Teun Remmers

Relationship between PA and the development of BMI in children | 17 Introduction The Netherlands is no exception to the worldwide trend of rising childhood overweight and obesity. In 2-21 year-old Dutch boys, overweight and obesity rose from 9.4% in 1997 to 13.3% in 2009. In girls of the same age, prevalence rose from 11.9% to 14.9% (32). Childhood overweight and obesity leads to various risk factors, both short term (e.g. decreased cardiovascular fitness, depression, body dissatisfaction, and social isolation) (11) and long term consequences (e.g. decreased insulin sensitivity) (7), resulting in a higher combined metabolic- and cardiovascular risk. In addition, childhood overweight and obesity, and the accompanying consequences, tend to persist into adolescence and adulthood (34), leading to considerably higher healthcare costs for the general population (35). Hence, the improvement of childhood body composition will provide major benefits for the current and future health of these children. Examining key determinants and critical periods for the prevention of an unhealthy body composition is an important topic in pediatric research. Children may be more vulnerable to the development of overweight at certain stages of childhood (23). After a rapid increase in body fat in the first two years, a second naturally occurring increase in body fat (i.e. the adiposity rebound), is identified as a critical period in the development of subsequent adiposity (9). In normal circumstances, the adiposity rebound initiates at around the age of 5-6 years. An earlier occurrence of this rebound period has been found to be associated with a greater skinfold thickness and body mass index (BMI) at later ages (27, 39). Hence, 5-6 year-old children are an important target group for examining the effect of determinants on the development of body composition (9). Considering the relatively high incidence of overweight in children in the last two decades, the etiology of the present epidemic is more likely to be caused by energy metabolism- related behaviors than genetic influences (25). Earlier studies that have investigated the relationship between energy expenditure and body composition have generally found that higher energy expenditure is related to less accumulation of body fat in children (14), however more recent evidence for this association is less clear (40). Research concerning more specific measures of energy metabolism-related behaviors, such as physical activity (PA), may therefore provide more meaningful results (14). PA behavior can be specified according to its duration, frequency, type, and intensity (23). The PA behavior of children is considered less structured than of adults, containing relatively short bouts of intense, spontaneous PA behavior (23, 40). Because this type of PA behavior is difficult to observe, the usage of subjective measurements (e.g. parental reports) are prone to significant measurement errors (30). Objective measurements (e.g. accelerometers) are independent of these biases, and are therefore able to provide more reliable and valid estimates of a child’s daily PA behavior (8, 20).