Interrelationships between muscle fibre type, substrate oxidation and body fat

Abstract

OBJECTIVE: To investigate interrelationships between muscle ®bre type, respiratory exchange ratio (RER) during exercise at a ®xed 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 V Á O 2max , muscle ®bre 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 ®bres (r 7 0.55, P`0.02). In addition percent trunk fat was negatively correlated with percent type I ®bres (r 7 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 ®bre type composition or percent body fat. CONCLUSION: Body fat and percent type I muscle ®bres were correlated, supporting skeletal muscle ®bre 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.Keywords: type I; body fat; fat distribution; stress IntroductionSkeletal muscle plays a central role in whole-body energy metabolism, contributing an estimated 20% of total energy expenditure in humans. 1 It is the major tissue of insulin-stimulated glucose uptake and contains substantially more glycogen than the liver. 2 Equally, muscle utilises substantial amounts of circulating lipid for energy and contains signi®cant triacylglycerol stores. Skeletal muscle contains a mixture of ®bre types with a range of oxidative capacities. Type I ®bres are of the high oxidative, slow-twitch variety with an excellent capacity for using lipid for fuel at rest and during moderate exercise. Type IIb are glycolytic, fast-twitch ®bres with an almost exclusive reliance on glucose and glycogen for fuel. Type IIa ®bres are intermediate with an oxidative capacity that often overlaps that of type I ®bres. Since fat balance is central to long-term energy balance, and hence the potential for obesity, it is not surprising that there is now some 3 ± 8 (but not unequivocal 9,10 ) evidence for a positive relationship between the relative percentage of type IIb ®bres and adiposity in humans. One of these studies appeared to provide a mechanism underpinning these observations by further observing that individuals with a higher proportion of type I ®bres had a lower respiratory exchange ratio (RER) during low-intensity bicycle exercise; that is a greater reliance on lipid for fuel. 4 Conversely, individuals with a higher proportion of type IIb ®bres had a higher RER during exercise indicating low fat usage. This study, while interesting, was intrinsicallȳ awed because the exercise was carried out at a ®xed workload unadjusted for individual V Á O 2maxvalues. Since the more obese individuals would, ba...

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