Journal of Adolescent Health
Volume 47, Issue 3 , Pages 221-222, September 2010

Encouraging Physical Activity and Discouraging Sedentary Behavior in Children and Adolescents

Menzies Research Institute, University of Tasmania, Hobart, Tasmania, Australia

Article Outline

 

See Related Article p. 309

The health benefits that children and adolescents gain from participation in regular moderate to vigorous intensity physical activity have been well elucidated in previously published epidemiological studies [1], [2], [3]. This considerable amount of evidence-based data have resulted in the development of national guidelines in many countries, which aim to disseminate key messages to the public and to practitioners on the optimal levels of physical activity recommended for children and adolescents. These guidelines generally stipulate that children need to be at least moderately active for a minimum of 60 minutes per day, every day [4], [5], [6]. More recently, these guidelines have included recommendations to limit the amount of time spent using electronic media, such as watching television (TV), playing electronic games, or using the internet, to no more than 2 hours per day [4]. These sedentary behavior guidelines have resulted from emerging scientific evidence, which suggest that sedentary behavior and physical activity are unique behavioral constructs that have independent effects on health outcomes and mortality [7], [8], and are hypothesized to work through different physiological mechanisms [9].

Overweight is one of the key health outcomes consistently associated with low levels of physical activity and high levels of sedentary behavior. Overweight is of particular concern because it is common [10], tracks strongly from childhood into adulthood [11], [12], and is associated with an increased risk of health problems in adulthood independent of adult weight status [13]. For these reasons, it is of substantial public health importance that we understand how physical activity and sedentary behavior combine to influence the risk of overweight among children and adolescents, and whether there are particular at-risk groups that would benefit from targeted preventive interventions.

In this issue of the Journal of Adolescent Health, Sisson et al used data from a sample of more than 50,000 North American 6–17-year-olds who were involved in the 2003 National Survey of Children’s Health to attempt to disentangle associations between physical activity, sedentary behavior, and overweight. They address the “displacement theory,” which hypothesizes that the time spent in sedentary activities displaces the time that could have been spent in more active pursuits. This theory has been investigated in adults, with few studies finding support for this theory because of poor correlations between physical activity and sedentary behavior [14], [15]. There has been little research examining the combined influence of physical activity and sedentary behavior on the risk of overweight and obesity in children and adolescents.

Sisson et al found that only one quarter of the sample engaged in 20 minutes of vigorous intensity physical activity per day over the past 7 days, and that approximately half of the sample spent 2 or more hours per day in screen-based sedentary behavior. An inverse association between the two behaviors was evident, particularly among boys, thereby supporting the displacement hypothesis. Although the combined influence of physical activity and sedentary behavior on overweight was generally in the expected direction, some gender differences were evident. Among boys, there was a clear dose–response relationship, with those least active and most sedentary at highest risk for overweight. Although this general pattern was also evident among girls, associations with overweight were strongest in those in the middle category of sedentary time (2–3 hours/day) and vigorous activity on 4 days or less. This different pattern might be explained by the measures of physical activity (which may be more sensitive among boys because of the nature of the examples provided), the measures of sedentary behavior (which may be less sensitive in girls who may spend their sedentary time in nonscreen-based activities such as speaking on the telephone or reading), the differences in the reference period for the physical activity (past week) and sedentary behavior (an average school day) measures, and the possibility of age interactions (which were not investigated, but plausibly the association of physical activity and sedentary behavior may differ with age). Notably in both boys and girls, vigorous intensity physical activity on 5–7 days of the week was associated with markedly lower odds of overweight, even in those with 4 or more hours per day of leisure-time sedentary behavior.

Findings from the study should be considered in light of limitations, such as the cross-sectional study design, which precludes inferences related to causality. The use of parental reports of height and weight to calculate body mass index may result in misclassification of overweight [16]. However, 35.3% of the participants in this study were classified as overweight (using Centers for Disease Control and Prevention growth charts at the 85th percentile), which is similar to the 31.7% classified using the same criteria in the 2007–2008 National Health and Nutrition Examination Survey of 2–19-year-olds [10]. No information has been provided about the reliability or validity of the parent responses to questions related to their child’s vigorous intensity physical activity (moderate intensity activity was not considered) and sedentary behavior, even though measurement error would most likely bias results toward null. Leisure-time sedentary behavior was measured as the number of hours spent watching TV, watching videos, and playing video games. This measure is unlikely to have captured computer use for leisure or educational purposes. There is evidence of secular increases in computer use among adolescents, which could be displacing TV viewing as a result of the growth in social networking and should be measured in future studies [17]. The analyses were not adjusted for overall dietary intake, nor did they consider foods and beverages consumed during the period of being sedentary, which have been shown to partially explain the association between TV viewing time and abdominal obesity in young adults [14].

Despite these limitations, the study by Sisson et al has important strengths including the large and nationally representative sample size, the wide age range of participants, and consideration of the combined influence of physical activity and sedentary behavior. The study provides an important foundation for further investigations of the combined influence of physical activity and sedentary behavior on overweight among children and adolescents. Purposefully designed prospective studies that use valid and reliable measures of key exposure and outcome variables and consider a range of potential confounding and mediating factors are necessary to fully comprehend the combined influence of a range of physical activities and sedentary behaviors on overweight. Future work should also consider the effect of age as a potential mediator of associations, and the role of overall energy intake and specific eating behaviors during sedentary time as possible alternate explanations for the physical activity displacement hypothesis.

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References 

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PII: S1054-139X(10)00314-9

doi:10.1016/j.jadohealth.2010.06.022

Refers to article:

  • Screen Time, Physical Activity, and Overweight in U.S. Youth: National Survey of Children's Health 2003 , 28 April 2010

    Susan B. Sisson, Stephanie T. Broyles, Birgitta L. Baker, Peter T. Katzmarzyk
    Journal of Adolescent Health September 2010 (Vol. 47, Issue 3, Pages 309-311)

Journal of Adolescent Health
Volume 47, Issue 3 , Pages 221-222, September 2010

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