MUSCLE-FIBER TYPE COMPOSITION IN INFANT AND ADULT-POPULATIONS AND RELATIONSHIPS WITH OBESITY

OBJECTIVES: To investigate the fibre type composition of skeletal musc le in infants and young children and to compare the findings to an adu lt population. To relate the fibre type profile of skeletal muscle in adults to measures of adiposity. DESIGN: Cross-sectional studies of sk eletal muscle fibre composition in infants and adults with measures of adiposity in the adults. SUBJECTS: 21 healthy infants and young child ren (age: 3-21 months) and 40 healthy adult Australian Caucasians (age : 26-62 y; BMI: 18-48 kg/m(2)). MEASUREMENTS: Skeletal muscle fibre ty pe composition (by myosin ATPase method) and relative body fatness (BM I, waist circumference and waist/hip ratio (WHR)). RESULTS: Infants an d young children had significantly lesser proportions of glycolytic Ty pe 2b fibres (6.2 +/- 1.1%; range 0.3-18.9%) compared with adults (20. 5 +/- 1.6%; range 4.9-36.0%) (p < 0.0001). The percentage of Type 2b f ibres was directly related to BMI (r = 0.44, p = 0.02), waist circumfe rence (r = 0.49, p = 0.009) and WHR (r = 0.44, p = 0.02) in adults. A significant, direct relationship was also found between the proportion of glycolytic Type 2b fibres and age in the adults (r = 0.45, p = 0.0 1). CONCLUSION: Skeletal muscle fibre type composition is different in infants and adults and there is an age-dependent increase in Type 2b fibres over the lifespan. An increased proportion of glycolytic Type 2 b fibres is associated with obesity in adults. Results support a gene- environment interaction on fibre type composition in human skeletal mu scle.