Teun Remmers

18 | Chapter 2 Because reversed causation plays a major role in the interpretation of cross-sectional research concerning the relationship between PA and body composition, longitudinal designs are superior. Only a small number of longitudinal studies investigating primary prevention of weight gain have measured PA objectively, which makes the available evidence insufficient and inconclusive (40). Three studies have previously examined this relationship longitudinally in pre-pubertal children, using objective measurements of PA. First, the study of Moore et al. (22) followed a sample of 103 four year-old children for seven years. They reported that higher levels of PA at baseline were associated with lower BMI scores and lower skinfold thicknesses in later childhood. Second, the study of Jago et al. (17) reported a significant inverse relationship between PA and the development of BMI over three years in 3-4 year-old children. Finally, Metcalf et al. (21) did not find a significant relationship between complying with health-enhancing PA recommendations and development of BMI scores, waist circumferences, and skinfold thicknesses over three years in 5-8 year-old children. The effect of PA on the development of body composition has a different clinical implication for children with a heavier initial (baseline) body composition, compared to children with a leaner initial body composition. Because possible influences from dietary compensation or the additional accumulation of muscle mass as a result of strenuous PA may influence the association between PA and BMI development, acknowledging a differential effect in these subgroups may provide new insights. As energy intake is considered to cluster considerably with PA, controlling for energy intake would lead to unwanted indirect controlling for PA. Moreover, as a higher level of PA leads to higher energy intake, controlling for energy intake would annul the observed relations. Therefore, we decided to focus solely on the effect of PA, and energy intake was not controlled for in this study. The acknowledgment of potential gender differences is also important in studies concerning PA and the development of body composition in children, because girls are considered to be less active compared to boys (38) and display a considerable decline in activity energy expenditure before puberty compared to boys (15). The aim of this study was to contribute towards a better understanding of how PA and the development of BMI are related. Compared to other studies, this study is unique in investigating a time span including the adiposity rebound period (i.e. 4-9 years old), its acknowledgment of potential gender differences, the investigation of differences in children with a high initial versus low initial BMI, and the categorization in various intensities of objectively measured PA. We hypothesize that the relationship between PA and BMI decreases in initially heavier boys and girls.