Some exciting news this week – the world’s first systematic review on the relationship between sedentary behaviour and health in school-aged children has just been published online in the International Journal of Behavioural Nutrition and Physical Activity. I am one of 8 authors on the review (nestled nicely in the middle), which was created to inform the Canadian Sedentary Behaviour Guidelines, released earlier this year.
What did we do?
The review looked at the following broadly defined health indicators: body composition, fitness, metabolic syndrome and cardiovascular disease, self-esteem, pro-social behaviour and academic achievement. We identified 6,718 studies of possible relevance, which we eventually whittled down to 232 separate studies examining our outcomes of interest. In total, these studies included 983,840 participants between the ages of 5 and 17.
What did we find?
In general, the more sedentary behaviour that a child accumulates, the worse their health.
Here is what we found more specifically for each outcome.
Of the 8 RCTs, 7 showed that decreases insedentary time lead to reductions in body weight.…Intervention studies reported desirable changes in body weight, BMI, and weight status among children and youth who successfully decreased their sedentary time.…Of the 119 cross sectional studies, 94 reported that increased sedentary time was associated with one ormore of increased fat mass, increased BMI, increased weight status and increased risk for being overweight[28, 90-182]. Risk for obesity increased in a dose response manner with increased time spent engaging insedentary behaviours.
Increased time spent being sedentary was associated with decreased scores for overall physical fitness, VO2 max, cardiorespiratory fitness, and musculoskeletal fitness.
Metabolic Syndrome (MS) and CVD Risk:
Eleven studies assessed the relationship between time spent engaging in sedentary behaviour and risk factors for MS and CVD (Table 5). All of the studies reported that increased sedentary time was associated with increased risk for MS or CVD. However, the results of these studies should be viewed with caution as the proportion of children and youth who have measurable health risk factors for MS or CVD is quite lo
Intervention studies that targeted changes in sedentary behaviour produced inverse changes in physical self-worth and self-esteem [52, 54]. Cross sectional studies showed that increased screen time was associated with higher depressive symptoms, low self-esteem, and decreased perceptions of self-worth.
Eighteen studies assessed the relationship between time spent engaging in sedentary behaviour and pro-social behaviour (Table 7). The one longitudinal study examining the relationship between sedentary behaviour and pro-social behaviour found that sustained TV exposure (i.e. ≥2 hours per day) was a significant risk factor for behavioural problems . Cross sectional studies reported similar findings. Those who watched less TV were more emotionally stable, sensitive, imaginative, outgoing, self-controlled, intelligent, moralistic, college bound, and less likely to be aggressive or to engage in risky behaviour
The longitudinal studies included in this review found that children who watched higher amounts of TV had greater difficulties with attention as teenagers , showed lower progression for reading level , and performed worse on cognitive tests  than those watching less than one hour of television per day. The majority of cross sectional studies (75%) reported that children and youth who watched higher levels of TV tended to spend less time doing homework, studying, and reading for leisure which may lead to a decrease in academic achievement
Strengths and Limitations
As with all papers, this new review has some limitations. Due to variations in study design and measurement, we were only able to perform a meta-analysis for body composition, and even then it was only on a small subset of available body composition studies. Further, the majority of currently available data comes from cross-sectional studies, which are obviously less useful when trying to determine whether sedentary behaviour causes these poor health outcomes, or is simply along for the ride. Finally, up until recently very few studies have used objective measures of sedentary behaviour. As I’ve mentioned before, objective measures of sedentary behaviour can be wildly different from subjective measures (e.g. simply asking a kid how much time they spend sitting), and it’s important that new studies use objective measures whenever possible.
Despite the above-mentioned limitations, this study still represents the best available information on the relationship between sedentary behaviour and health in school-aged kids. It was an exhaustive process, and was used to inform the world’s first evidence-based sedentary behaviour guidelines. Finally, it also made clear that there have been way too many cross sectional studies looking at sedentary behaviour and body weight (we found 119 of them!), and far too few longitudinal or intervention studies looking at other aspects of health. So if you’re interested in studying sedentary behaviour, I’d strongly encourage you to look at a health outcome other than body weight.
What’s the take-home message?
From our conclusion:
Based on this systematic review of 232 studies, sedentary behaviour (assessed primarily through increased TV viewing) for more than 2 hours per day was associated with unfavourable body composition, decreased fitness,lowered scores for self-esteem and pro-social behaviour and decreased academic achievement in school-aged children and youth (5-17 years). This was true for all study designs, across all countries, using both direct and indirect measurements, and regardless of participant sample size. All studies examining risk factors for MS and CVD disease reported that increased sedentary time was associated with increased health risk; however, the included studies examined a wide range of risk factors, and thus there was insufficient evidence to draw conclusions on the relationship for metabolic risk as a whole.
The study is available for free (and has a creative commons license) through the International Journal of Behavioural Nutrition and Physical Activity. You can also read a brief statement from lead-author Mark Tremblay on the Healthy Active Living and Obesity Research Group website here.
Tremblay, M., LeBlanc, A., Kho, M., Saunders, T., Larouche, R., Colley, R., Goldfield, G., & Connor Gorber, S. (2011). Systematic review of sedentary behaviour and health indicators in school-aged children and youth International Journal of Behavioral Nutrition and Physical Activity, 8 (1) DOI: 10.1186/1479-5868-8-98
To get future posts delivered directly to your email inbox or to your RSS reader, be sure to subscribe to Obesity Panacea.