New Systematic Reviews Find Strong Prospective Relationships Between Sedentary Behaviour and Mortality
[Travis' Note: This article has been adapted with permission from a post published on Obesity Panacea on July 12, 2011.]
In the past few years several papers have examined the prospective links between sedentary behaviour and chronic disease morbidity and mortality. Two recent systematic reviews have attempted to synthesize that research in order to give a clearer picture of the relationship between sedentary behaviour and disease. The first, published in the American Journal of Preventive Medicine by Karin Proper and colleagues, focused on the prospective association between sedentary behaviour and obesity, CVD and diabetes risk, as well as mortality. Somewhat surprisingly, they found little evidence that sedentary behaviour was associated with increased body weight or other health risk factors, despite consistent associations between sedentary behaviour and risk of death. From the paper:
Based on the inconsistent findings among the  prospective studies identified, there is insufficient evidence for a longitudinal relationship between sedentary behavior and body weight/BMI gain.
Risk of Being Overweight or Obese:
Based on the inconsistent findings among the  studies, there is insufficient evidence for the relationship between sedentary behavior and the risk for overweight or obesity.
Increased Waist Girth:
Based on this single study, there is insufficient evidence for the relationship between sedentary behavior and waist gain.
Risk of Developing Diabetes:
Based on the consistent findings of… two low-quality studies, there is moderate evidence for a significant positive relationship between the time spent sitting and the risk for type 2 diabetes.
Risk of Cardiovascular Disease:
Based on the findings of the 4 studies identified, there is insufficient evidence for a significant relationship between sedentary behavior and various CVD risk factors.
Based on the inconsistencies found between and within the two studies identified, there is insufficient evidence for the relationship between sedentary behavior andendometrial cancer.
As I said, there’s really not much evidence so far that sedentary behaviour is prospectively linked with these various markers of health. However, it does seem to be associated with increased risk of death from various causes:
Based on the findings of the two high-quality studies, there is strong evidence for a relationship between sedentary behavior and mortality from all causes and from CVD, but no evidence for the relationship between sedentary behavior and mortality from cancer.
Another review published late last year focused specifically on sedentary behaviour and cancer outcomes. It’s conclusions were a bit stronger than the review above, but the evidence for a relationship between sedentary behaviour and cancer still seems pretty weak. From the review:
The literature review identified 18 articles pertaining to sedentary behavior and cancer risk, or to sedentary behavior and health outcomes in cancer survivors. Ten of these studies found statistically significant, positive associations between sedentary behavior and cancer outcomes. Sedentary behavior was associated with increased colorectal, endometrial, ovarian, and prostate cancer risk; cancer mortality in women; and weight gain in colorectal cancer survivors. The review of the literature on sedentary behavior and biological pathways supported the hypothesized role of adiposity and metabolic dysfunction as mechanisms operant in the association between sedentary behavior and cancer.
What’s the take-home message?
Sedentary behaviour seems very likely to be associated with increased risk of all-cause and cardiovascular disease mortality, while the relationship between sedentary behaviour and cancer mortality remain quite speculative. Interestingly, prospective studies have yet to find much strong evidence linking sedentary behaviour with prospective risk of cardiovascular disease, despite being associated with increased risk of cardiovascular disease mortality. Clearly there’s still a lot to be worked out here, especially given that so few prospective studies have been performed to date. Further, 17 of 19 studies in the Proper review used self-report measures of sedentary behaviour, which can be dramatically different from directly measured sedentary behaviour.
The prospective relationships between sedentary behaviour and risk markers is likely to become more clear with time. In the meantime, it seems reasonably clear that the more you sit, the greater your risk of mortality.