The relationship between physical activity and body composition changes over time in children

Abstract

Physical inactivity is a risk factor for chronic disease and obesity. The objective of this three year longitudinal study was to determine how body composition and physical activity (PA) change over time. Body fat (Slaughter equation) and body mass index (BMI) were determined for children (8 to10 years) over one year (n=41) and 2 years (n=15). Triaxial accelerometry assessed daily PA (kcal/kg/d). Physical activity increased by 1.78 ±0.68 kcal/kg/d (P=0.007) over one year, while PA increased by 2.69±1.07 kcal/kg/d over two years (P=0.025). There was no change in body fat over one year (P=0.204) or two years (P=0.425). When controlling for initial body fat and sex, there was a relationship between changes in body fat and final PA (r2=0.302, P=0.017). Height (P<0.0001) and mass (P<0.012) significantly increased over the one and two year periods. While BMI increased over one year (P=0.001) but not over two years (P=0.416). Body mass index did not significantly change over time after adjusting for changes expected due to growth. When controlling for initial BMI and sex, there was no relationship between changes in BMI and final PA (r2=0.157, P=0.726). Thus, changes in body fat, but not BMI are predicted by the final level of PA achieved by children. Supported by Manitoba Institute of Child Health.