Cross‐sectional and prospective impact of reallocating sedentary time to physical activity on children's body composition

Summary Background The amount of time children spend in sedentary behaviours may have adverse health effects. Objective To examine the substitution effects of displacing a fixed duration of sedentary time with physical activity (PA) on children's body composition. Methods We included 386 children (197 boys). Outcomes were body mass index, waist circumference, total body fat mass and trunk fat mass assessed by dual‐energy X‐ray absorptiometry. Sedentary time and PA were measured with accelerometers. Data were analysed by isotemporal analyses estimating the effect of reallocating 15 and 30 min d −1 of sedentary time into light (light physical activity), and moderate‐to‐vigorous (MVPA) PA on body composition. Results Reallocating 15 and 30 min d −1 of sedentary time into MVPA was negatively associated with body fatness in cross‐sectional analyses. Prospectively, reallocating 30 min of sedentary time into 30 min of MVPA was negatively associated with waist circumference (β = −1.11, p < 0.05), trunk fat mass (β = −0.21, p < 0.05), and total body fat mass (β = −0.48, p < 0.05) at follow‐up (20 months). The magnitude of associations was half in magnitude and remained significant ( p < 0.05) when reallocating 15 min of sedentary time into MVPA. Reallocating sedentary time into light physical activity was not related ( p > 0.05) with body fatness outcomes. Conclusions Substituting sedentary time with MVPA using isotemporal analysis is associated with positive effects on body composition.