Interrelationships between muscle fibre type, substrate oxidation and body fat

Abstract

OBJECTIVE: To investigate interrelationships between muscle fibre type, respiratory exchange ratio (RER) during exercise at a fixed workload and adiposity.

DESIGN: Cross-sectional study.

SUBJECTS: 21 untrained, healthy male subjects.

MEASUREMENTS: Body fat composition by dual-energy X-ray absorptiometry (DEXA). Exercise test at 55% of VO_2max, muscle fibre type composition, 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 type I muscle fibres (r=−0.55, P<0.02). In addition percent trunk fat was negatively correlated with percent type I fibres (r=−0.58, P<0.01) while this relationship was not present for percent leg fat. There was no relation 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, supporting skeletal muscle fibre type as a potential etiological factor in obesity. No correlation was observed between percent body fat and substrate oxidation at rest or during moderate exercise, indicating that muscle fuel substrate mix does not appear to provide a mechanism for this relation under either condition.