We studied the effects of body composition and maximal aerobic power on insulin sensitivity in 23 normal-weight, healthy male subjects. Eight were weight lifters, eight were long-distance runners, and seven were untrained controls. In each subject, the percentage of body weight (BW) made up of muscle and fat tissue (% muscle and % fat, respectively), the maximal aerobic power (O2max), and the tissue sensitivity to insulin were measured. The weight lifters were characterized by 35% higher % muscle as compared with the runners or controls (P < 0.01). O2max in the runners was 30–40% higher than in the weight lifters or controls (P < 0.001). During the euglycemic clamp studies, similar steady-state plasma glucose and insulin levels were achieved in each group. When calculated per total BW, the rate of glucose metabolism (M) was virtually identical in the weight lifters (10.26 ± 1.02 mg/kg BW/min) and the runners (10.03 ± 0.86 mg/kg BW/min), and 40–45% higher than in the controls (7.10 ± 0.75 mg/kg BW/min, P < 0.05). When calculated per muscle mass (Mm), only the runners had a higher than normal rate of glucose metabolism (P < 0.02). M was directly proportional to % muscle (r = 0.54, P < 0.01) and inversely related to % fat (r = −0.72, P < 0.001). The multiple linear regression analysis revealed a highly significant multivariate correlation between M and the combined effect of % muscle, % fat, and VO2max (r = 0.78, P < 0.0005). In addition, a regression equation for the predicted M was obtained as follows: M (mg/kg BW/min) = 0.697 + 0.155 % muscle − 0.312 % fat + 0.065 VO2max (ml/kg/min).
In conclusion, first, body sensitivity to insulin is directly related to muscle mass and inversely proportional to adiposity. Second, it may be possible in healthy man to predict the rate of glucose metabolism when body composition and O2max are known.