Do Cardiorespiratory Fitness and Abdominal Obesity Mediate the Exercise-Induced Change in Insulin Sensitivity in Older Adults?
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Aging is associated with increased insulin resistance, a condition in which the tissue response to insulin-stimulated glucose uptake is reduced. Insulin resistance is a strong predictor of disease and mortality. Aging is also associated with a decline in physical activity, lower cardiorespiratory fitness (ability to deliver oxygen to active muscles during exercise), and increase in abdominal fat. Both low cardiorespiratory fitness (CRF) and excess abdominal fat are associated with reduced insulin sensitivity in older adults. Improvements in CRF and abdominal obesity through exercise training may be responsible for improvement in insulin sensitivity. Several investigations have reported that changes in CRF and abdominal obesity through exercise are associated with changes in insulin sensitivity. To our knowledge, no prior study has assessed whether change in CRF or abdominal fat alone explains the association between exercise and improvement in insulin sensitivity in older adults. Our findings suggest that improvement in CRF may not explain the exercise-induced change in insulin sensitivity. The improvement in insulin sensitivity from exercise is explained through a decrease abdominal fat that also occurs with exercise. Additionally, improvements in waist circumference, a surrogate measure for abdominal obesity, and body mass index together explained a large portion of exercise-induced change in insulin sensitivity compared to either variable alone. Our findings suggest that exercise combined with a healthy diet will improve insulin resistance, a risk factor for development of type 2 diabetes and cardiovascular disease in older adults. Our findings suggest that the reduction in abdominal obesity is the conduit by which exercise improves insulin sensitivity in older adults. Although CRF is not related to exercise-induced change in insulin sensitivity, change in CRF from exercise has been reported to decrease risk for other health conditions, such as hypertension and all-cause mortality. Our findings suggest that clinicians should measure both waist circumference and body mass index when evaluating the effectiveness of a lifestyle-based treatment strategy for improving insulin resistance and its associated health outcomes in older adults.