Abdominal Adiposity Is a Stronger Predictor of Insulin Resistance Than Fitness Among 50–95 Year Olds

OBJECTIVE: Physical inactivity and increased adiposity contribute to insulin resistance; less is known, however, about the relative contributions of these factors in older adults. The aim of this study was to determine whether cardiovascular fitness, whole-body adiposity, or abdominal adiposity is the strongest predictor of insulin resistance into old age. RESEARCH DESIGN AND METHODS: Subjects included 407 men and women aged 50-95 years (means +/- SD 69 +/- 11 years). Insulin resistance was estimated using the insulin sensitivity index (ISI) of Matsuda and DeFronzo ISI = 10,000/square root of (fasting glucose x fasting insulin) x (mean glucose x mean insulin during an oral glucose tolerance test); lower ISI = greater insulin resistance. Fitness was determined with a treadmill maximal oxygen consumption (Vo(2max)) test. Whole-body adiposity measures included BMI and percent fat by dual-energy X-ray absorptiometry or hydrodensitometry; abdominal adiposity was estimated by waist circumference. RESULTS: Waist circumference was the strongest independent correlate of ISI (r = -0.52, P < 0.0001), explaining 28% of the variance when controlling for sex, BMI, percent fat, and Vo(2max). BMI (r = -0.45), percent fat (r = -0.40), and Vo(2max) (r = 0.22) independently predicted ISI (all P < 0.0001); however, after controlling for waist circumference, only Vo(2max) remained significant (r = 0.13, P = 0.009). CONCLUSIONS: Adiposity and fitness continue to be significant predictors of insulin sensitivity into old age, with abdominal obesity being the most important single factor. These findings support the measurement of waist circumference to assess health risk among older adults.