A single bout of moderate-intensity exercise increases whole-body insulin sensitivity for 12–48 h post-exercise; however, the relationship between exercise energy expenditure and the improvement in insulin sensitivity is not known. We hypothesized that the exercise-induced increase in whole-body insulin sensitivity, assessed with HOMAIR (homoeostasis model assessment of insulin resistance), is directly related to the energy expended during exercise. We studied 30 recreationally active non-obese men (age, 27±5 years; body mass index, 24±2 kg/m2) in the post-absorptive state on two separate occasions: once after exercising at 60% of V̇O22peak (peak oxygen consumption) for 30–120 min on the preceding afternoon (expending a total of 1.28–5.76 MJ) and once after an equivalent period of rest. Blood samples were obtained the following morning. Exercise-induced changes in HOMAIR were curvilinearly related to exercise energy expenditure (r=−0.666, P=0.001) with a threshold of approx. 3.77 MJ (900 kcal) for improvements in HOMAIR to be manifested. In particular, HOMAIR was reduced by 32±24% (P=0.003) in subjects who expended more than 3.77 MJ during exercise, but did not change for those who expended fewer than 3.77 MJ (−2±21%; P=0.301). Furthermore, the magnitude of change in HOMAIR after exercise was directly associated with baseline (i.e. resting) HOMAIR (r=−0.508, P=0.004); this relationship persisted in multivariate analysis. We conclude that improved whole-body insulin resistance after a single bout of exercise is curvilinearly related to exercise energy expenditure, and requires unfeasible amounts of exercise for most sedentary individuals.

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