Teun Remmers

24 | Chapter 2 Table 1: Timing and results of measurements of physical activity and Body Mass Index Baseline Follow-up boys girls boys girls First period T 0 ( n = 150) T 0 ( n =147) T 1 ( n =150) T 1 ( n =147) % time spent in light: mean (SD) 46.19 (5.42) 44.37 (4.53) - - % time spent in MVPA: mean (SD) 7.97 (2.73) 7.19 (2.38) - - Age at measurement: mean (SD) 4.86 (0.34) 4.86 (0.31) 7.22 (0.38) 7.22 (0.35) BMI z-score: mean (SD) * -0.07 (0.84) 0.02 (0.92) -0.26 (0.94) -0.20 (0.93) Second period T 1 ( n =133) T 1 ( n =145) T 2 ( n =133) T 2 ( n =145) % time spent in light: mean (SD) 39.88 (5.04) 39.19 (3.90) - - % time spent in MVPA: mean (SD) 8.00 (2.49) 7.32 (2.34) - - Age at measurement: mean (SD) 7.21 (0.47) 7.12 (0.39) 9.09 (0.67) 9.04 (0.65) BMI z-score: mean (SD) * -0.26 (0.99) -0.16 (0.90) -0.03 (0.97) -0.07 (0.96) Values are presented as mean ± SD. *: standardized against the age and gender specific reference values of the Fourth Dutch Growth Study (11). Longitudinal relationship between PA behavior and BMI development First, the effect of potential effect modifiers (age x PA, gender x PA and period x PA) was checked. We found that interaction with PA was not statistically significant for age and period (p > 0.05 for all PA categories). Gender significantly interacted with PA in heavier children (p < 0.05 for all PA categories except MVPA), and normal weight children (p < 0.05 for MVPA). Consequently, we decided to perform both pooled (top part of Table 2) and stratified GEE analyses for gender, and identify significant gender interactions with a dagger (see Table 2). In heavier children, a 5% increment of MVPA was associated with a 0.16 decrease in BMI z-score, and this was of similar size in boys (0.17) and girls (0.14), and statistically significant in both (Table 2). In heavier boys, a 5% increment of light PA was also related to a statistically significant decrease of 0.13 BMI z-scores per year. When combining light PA and MVPA into total PA, this amounted to a 0.11 BMI decrease in heavier boys for a 5% increment in total PA. In normal weight boys, only MVPA was associated with a decrease of BMI z-score. Girls generally did not show any decrease in BMI related to PA in the adjusted models, except for MVPA in heavier girls (similar to boys as noticed above).