Muscle Fiber Type Adaptations To Exercise Differ In Obese And Non-obese Volunteers

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Human skeletal muscle is composed of slow fibers (type I), fast fibers (IIa and IIx), and a continuum of hybrid fibers co-expressing different myosin heavy chains. Cellular responses to exercise involve changes in fiber type proportions and cross sectional area (CSA). To our knowledge, the impact of obesity on these responses is not yet known. PURPOSE: To determine if obesity impacts changes in fiber type proportions and CSA in response to endurance exercise in a sedentary population. METHODS: Twenty-two obese sedentary healthy men and women (O, BMI>30kg/m2) and 15 non-obese (Lean, L, BMI<26kg/m2) volunteers participated in a 4 months supervised endurance exercise intervention. 18 endurance trained volunteers matched by gender and age (60-79) served as controls (C). Vastus lateralis muscle biopsies were analyzed using immunohistochemistry to determine fiber type distribution and CSA. Baseline group comparisons were made using 1-way ANOVA. Pre/post-intervention changes were assessed by paired t tests. RESULTS: At baseline, both L and O had less type I fibers than C. No difference was found in proportions of type IIa, IIx or hybrid I-IIa fibers. The proportion of hybrid type IIa-IIx was higher in O than C. Type I CSA was larger in O than L. No difference in CSA was found for type IIa, IIx or hybrid fibers. With intervention, proportions of type I, IIa, IIx and hybrid type IIa-IIx were not modified. An increase in proportion of hybrid type I-IIa was seen in L but not in O. Type I CSA increased in O and L, while IIa CSA increased only in L. CONCLUSION: Different adaptations in proportion of fiber types and CSA were observed in O and L previously sedentary volunteers. The increased proportion of hybrid I-IIa fibers with intervention observed in L could be interpreted as a shift of fibers towards a more oxidative muscle such as the profile of C. This transition was not observed in O. As exercise dose was similar in both groups, this unequal shift may be time-dependent, thus not yet apparent here in O. Although fiber CSA was on average 1.5x larger in O than L at baseline, similar CSA increments were observed with endurance exercise in both groups. In previously sedentary seniors, increments of CSA with exercise, even if concurrent to a significant weight loss in the obese subjects, are crucial to prevent age related muscle atrophy.