Purpose: Increased body fatness, especially abdominal obesity, and low levels of fitness are associated with decreased insulin sensitivity. Men and women differ in obesity, body fat distribution, and fitness levels. This cross-sectional study evaluated sex differences in the relationships of insulin sensitivity with fatness and fitness and obesity. Methods: Subjects were nonsmoking, nondiabetic, sedentary men (n = 50) and women (n = 61) aged 55-75 years with mild hypertension. Study measures were insulin sensitivity (QUICKI: 1/[log(fasting insulin) + log(fasting glucose)]), lipids and lipoproteins, total body fatness using dual energy x-ray absorptiometry (DXA), anthropometrics, abdominal obesity using magnetic resonance imaging (MRI), and aerobic fitness assessed as Vo2 peak during treadmill testing. Results: Women had a higher percentage of body fat and more abdominal subcutaneous and less visceral fat than men. Among women, QUICKI correlated negatively with body mass index (BMI), percent body fat, abdominal total fat, subcutaneous fat, and visceral fat but not with lipids. Among men, QUICKI correlated negatively with total and abdominal fatness and triglycerides. QUICKI correlated with fitness in men only. Using stepwise regression, among women, decreased total abdominal fat accounted for 33%, and postmenopausal hormone therapy accounted for an additional 5% of the variance in QUICKI. Among men, only a higher level of fitness independently correlated with insulin sensitivity, accounting for 21% of the variance (p < 0.01). Conclusions: Abdominal obesity among women and fitness among men were the strongest determinants of insulin sensitivity in this older cohort. This raises the question whether there are sex differences in the lifestyle changes that would be most effective in improving insulin sensitivity.
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