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86 Chapter 3 There are multiple factors that may partially explain these different and conflicting results. As is shown in Tables 2, 3, and 4, the definitions of overweight and obesity differed among studies. Some studies have used the 85th and 95th percentiles of BMI for age and sex published in a study from the United States as cutoff points to identify overweight and obesity. 31 Other studies have used centile curves on the basis of data from multiple countries (the International Obesity Task Force cutoff values) 32 or the Centers for Disease Control and Prevention growth charts. One study used the cutoff values provided by the World Health Organization (WHO), these gender-specific BMI-for-age percentile curves use z scores. 33 The WHO Child Growth Standards are now widely implemented worldwide in clinical care. 34,35 It has been shown that using different definitions of overweight and obesity may lead to different results in epidemiologic studies. 36,37 This could partially explain the different findings among the studies included in this systematic review. In addition, studies used different definitions for FDDs. Although in all studies, the diagnosis of FDDs was based on the Rome criteria, some used the Rome II criteria, others used the Rome III criteria, and some studies had modified the criteria. It has been shown that using different criteria can lead to major differences in the evaluation of the prevalence of FDDs. 38 Furthermore, only one study was rated to be of good quality on the basis of an assessment of the internal validity and risk of bias, whereas most studies were rated to be of fair or poor quality. Thus, most of these studies are at some risk of bias and should be interpreted with caution. Future high-quality studies are needed to shed more light on this issue. Although evidence from studies performed in tertiary hospital settings seems indicative for an association between FC and overweight, evidence from population-based studies is much less convincing. Potentially, patients in tertiary care centers may not be representative of the population as a whole. These patients may represent a subset of patients with risk factors for FDDs and overweight/obesity that were not accounted for in the studies. Lifestyle factors such as diet and a physical activity are assumed to play an important role in the pathophysiology of both FC and overweight, 20,39–43 which may explain why some studies have revealed an association between these disorders. For overweight and obesity, the pathophysiological importance of dietary factors (e.g., high-caloric diet and low fiber intake) and a sedentary lifestyle is well recognized. 20,44–48 Therefore, treatment of childhood obesity mainly consists of dietary and physical activity modifications, often utilizing behavioral interventions. 49,50 The suggested role of dietary factors, especially the role of fiber, in the pathogenesis of FC is generally well-accepted, although pediatric data are scarce. 20,39,40,51,52 The pathophysiological role of physical exercise is less well described and may be disputable. 41–43,51 Studies on fiber supplementation in the treatment of FC in children have resulted in conflicting results 53 and no randomized controlled trials on the effect of increased physical activity on FC in children have been performed. 54 Interestingly,