Interrelationships between muscle fibre type, substrate oxidation and bodyfat

OBJECTIVE: To investigate interrelationships between muscle fibre type, res piratory exchange ratio (RER) during exercise at a fixed workload and adipo sity. DESIGN: Cross-sectional study. SUBJECTS: 21 untrained, healthy male subjects. MEASUREMENTS: Body fat composition by dual-energy X-ray absorptiometry (DEX A). Exercise test at 55% of (V)over dot O-2max, muscle fibre type compositi on, muscle NADH and citrate synthase enzyme activity levels; serum insulin, glucose and cortisol concentrations. RESULTS: Percent body fat was inversely correlated to the proportion of typ e I muscle fibres (r = -0.55, P < 0.02), In addition percent trunk fat was negatively correlated with percent type fibres (r = -0.58, P < 0.01) while this relationship was not present for percent leg fat. There was no relatio n between RER at rest or during exercise and muscle fibre type composition or percent body fat. CONCLUSION: Body fat and percent type I muscle fibres were correlated, supp orting skeletal muscle fibre type as a potential etiological factor in obes ity. No correlation was observed between percent body fat and substrate oxi dation at rest or during moderate exercise, indicating that muscle fuel sub strate mix does not appear to provide a mechanism for this relation under e ither condition.