Teun Remmers

Relationship between PA and the development of BMI in children | 23 PA and time period x PA) were checked separately from the regression coefficients of the interaction terms, in a complete model containing the main effects and the interaction term. When effect modification was not statistically significant, the interaction term was deleted from the model. When statistically significant interaction was present, we subsequently stratified the analyses. The theoretically considered interaction between initial weight status and PA was stratified irrespective of significance of the interaction term. Goodness of fit statistics were computed to compare models. In all analyses, statistical significance was assumed at p < 0.05. Results In total, 334 children provided valid PA measurements at T 0, and 297 of these participants also provided valid BMI scores at T 0 and T 1 . At T 1 , 317 children provided valid PA measurements, and 278 of these participants also provided valid BMI scores at T 1 and T 2 . Children participating in either period 1, 2, or both periods were comparable with the total KOALA Birth Cohort ( N = 2834) in terms of gender (49.7% versus 51.2% boys, respectively), and recruitment group (87.7% versus 82.7% participants with a conventional lifestyle, respectively). In total, 105 participants provided valid measurements for all PA and BMI variables. Participant characteristics The characteristics of PA behavior and BMI measurements are presented in Table 1. The mean duration of the first period was a little longer than two years (mean age at T 0 : 4.86 years; mean age at T 1 : 7.22 years for both boys and girls), while the mean duration of the second period was a little shorter than two years (mean age at T 1 : 7.21 years for boys and 7.12 years for girls; mean age at T 2 : 9.09 years for boys and 9.04 years for girls) (Table 1). At both T 0 (i.e. baseline period 1) and T 1 (i.e. baseline period 2), boys spent a somewhat higher percentage of time on light PA and MVPA than girls, but in boys, light and MVPA showed more variation (higher standard deviations, Table 1). Except for T 2 , boys showed slightly lower BMI z-scores than girls. At all times the study participants were somewhat leaner than the Dutch reference population (BMI z-scores below 0, with the exception of girls at T 0 , BMI z-score +0.02). The leaner category corresponded to ≤ -0.70 BMI score standard deviations, and the heavier category corresponded to ≥ +0.42 BMI score standard deviations. In total, 63 boys (22.3%) and 81 girls (27.7%) were classified as heavier, whereas 74 boys (26.1%) and 71 girls (24.3%) were classified as leaner. Boys and girls did not differ significantly in terms of BMI z-scores within all periods. All remaining participants were classified as normal weight. When this classification was compared against the international obesity taskforce (IOTF) thresholds, 46.20% of leaner children and 31.25% of heavier children were classified as normal weight according to the IOTF (4).