Today’s post comes from friend and colleague Dr Stephanie Prince, discussing her important new paper published in Obesity Reviews (available free here). More on Stephanie can be found at the bottom of this post.
Physical activity has long been established as a strong predictor of cardiometabolic health, disease and mortality. For decades now researchers have been trying to develop interventions that will increase our physical activity levels, most notably at higher intensities – think moderate-to-vigorous physical activity (otherwise known as MVPA). In Canada, we have physical activity guidelines which promote that adults (18-64 years) aspire to achieve a minimum of 150 minutes of MVPA per week in bouts of 10 minutes or more, with the idea that more physical activity is always better. Canadians also have sedentary behaviour guidelines for children and youth which propose limits on screen time.
There is a rapidly growing body of literature which suggests that greater amounts of time spent being sedentary (watching TV, screen time, sitting time, total sedentary time) are associated with an increased risk for markers of cardiometabolic disease, cardiovascular disease, diabetes, some cancers and premature death (references here and here). Sedentary behaviours refer to the time spent sitting or lying down and in a state where there is relatively minimal energy expended (not just the absence of MVPA).
While there has been a lot of research looking at the effectiveness of physical activity interventions, little has been done to examine whether these interventions also decrease the time we spend being sedentary. Prior to this review I often wondered whether the opposite might occur whereby individuals who increased their physical activity levels may become more sedentary almost has a consequence of the satisfaction that they had already undertaken the necessary amount of physical activity needed to meet physical activity guidelines. Although in its infancy compared to the physical activity field, the sedentary behaviour field is now proposing and testing a variety of interventions targeting sedentary behaviours exclusively (with the majority to date having focused on office environments, where many of us spend the majority of ours days sitting at desks in front of computers). While the efficacy of physical activity interventions is more known, it remains unknown whether these intervention also have the capacity to decrease sedentary time. Further it is unknown whether interventions which target sedentary behaviours are any better at reducing these behaviours than physical activity interventions.
Therefore, the objective of our research was to systematically review the literature and compare the effectiveness of interventions with a focus on physical activity and/or sedentary behaviours (e.g.physical activity only interventions vs. physical activity + sedentary behaviour interventions vs. sedentary behaviour only interventions) for reducing time spent being sedentary in adults.
What did we do?
To examine all available literature we conducted a systematic review which searched six different bibliographic databases (you can see more details on this in the full article) looking for any study which described a physical activity and/or sedentary behaviour intervention and reported on changes in total sedentary time or time spent watching TV or sitting among adults. All of the studies included in the review can be found in the supplemental tables found here. We were also very fortunate to obtain several unpublished results provided to us upon request from authors of original studies where sedentary time was captured, but not reported upon in previous articles.
We assessed the quality and risk of bias of every individual study and also used the GRADE (Grading of Recommendations Assessment, Development and Evaluation) framework, essentially the gold standard for assessing the overall quality of the evidence and for rendering a recommendation based on the current evidence.[Travis’ note: This was a lot of work!!!]
What did we find?
After we removed duplicate papers, we ended up retrieving 9,107 relevant papers. That was a lot of papers to screen! After many hours and days looking at the abstracts and then the full articles, 63 studies were kept in the review having met our inclusion criteria. Of these 63, 33 were used in the meta-analyses which allowed us to group quantitative results to yield an overall effect of the interventions. See Figure 1 for the PRISMA flow diagram to get a sense of how we screened out papers.
When we examined all of the evidence looking at both the more descriptive results of what the studies found and also at the quantitative results (see Figure 3 above) we found that on average the physical activity interventions resulted in significant, though very small reductions in sedentary time (average difference of 19 minutes/day). When interventions had both a physical activity and sedentary behaviour component they resulted in significant, but slightly greater reductions in sedentary time (average difference of 35 minutes/day) (Figure 4). Most promising is that when interventions focused solely on the behaviour itself, they yielded an average reduction of 91 minutes/day.
Interestingly, the reductions observed in the physical activity and physical activity + sedentary behaviour interventions were similar to those reported in a systematic review looking at increases in physical activity following a physical activity intervention. These findings suggested that individuals in these interventions are likely reallocating time from sedentary behaviours to MVPA, which is ideal. They are also supported by two recent studies that also showed that reductions in sedentary time do not necessarily exceed gains in physical activity (details here, here and here).
Although the reductions may not exceed gains in MVPA, a recent study by Buman et al. using National Health Examination Survey study data showed that for every 30 minutes of sedentary behaviours that are reallocated to MVPA there is a 2-25% improvement in biomarkers of risk for disease. Therefore, even the small reductions in sedentary behaviour seen in the physical activity + sedentary behaviour interventions likely resulted in a reduction of disease risk. The reductions observed in the sedentary behaviour focused interventions ultimately likely yielded the greatest health benefits, but they were all fairly short in duration and all but one reported on the immediate post-intervention results. It remains unknown if these results could be maintained over the long term.
The results of our review provide a comprehensive and objective summary of the effectiveness of physical activity and/or sedentary behaviour focused interventions on reducing time spent being sedentary among adults. Ultimately the greatest reductions were observed in the sedentary behaviour focused interventions, but it remains to be seen whether these reductions can be maintained in the long term. Further, the reductions observed in the intervention with some sedentary behaviour component also likely lead to clinically meaningful reductions in cardiometabolic disease risk.
Given that it is likely easier for individuals to reallocate time spent being sedentary to light intensity physical activity (more incidental movement), and that beneficial changes in metabolic health can be achieved with these reallocations, reducing sedentary behaviours in addition to increasing MVPA is an important public health message. Interventions which placed an increased focus on these behaviours are more likely to see reductions in sedentary time.
About the author:
Stephanie Prince is an Epidemiologist and Population Health scientist in the field of physical activity and health behaviours. She is currently a CIHR post-doctoral fellow at the University of Ottawa Heart Institute in the Division of Prevention and Rehabilitation where her research focuses on sedentary behaviours in both the primary and secondary prevention of cardiometabolic health and disease. She is the lead author on the systematic review comparing the efficacy of physical activity and/or sedentary behaviour focused intervention on reducing sedentary time in adults and is a member of the Sedentary Behaviour Research Network.