Teun Remmers

General introduction | 3 The Importance of Physical Activity in Youth The importance of regular Physical Activity (PA) for children's health and well-being is well understood, both worldwide and in the Netherlands. For example, the World Health Organization's Global Recommendations for Physical Activity and Health state that children should participate in at least 60-minutes per day of moderate-to-vigorous PA (MVPA) (1, 2). In the Netherlands, this resulted in comparable PA guidelines of 60-minutes of MVPA per day (3). These guidelines additionally advocate that the relationship between PA and health is continuous; any increment of PA irrespective of the type, intensity, frequency or duration is related to additional benefits for children's general health and well-being. Also, activities should be promoted that strengthen muscles and bones, whereas prolonged sedentary time should be avoided (3). These guidelines reflect recognition of the importance of PA (3). According to national and international monitoring studies, many children are insufficiently active (4, 5). Increasing evidence suggests that PA declines considerably as children age, and that this inactivity tracks from childhood into adolescence and adulthood (6, 7). Several studies have indicated that inactivity is directly linked with various detrimental consequences for health and well-being, such as cardiovascular risk factors and bone health (8, 9). Increasing evidence also suggests that PA in children may enhance academic performance in schools (10), general cognitive functioning (11, 12), and social capabilities in children (13). Besides these potential direct influences, PA may also be beneficial for the prevention of overweight and obesity (8, 14), which in turn may lead to the prevention of type 2 diabetes, hypertension and decreased self-esteem (15-17). Over the last decades, overweight and obesity have dramatically increased among youth in Western countries (18). In the Netherlands, the prevalence of overweight and obesity increased by two- to three-fold in the last three decades, with prevalence rates around 13.3% and 14.9% for boys and girls in 2009, respectively (19). Although previous experimental studies have shown promising results of providing exercise programs for overweight children, results from observational studies investigating relationships between PA and body composition are still mixed (8, 14). This may be primarily caused by differences in assessing PA (e.g. parental-reports, self-reports or objective measurements), or differences in study design (i.e. the majority of studies used cross-sectional designs). As the possibility of reversed causation cannot be ruled out in interpreting results from these observational studies (i.e. PA influencing body composition versus body composition influencing PA), longitudinal designs are warranted to further unravel the potential role of PA in the prevention of obesity. Hence, the first aim of this thesis was to investigate longitudinal associations between objectively measured PA and body mass index (BMI) development in 4-9-year-old children, and to assess variations in this association by age, gender, and initial weight status.