Does Exercise Without Weight Loss Improve Insulin Sensitivity?

It has long been established that a single exercise session can increase insulin-stimulated glucose uptake in previously sedentary adults (1). A single bout of moderate intensity exercise can increase glucose uptake by at least 40% (2). The benefits of exercise diminish rather quickly, however, as the effects generally dissipate within 48 to 72 h of the last exercise session (3). This observation is reinforced by investigations demonstrating that cessation of exercise in trained persons is associated with a marked and rapid decrease in insulin sensitivity (4). A detailed review of the putative mechanisms by which exercise enhances insulin action is the subject of several excellent reviews (5,6). Although a single bout of exercise improves glucose metabolism acutely, the impact of exercise training (e.g., the summation of acute bouts of exercise) on glucose metabolism is unclear. The issue is not whether exercise training is associated with improvement in glucose tolerance and insulin action. Recognition that exercise training enhances the ability of skeletal muscle to glucose uptake was reported over 30 years ago when Bjorntorp et al. (7) observed that insulin action is greater in physically active than sedentary individuals. Rather, the issue is whether the impact of exercise training on glucose metabolism is a direct effect or an indirect effect that depends on concomitant reduction in body fat …