Cross‐sectional and prospective associations between physical activity, body mass index and waist circumference in children and adolescents

OBJECTIVE To study the cross-sectional and prospective associations between physical activity (PA) of different intensities, body mass index (BMI) and waist circumference (WC) in children and adolescents using isotemporal substitution modelling. METHODS Physical activity (accelerometry), BMI and WC were assessed in 6- (n = 970), 9- (n = 2,423) and 15-year-olds (n = 1,544) in 2005/2006 and 2011/2012. Participants aged 9 years in 2005/2006 were followed prospectively to 2011/12 (age 15). Associations between PA of different intensities (light, moderate and vigorous), BMI and WC were examined using isotemporal substitution models. RESULTS Substituting 10 min per day of sedentary time with light PA was associated with higher WC (0.17 to 0.29 cm, p ≤ 0.003)) in all age groups. Substituting 10 min per day of sedentary time with moderate PA was associated with lower WC in 6- and 9-year-olds (-0.32 to -0.47 cm, p ≤ 0.013)). Substituting 10 min per day of sedentary time with vigorous PA was associated with lower WC in 9- and 15-year-olds (-1.08 to -1.79 cm, p ≤ 0.015)). Associations were similar with BMI as the outcome. In prospective analyses, substituting sedentary time with light, moderate or vigorous PA at age 9 was not associated with BMI or WC at age 15. CONCLUSION Substituting sedentary time with moderate PA appears favourably associated with adiposity in children, whereas vigorous PA may be required in adolescents. Cross-sectional associations were not replicated in prospective analyses.