resistant states, such as obesity and maturity-onset diabetes, that is independent of its effects on body weight. (N Engl J Med 301:1200–1204, 1979) TRAINED athletes typically have normal glucose tolerance together with diminished plasma insulin levels, suggesting enhanced body sensitivity to insulin.1,2 Recent studies also indicate that insulin binding to monocytes is higher in athletes than in untrained subjects.2,3 However, the relation between circulating glucose and insulin levels during a glucose-tolerance test and measurements of insulin binding to monocytes provide, at best, only indirect assessments of tissue sensitivity to insulin. The effect of physical training on in vivo insulin sensitivity has not been examined with use of the direct measurements of glucose uptake during physiologic hyperinsulinemia that can be obtained with the insulin.
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