Teun Remmers

16 | Chapter 2 Abstract Purpose : Studies estimating the contribution of physical activity (PA) to the development of body mass index (BMI) in critical periods of childhood are warranted. Therefore, we have prospectively investigated this relationship in boys and girls of the KOALA Birth Cohort study, the Netherlands, in the period around adiposity rebound (i.e. 4-9 years old). Methods : PA was assessed in 470 children (231 boys, 239 girls) using accelerometers at the ages of 5 and 7 years, and height and weight were measured at 5, 7, and 9 years. BMI z-scores were calculated to standardize for age and gender. Leaner and heavier children were classified according to the 25 th and 75 th percentile of our study sample. To examine longitudinal relationships between PA and BMI z-scores, generalized estimating equation analyses were performed, stratified for gender and baseline weight status (leaner, normal weight, and heavier children). Results : In heavier children, an increment of 6.5 minutes of moderate to vigorous physical activity (MVPA) was related to a subsequent decrease of 0.03 BMI z-scores both in boys (95% CI = -0.07 to -0.001) and girls (95% CI = -0.05 to -0.002). Light physical activity was also associated with a decrease of BMI in heavier boys but not girls. In normal weight children, MVPA was associated with decrease of BMI in boys but not girls. Conclusion : In both heavier boys and girls, increments in MVPA between the ages of 4-7 years were associated with beneficial BMI development. Promoting MVPA should remain a major prevention vehicle for improving body composition in 4-9 year-old children.