Skeletal Muscle Fiber Type Distribution May Predict Phenotype At Risk Of Developing Metabolic Syndrome

PURPOSE: Skeletal muscle fiber type has been associated with metabolic health. Individuals with a high proportion of type II fibers have a greater body mass index (BMI); but subjects with obesity, and a higher proportion of type I fibers, experience the greatest weight loss. Thus, we hypothesize that skeletal muscle fiber type may predict the metabolic phenotype of individuals at risk of developing the metabolic syndrome even in non-obese individuals. The purpose of this study was to determine if fiber type distribution associates with markers of the metabolic syndrome in subjects classified as overweight. METHODS: A skeletal muscle biopsy from the vastus lateralis and blood samples were taken from overweight (BMI 25.0-29.9 kg/m2), but otherwise healthy individuals (free from cancer, hypertension, and diabetes) after an overnight fast. HOMA-IR was calculated from fasting glucose and insulin. Visceral adiposity was measured by DXA. Participants were subject to a graded exercise test to determine VO2max. Skeletal muscle fiber type (myosin heavy chain) distribution was assessed via immunohistochemistry. In the absence of normality, bias corrected and accelerated bootstrap Pearson correlations and associated p-values were used. RESULTS: A total of 38 participants (20 female, 18male), aged 29 ± 8 years, were included in the study. Participants were type II fiber dominant (65.1 ± 12.9%) and sedentary (VO2max 31.3 ± 8.1 mL/kg/min). No differences in fiber types existed between the sexes (p = 0.14). Visceral adipose tissue (VAT) (r = 0.48, p = 0.004), android:gynoid (r = 0.46, p = 0.005), and HOMA-IR (r = 0.45, p = .01) were positively associated while HDL (r = -0.33, p = 0.05) and VO22max (r = -0.37, p = 0.03) were negatively associated with type II skeletal muscle fibers. Additionally, fasting lactate, an early predictor of metabolic disease risk, positively associated with type II fibers (r = 0.44, p = 0.009). CONCLUSIONS: Young, overweight individuals with a high proportion of type II fibers may already be developing visceral obesity and risk factors of the metabolic syndrome. Thus, having a high percentage of type II skeletal muscle fibers may be an innate driver of metabolic disease. Supported by: NIH R01 DK120296