Abstract
Abdominal obesity and low cardiorespiratory fitness (CRF) are associated with insulin resistance in older adults. Exercise is associated with improvement in insulin sensitivity. Whether this association is mediated by change in CRF and/or abdominal obesity is unclear. The current study is a secondary analysis of data from a randomized controlled trial in Kingston, Ontario. Sedentary older adults (60–80 years) (N = 80) who completed the exercise (N = 59) or control (N = 21) conditions for 6 months were included. CRF was measured using a treadmill test, adipose tissue (AT) by magnetic resonance imaging, and insulin sensitivity by hyperinsulinemic-euglycemic clamp. Waist circumference (WC) was measured at the iliac crest. Mediation analyses were used to assess whether abdominal AT and/or CRF mediated the exercise-induced change in insulin sensitivity. By comparison to controls, reduction (mean ± SD) was observed for visceral (-0.4 ± 0.4 kg) and abdominal subcutaneous (-0.4 ± 0.4) AT depots, WC (-4.1 ± 3.2 cm) and BMI (-0.9 ± 0.8 kg/m2) (p < 0.05). Insulin sensitivity (4.2 ± 5.2 M/I) and CRF (0.2 ± 0.3 L/min) improved in the exercise group (p < 0.05). All AT variables, BMI and WC were mediators of the change in insulin sensitivity (p < 0.05). After adjustment for change in total AT, abdominal AT remained a mediator with an effect ratio of 0.79 (p < 0.05), whereas total AT was not significant when adjusted for abdominal AT (p > 0.05). The effect ratio for change in WC and BMI combined (0.63, p<0.05) was greater than either alone. In conclusion, CRF did not mediate the exercise-induced change in insulin sensitivity in older adults. Abdominal adiposity was a strong mediator independent of change in total adiposity.
Highlights
Insulin resistance is a strong predictor of morbidity and mortality [1] and aging is associated with a reduction in insulin sensitivity [2]
The primary finding of this study is that cardiorespiratory fitness (CRF) does not mediate the exercise-induced change in insulin sensitivity in older adults whereas change in abdominal adipose tissue (AT) is a strong mediator independent of total AT
Waist circumference (WC) and BMI together mediate the association between exercise and insulin sensitivity, suggesting both be measured in clinical settings to assess the efficacy of exercise as a strategy for improving blood glucose management in older adults
Summary
Insulin resistance is a strong predictor of morbidity and mortality [1] and aging is associated with a reduction in insulin sensitivity [2]. It is well established that in older adults, chronic exercise is associated with improvement in insulin sensitivity [8] and CRF [9] as well as a reduction in abdominal obesity [9]. Whereas the findings from some short-term intervention studies suggest that changes in CRF are associated with exercise-induced change in insulin sensitivity [7, 11], others do not [10, 12, 13]. Findings from a prior study in middle-aged adults suggest that exercise-induced reduction in insulin resistance was mediated by changes in abdominal obesity, but not CRF [14]. Absent from the literature are data from randomized controlled trials that permit determination of whether changes in abdominal obesity and CRF independently mediate the exercise-induced improvement in insulin sensitivity in older adults
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