INTERRELATIONSHIPS BETWEEN MUSCLE MORPHOLOGY, INSULIN ACTION, AND ADIPOSITY

There is evidence that insulin resistance and obesity are associated w ith relative increases in the proportion of glycolytic type IIb muscle fibers and decreases in the proportion of oxidative type I fibers. Fu thermore, insulin resistance and obesity are associated with the fatty acid (FA) profile of structural membrane lipids. The present study wa s undertaken to define interrelationships between muscle fiber type an d oxidative capacity, muscle membrane FA composition, and insulin acti on and obesity. Muscle morphology, insulin action, and body fat-conten t were measured in 48 male nondiabetic Pima Indians. Percent body fat (pFAT, determined by hydrodensitometry) correlated negatively with per centage of type I fibers (r = -0.44, P = 0.002) and positively with pe rcentage of type IIb fibers (r = 0.40, P = 0.005). Consistent with thi s finding, pFAT was also significantly related to oxidative capacity o f muscle, as assessed by NADH staining (r = -0.47, P = 0.0007) and cit rate synthase (CS) activity (r = -0.43, P = 0.008). Insulin action was correlated with oxidative capacity (CS; r = 0.41, P = 0.01) and weekl y correlated with percentage of type IIb fibers (r = -0.29, P = 0.05). In addition, relationships were shown between muscle fiber type and F A composition (e.g., percentage of type I fibers related to n-3 FA; r = 0.37, P = 0.01). Thus leaness and insulin sensitivity are associated with increased oxidative capacity and unsaturation of membranes in sk eletal muscle. Present studies support the hypothesis that muscle oxid ative capacity and fiber type may play a genetically determined or an environmentally modified role in development of obesity and insulin re sistance.