Relationship between physical activity and the development of body mass index in children.

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 yr old). METHODS: PA was assessed in 470 children (231 boys, 239 girls) using accelerometers at the ages of 5 and 7 yr, and height and weight were measured at 5, 7, and 9 yr. BMI z-scores were calculated to standardize for age and sex. Leaner and heavier children were classified according to the 25th and 75th percentile of our study sample. To examine longitudinal relationships between PA and BMI z-scores, generalized estimating equation analyses were performed and stratified for sex and baseline weight status (leaner, normal weight, and heavier children). RESULTS: In heavier children, an increment of 6.5 min of moderate to vigorous PA (MVPA) was related to a subsequent decrease of 0.03 BMI z-scores both in boys (95% confidence interval = -0.07 to -0.001) and girls (95% confidence interval = -0.05 to -0.002). Light PA 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: Increments of MVPA were associated with decreases in BMI z-score in heavier children, both boys and girls. Promoting MVPA should remain a major prevention vehicle for improving body composition in 4- to 9-yr-old children. Copyright © 2013 by the American College of Sports Medicine.