Is sitting the new smoking?

Despite the health-enhancing benefits of physical activity, this alone may not be enough to reduce the risk of disease and illness. Population-based studies found more than half of an average person’s working day involves sedentary activities associated with prolonged sitting. 

Despite the health-enhancing benefits of physical activity, this alone may not be enough to reduce the risk of disease and illness. Population-based studies found more than half of an average person’s working day involves sedentary activities associated with prolonged sitting. Studies suggest long periods of sitting have deleterious health effects independent of adults meeting physical activity guidelines.

The objective of this meta-analysis was to quantitatively evaluate the association between sedentary time and health outcomes independent of physical activity participation among adult populations. It was hypothesized that sedentary time would be independently associated with both cardiovascular and non-cardiovascular outcomes after adjusting for participation in physical activity. A further predication was that the relative hazards associated with sedentary times would be attenuated in those who participate in higher levels of physical activity compared to lower levels.

A total of 20,980 studies were identified through database searching and 25 studies were added after hand-searching in-text citations. No randomized controlled trial met the selection criteria. Most studies used prospective cohort study designs and three studies used cross-sectional and case-control study designs. Definitions of sedentary time varied across studies.

There was statistical evidence of publication bias among studies reporting all-cause mortality. No statistical evidence of publication bias was found for cardiovascular disease mortality and cancer mortality. Publication bias was not assessed for cardiovascular disease incidence and type II diabetes incidence because the small number of studies may overestimate the effects of bias.

Heterogeneity across studies reporting all-cause mortality and cardiovascular disease incidence, as outcomes, may be high. Heterogeneity was low for cardiovascular disease mortality, cancer mortality, cancer incidence and type II diabetes incidence.

Pertinent results
Greater sedentary time was found to be positively associated with an increased risk of all-cause mortality, cardiovascular disease mortality, cancer mortality, cardiovascular disease incidence, cancer incidence and type II diabetes incidence. The largest statistical effect was associated with the risk for type II diabetes [pooled hazard ratio (HR) 1.91 (CI 1.64-2.22)]. Among studies assessing cancer mortality and incidence, significant associations were found for breast, colon, colorectal, endometrial and epithelial ovarian cancer. One study evaluated whether sedentary behaviour was correlated with potentially modifiable hospitalization, revealing that self-reporting fewer than eight hours of sitting time per day resulted in a 14 per cent lower risk of potentially preventable hospitalization [HR 0.86 (CI 0.83-0.89)].

Ten studies reported the effects of prolonged bouts of sedentary time modified by the highest or lowest reported participation in physical activity and health outcomes. The relative hazards associated with sedentary time on outcomes varied according to physical activity levels and were generally more pronounced at lower levels than at higher levels. Sedentary time was associated with a 30 per cent lower relative risk for all-cause mortality among those with high levels of physical activity [pooled HR 1.16 (CI 0.84-1.59)] as compared with low levels of physical activity [pooled HR 1.46 (CI 1.22-1.75)].

Conclusions, application
This study demonstrated sedentary time was independently associated with a greater risk for all-cause mortality, cardiovascular disease incidence or mortality, cancer incidence or mortality and type II diabetes in adults. The deleterious outcomes associated with sedentary time generally decreased in magnitude among persons who participated in higher physical activity compared to lower levels.

In other words, prolonged sedentary time, independent of physical activity, is positively associated with various deleterious health outcomes. The results reaffirm the need for greater public awareness about the hazards associated with sedentary behaviours. The results also justify the necessity for further research to explore the effectiveness of interventions designed to target sedentary time independently, and in addition, to those targeting physical activity.

The most important take home message for your practice is GET YOUR PATIENTS MOVING!

Dr. Shawn Thistle, DC, is the founder and CEO of RRS Education (rrseducation.com), which helps busy clinicians integrate current research evidence rationally into practice. He also maintains a practice in Toronto, lectures at CMCC and provides chiropractic medicolegal consulting services. Reach him at: shawn@rrseducation.