Tuesday, September 23, 2014

Sedentary Behaviour and Depression: Not just about less physical activity

In the world of sedentary behaviour research one question usually permeates any discussion among experts and non-experts alike.

It goes something like this: "So I see you've shown an association (causal or otherwise) between sedentary behaviour and health risk X. But isn't this really just about less physical activity?"

We all know that not getting enough physical activity means that there's a higher risk of being unhealthy, generally speaking. We now know, or at least have enough evidence to date to suggest, that excess sedentary behaviour is also associated with poorer health. Sedentary behaviour just refers to low-movement behaviours while in a sitting or reclined posture - in real world terms, this is just called sitting too much.

So I wasn't surprised to find yet another study on the ill-effects of sitting on our health. The authors pooled data from 13 cross-sectional studies and 11 longitudinal studies, or 193 166 people, which examined the association between sitting and the risk of being diagnosed with depression. The "main" finding was that individuals were 25% more likely to be diagnosed with depression if they were in the highest sitting group, as compared to the lowest sitting group. Of course, we don't know (and may never know) whether more sitting causes depression or depression causes more sitting. In my opinion, there seems to be evidence for both directions and the association may truly be bi-directional.

What's most interesting to me about this study - above and beyond the main finding - is a sub-analysis that they ran. The authors compared the effect of sitting on depression between studies that accounted for physical activity level, and those that did not. In the studies that did not account for physical activity, the likelihood was 34% [1.34 (1.22 to 1.48)], and for those that did account for physical activity, the likelihood was 12% [1.12 (1.06 to 1.18)]. This is to be expected, as physical activity is typically thought to be protective against depression, so adjusting for it should partition out some of the effect. However, even after adjusting for physical activity, there is still an association between sedentary behaviour and depression!

This underscores the importance of considering physical activity levels when examining the links between sedentary behaviour and health outcomes. Of course it's just good science, but even if only to silence the critics. I do believe that, conceptually speaking, physical activity and sedentary behaviour represent distinct and separate constructs. However, practically speaking these may be difficult to disentangle, especially with respect to the links with health and disease - it's just intuitive that a change in one necessitates an inverse change in the other. As researchers, we need to provide convincing evidence that this intuitive association between sedentary behaviour and physical activity does not necessarily exist.

Remember the conversation above? Some of the best evidence we have today would seem to suggest that the link between sedentary behaviour and depression is not just about less physical activity, but also about sitting too much. Practically speaking, the distinction is like splitting hairs, but conceptually speaking these two are "sitting" in quite different camps.