Teun Remmers

28 | Chapter 2 We decided to focus solely on the effect of PA and not to control for energy intake as this would lead to unwanted indirect controlling for PA, and therefore would annul the observed associations. However, as some children compensate MVPA with specific unhealthy dietary habits (e.g. sugar sweetened drinks), this would be interesting to investigate in future studies, but was beyond the scope of the present study. We used 25 th and 75 th percentiles for the classification of relative leaner and heavier children, respectively. We were unable to use the IOTF thresholds (4) for classification of overweight, underweight and normal weight because this would have resulted in too small group sizes. This study showed that children with relatively smaller deviations from normal weight can benefit from MVPA. When interpreting results from accelerometers, some discussion arises concerning thresholds for PA intensities. In the present study, the thresholds of Evenson (12) were used because of the similarity of the age of children in our sample and Evenson’s validation study, and the advantage of using one threshold for all age categories. However, Evenson’s thresholds are notably lower compared to other thresholds (12). Our results, therefore, may have overestimated the percentage of light PA in children, and underestimated the percentage of sedentary behavior. Some studies suggest that differences in accelerometry thresholds predominantly occur in teenage years (29); other studies suggest that these differences are also present in the age categories in our study sample (26, 38). However, because the age range was rather narrow in our study sample, the magnitude of this potential bias was expected to be small. In addition, one may suggest that small but significant differences exist between the two Actigraph accelerometer models used in our study. One previous study found that the GT1M model reported an average 9% lower counts per epoch compared to the 7164 model. However, both models did not significantly differ when counts were conceptualized as time spent in moderate or vigorous category (5). This comparability is supported by two other studies, of which one reported no differences at all (18), and the other reported only slight differences that were not meaningful in activity intensity classification (19). If one did suppose that differences existed, then this potential bias would be randomly distributed over the entire sample, as we were unaware of a child’s BMI when handing out the accelerometers. Discrepancies between the two Actigraph models are therefore unlikely to have significantly biased our results. Impact A 5% daily increment of MVPA per year (i.e. 32.5 minutes) may be difficult for children to achieve. Alternatively, a 1% daily increment of MVPA per year corresponds to only 6.5 minutes. In this case, a heavier child has to spend approximately 6.5 more minutes per day in MVPA in order to achieve an average subsequent decrease of 0.03 BMI z-scores per year. Even though this is a small decrease, when sustained through entire childhood, such a decrease may become relevant for the primary prevention of obesity. In conclusion, we found that increments of MVPA were associated with decreases in BMI z-score in heavier children, both boys and girls.