Today’s post is a summary of a fascinating new paper published in Annals of Internal Medicine on the relationship between sedentary behaviour and premature mortality (available here). The paper has garnered a tremendous amount of media attention (see here, here and on Jimmy Kimmel here), and we are fortunate to have a summary of the paper written by lead author Avi Biswas. You can find more on Avi at the bottom of this post. This post is also available at the Sedentary Behaviour Research Network.
In a similar vein to other reviews that have examined the association between sedentary time and various health outcomes, our study sought to not only update the sedentary behavior literature but also focus exclusively on studies that adjusted for physical activity in order to assess the independent effects of sedentary time. This differentiated our meta-analysis from two others which had previously reported independent risks (Wilmot et al., 2012 and Schmid and Leitzmann, 2014), as investigating the independent effects of sedentary time (by reporting the effects from studies that at least adjusted for physical activity) was our core focus rather than among a limited subgroup of available studies. Additionally, our study examined the extent to which levels of physical activity may potentially modify the associations between sedentary time and outcomes, which to the best of our knowledge has not been systematically quantified in other reviews.
What we did
We devised a comprehensive, systematic search strategy of studies that assessed sedentary behavior in adults as a predictor variable and correlated to at least one health outcome. We also focused on direct clinical outcomes (death, disease incidence and health service use) rather than indirect surrogate outcomes such as quality of life, insulin sensitivity and weight gain) as we believed the later type would present inconsistent end points and cutoffs. We then collected the statistical effects associated with the longest reported sedentary time (vs. a baseline of the lowest sedentary time) and that adjusted for physical activity. These were used to pool the analysis of the overall effect for each outcome.
When examining whether physical activity modifies the effects associated with sedentary time, we reanalyzed all available studies that met our selection criteria to see if they reported statistical effects of the longest period of sedentary time with the highest duration and intensity of physical activity.
What we found
Our study found that long periods of sedentary time was positively associated with an increased risk for dying (from all-causes, cancer and cardiovascular diseases) and increasing the risk for certain forms of cancer (specifically breast, colon, colorectal, endometrial, and epithelial ovarian cancer), cardiovascular disease and type 2 diabetes. Our meta-analysis found that the largest association was with type 2 diabetes (91% increased risk).
Of the limited studies that reported the joint effects of physical activity and sedentary time (only quantified for dying from all-causes), our pooled analysis found that the risk of dying from all-causes due to long sedentary time was 30% greater in those who spent little to no time in regular physical activity than those who at least met their physical activity recommendations of 30 minutes/day. While the pooled effect corresponding to high levels of physical activity and long sedentary time were found not to be statistically significant, we believe the consistent and strong effects associated across all-cause mortality-assessed studies (and to a lesser degree, cardiovascular disease and mortality) suggests that there is a strong likelihood that physical activity alone does not completely reduce the negative effects from long sitting time. We believe more work needs to be done to tease out whether there is such an association.
Take home message
Our study provides the strongest evidence as so far, to suggest that long periods of sitting is associated with various health outcomes. Furthermore, while regular or high levels of physical activity seems to have protective effects on reducing sedentary time-associated risks, we believe there is sufficient evidence to suggest that a focus on promoting regular exercise and reducing sitting time to be a better public health message.
Avi is a PhD candidate at the Institute of Health Policy, Management and Evaluation, University of Toronto and the Toronto Rehabilitation Institute. Along with this meta-analysis study, his PhD work is focused on understanding the risks of sedentary behaviour among exercise-engaged cardiac rehabilitation patients, in lieu of developing non-sedentary interventions.