Subcutaneous Abdominal Fat and Thigh Muscle Composition Predict Insulin Sensitivity Independently of Visceral Fat

Whether visceral adipose tissue has a uniquely powerful association with insulin resistance or whether subcutaneous abdominal fat shares this link has generated controversy in the area of body composition and insulin sensitivity. An additional issue is the potential role of fat deposition within skeletal muscle and the relationship with insulin resistance. To address these matters, the current study was undertaken to measure body composition, aerobic fitness, and insulin sensitivity within a cohort of sedentary healthy men (n = 26) and women (n = 28). The subjects, who ranged from lean to obese (BMI 19.6-41.0 kg/m²), underwent dual energy X-ray absorptiometry (DEXA) to measure fat-free mass (FFM) and fat mass (FM), computed tomography to measure cross-sectional abdominal subcutaneous and visceral adipose tissue, and computed tomography (CT) of mid-thigh to measure muscle cross-sectional area, muscle attenuation, and subcutaneous fat. Insulin sensitivity was measured using the glucose clamp technique (40 mU · m∼² · min⁻¹), in conjunction with [3-³H]glucose isotope dilution. Maximal aerobic power (Vo_2max) was determined using an incremental cycling test. Insulin-stimulated glucose disposal (R_d) ranged from 3.03 to 16.83 mg · min⁻¹· kg⁻¹ FFM. R_d was negatively correlated with FM (r = -0.58), visceral fat (r = -0.52), subcutaneous abdominal fat (r = -0.61), and thigh fat (r = -0.38) and positively correlated with muscle attenuation (r = 0.48) and Vo_2max (r = 0.26, P < 0.05). In addition to manifesting the strongest simple correlation with insulin sensitivity, in stepwise multiple regression, subcutaneous abdominal fat retained significance after adjusting for visceral fat, while the converse was not found. Muscle attenuation contributed independent significance to multiple regression models of body composition and insulin sensitivity, and in analysis of obese subjects, muscle attenuation was the strongest single correlate of insulin resistance. In summary, as a component of central adiposity, subcutaneous abdominal fat has as strong an association with insulin resistance as visceral fat, and altered muscle composition, suggestive of increased fat content, is an important independent marker of insulin resistance in obesity.