Friday, April 24, 2009
Photo by independentman.
Ok, so the title of today's post is a little over the top, but stick with me on this. Over the past few years research has suggested that being sedentary (e.g. sitting or lying down) for extended periods of time has a negative impact on your health. We know that people who sit for long periods of time are less likely to be physically active, and more likely to be obese, so it's not that surprising that people who sit for extended periods of time would have increased health risk relative to people who sit less. But new research is suggesting that people who spend more time seated are at increased health risk even after control for these other factors. In other words, no matter your body weight or how much you exercise, if you also spend a lot of time sitting, you are likely to have a higher health risk than someone who performs the same amount of exercise but spends less time sitting.
A new study by Canadian researchers published in this month's Medicine and Science in Sport and Exercise looks at this very topic. The study, available here, was performed by Peter Katzmarzyk and colleagues at the Pennington Biomedical Research Center, and examined the links between time spent sitting (at school, work, and at home) and mortality in a representative sample of more than 17,000 Canadians. They report that time spent sitting was associated with increased risk of all-cause and cardiovascular disease mortality (there was no association between sitting and cancer deaths). In fact, individuals who sat the most were roughly 50% more likely to die during the follow-up period than individuals who sat the least, even after controlling for age, smoking, and physical activity levels.
But wait - could it be that the people who sit more are only at risk because they also happen to be obese? Unfortunately body weight was only available in a subset of individuals in the present study, making this a difficult question to address. However, the authors did examine the association between sitting and mortality after control for body weight in this subset of individuals, and report that sitting remained a significant predictor of mortality. This suggests that all things being equal (body weight, physical activity levels, smoking, alcohol intake, age, and sex) the person who sits more is at a higher risk of death than the peson who sits less. The authors go on to suggest that sitting for extended periods of time may alter certain physiological processes, such as lipoprotein lipase activity, which could explain the link between time spent sitting and mortality risk.
These findings have far-reaching implications. The vast majority of people in Western society are sedentary throughout the work day. Instead of focussing all our efforts on increasing time spent being physically active, should we instead try to convince people to be less sedentary? Should we drastically restructure our work environments? We also know that when individuals with obesity increase their levels of structured physical activity, they often compensate for this increase in exercise by becoming more sedentary in their leisure time. In obese youth, this compensation is so severe that in some studies children actually burn less calories once they start the exercise intervention! Is this yet another reason to consider new types of physical activity interventions (perhaps those focussing on Non Exercise Activity Thermogenesis) which may avoid this compensatory increase in sedentary behaviors?
This is only one cross-sectional study, and I should mention that it used self-report questionnaires to assess sitting time and physical activity levels. However, it raises several interesting questions, and will hopefully pave the way for longitudinal research on this area in the next few years. I would also like to point out that I first heard of this study on Amby Burfoot's Peak Performance blog, available here. Have a great weekend.
Katzmarzyk, P.T., Church, T.S., Craig, C.L., & Bouchard, C. (2009). Sitting Time and Mortality from All Causes, Cardiovascular Disease, and Cancer Medicine and Science in Sports and Exercise, 41 (5), 998-1005
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