Association between exercise, cardiorespiratory fitness and change in insulin sensitivity
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The ability of the body to manage blood sugar is referred to as insulin sensitivity. Reduced insulin sensitivity is both a risk factor and is involved in the pathogenesis of type-2 diabetes. Cardiorespiratory fitness (CRF) is an established predictor of insulin sensitivity. Whether this association persists following control for exercise is not known. The primary objective of this study was to determine the independent associations between changes in exercise, CRF and insulin sensitivity. The influence of changes in body weight and waist circumference (WC) on these associations was examined in secondary analyses. Participants were 140 middle-aged, inactive, abdominally obese adults that participated in a 24-week exercise program. Exercise was performed 5 times per week for the duration of the program and was measured as Calories of energy expended, herein referred to as exercise energy expenditure (exercise EE). CRF was measured using a treadmill test to exhaustion. Body weight was measured on a Detecto scale and WC was measured at the level of the hip bone. Physical activity performed outside of the exercise sessions was measured using portable activity monitors known as accelerometers. Caloric intake was monitored using daily diet records. Following oral consumption of a 75-gram glucose drink, 2-hour insulin area under the curve was calculated as a measure of insulin sensitivity. Change in insulin sensitivity was associated with exercise EE, change in CRF, change in body weight and change in WC. Exercise EE was associated with change in insulin sensitivity after we adjusted for change in CRF, whereas change in CRF was not associated with change in insulin sensitivity after we adjusted for exercise EE. After further adjustment for changes in body weight and WC, neither exercise EE nor change in CRF were associated with change in insulin sensitivity. Changes in body weight and WC were associated with change in insulin sensitivity after adjustment for exercise EE and change in CRF. The principle finding of this study was that the link between CRF and insulin sensitivity is largely explained by variations in exercise. Further, a reduction in obesity is one of the ways in which exercise improves insulin sensitivity.