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Objective: Insulin resistance and beta-cell dysfunction both are important contributors to the pathogenesis of type 2 diabetes. Exercise training improves insulin sensitivity, but its effects on beta-cell function are less well studied.Research Design and Methods: Sedentary, overweight adults were randomized to control or one of three 8-month exercise programs: 1) low amount/moderate intensity, 2) low amount/vigorous intensity, or 3) high amount/vigorous intensity. Of 387 randomized, 260 completed the study and 237 had complete data. Insulin sensitivity (S(i)), acute insulin response to glucose (AIRg), and the disposition index (DI = S(i) x AIRg) were modeled from an intravenous glucose tolerance test.Results: Compared with control subjects, all three training programs led to increases in DI. However, the moderate-intensity group experienced a significantly larger increase in DI than either of the vigorous-intensity groups and through a different mechanism. The high-amount/vigorous-intensity group improved S(i) and had a compensatory reduction in AIRg, whereas the moderate-intensity group had a similar improvement in S(i) but almost no reduction in AIRg. Importantly, the inactive control group experienced a significant increase in fasting glucose.Conclusions: To the extent that the DI accurately reflects beta-cell function, we observed that both moderate- and vigorous-intensity exercise training improved beta-cell function, albeit through distinct mechanisms. It is not clear which of these mechanisms is preferable for maintenance of metabolic health. While moderate-intensity exercise led to a larger improvement in DI, which may reflect a transition toward a more normal DI, longer-term investigations would be necessary to determine which was more effective at reducing diabetes risk. [ABSTRACT FROM AUTHOR]