Obesity
Don’t Just Sit There: Sitting, TV Viewing, and Mortality
Abstract & Commentary
By Russell H. Greenfield, MD
Synopsis: Findings from this unique investigation add a new wrinkle to concerns over the increasingly sedentary nature of American society. They also suggest that years could be added to general life expectancy simply by limiting time spent sitting and TV viewing.
Source: Katzmarzyk PT, et al. Sedentary behaviour and life expectancy in the USA: A cause-deleted life table analysis. BMJ Open 2012;2: pii: e000828. doi: 10.1136/bmjopen-2012-000828.
One would have to live under a rock (thereby getting very little exercise...) to be unaware of the importance of regular physical activity to overall health. The converse, that a couch potato lifestyle does not promote optimal health, is also widely recognized. Putting two and two together, it makes sense that regular exercise should counteract the adverse effects of a sedentary lifestyle — but what if they are independent, and not codependent, health risk variables?
The authors of this sobering investigation sought to determine the impact of sedentary behaviors, specifically sitting and television viewing, on life expectancy in the United States using a prevalence-based, cause-deleted life table analysis (a tool that estimates probability of death over time). Studies addressing the relationship between sitting or television viewing and all-cause mortality were identified through a systematic literature search of MEDLINE and were included if they provided relative risk (RR) estimates along with 95% confidence intervals (CIs). Trials that reported on the relationship between sedentary behaviors and outcomes other than all-cause mortality and those not using a prospective observational cohort design were excluded. Abstracts and papers were reviewed by both authors and discrepancies resolved by consensus. RR estimates from each study for sitting or television viewing and all-cause mortality were pooled using a random effects meta-analysis. Pooled RR estimates were obtained for two levels of sitting and two levels of television viewing, and age- and sex-adjusted RR estimates were used from each study. The prevalence of time spent sitting (2009-10) and television viewing (2005-6) were obtained from the National Health and Nutrition Examination Survey (NHANES). The population-attributable fraction (PAF) was computed from the prevalences of sitting and television viewing and the RR of all-cause mortality associated with those behaviors using an equation that reportedly produces internally valid estimates when confounding exists and adjusted RRs must be used. The specific PAF equation used requires the prevalence of sedentary behaviors among cases (those who have died, in this instance) rather than from the source population (in this case, NHANES prevalence). Potential gains in life expectancy associated with a reduction in sitting and television viewing were estimated using a cause-deleted life table analysis, focusing on adults aged 40-79 years. This approach estimates the number of years of life gained at birth if deaths from a specific cause are eliminated from the current death rates. The difference between current life expectancy and cause-deleted life expectancy thus represented any estimated gain in lifespan from reducing the prevalence of sitting and watching television.
The literature search yielded a total of 460 abstracts for review, of which almost all (455) were excluded, the primary reason being that the studies did not address either sitting or television viewing as an exposure and all-cause mortality as the outcome (n = 400). Two studies examined the association between sitting and all-cause mortality; the other three focused on the relationship between TV viewing and all-cause mortality. Together the investigations provided data from almost 170,000 subjects, almost 75% coming out of one trial.1
Pooled RRs for sitting and all-cause mortality were 1.18 (95% confidence interval [CI] 1.14 to 1.21) and 1.45 (95% CI 1.39 to 1.51) for the mid- and highest levels of sitting compared to the lowest, respectively. For the meta-analysis of television viewing and all-cause mortality, the pooled RRs were 1.17 (95% CI 1.04 to 1.32) and 1.49 (95% CI 1.22 to 1.82) for moderate and high levels of TV viewing compared with the lowest, respectively. PAFs for all-cause mortality associated with sitting and television viewing were 27% and 19%, respectively. Life table analyses showed that reducing the prevalence of sitting to < 3 hours per day could result in a gain of 2.00 years of life, while limiting TV viewing to < 2 hours a day could produce a gain of 1.38 years in life expectancy. The lower and upper limits from the sensitivity analyses were 1.39 and 2.69 years for sitting and 0.48 and 2.51 years for television viewing, respectively. The authors conclude that extended time sitting and television viewing likely reduces life expectancy in the United States.
Commentary
Increasing the level of physical fitness in our communities is, and has long been, a major public health initiative. And many have heeded the call, some by joining gyms, others by increasing their activity throughout the day. There seems to be a general movement toward increased movement. This is good news, because it is clear that regular physical activity contributes to improved physical and emotional health. Yet questions persist — for example, could high levels of sedentary behavior be an independent risk factor for morbidity and mortality? Think about it — most of you reading this article exercise on a regular basis (I hope), yet we also spend many hours in the seated position. Guidelines regarding physical activity to promote good health are explicit — should we also be sharing similarly explicit guidelines about sedentary activity with our patients, and following those guidelines ourselves? The results of the current study pointedly suggest that a renewed emphasis on limiting sedentary behaviors is in order.
The authors do a nice job reviewing the literature on the relationship between sedentary behavior and illness around the globe. Not surprisingly, the results support their findings — sitting and television viewing have a major negative impact on public health.
A prospective trial addressing this issue would be impractical at best, so the authors took a different approach. Their conclusions could be dinged for being based on results from only five trials; however, data were culled from a large number of participants. The researchers readily point out that their study relied on self-reported participation in sedentary behaviors, introducing concern over recall bias, and that some assumptions were made regarding NHANES data. They also explain that the equation used for PAF only provides a theoretical estimate of the effects of a given risk factor on a given outcome at the population level, the outcome in this instance being all-cause mortality, and that from the start they assumed a cause-effect relationship between sedentary behavior and mortality risk.
It’s important to keep in mind that even though the conclusions reached in this paper are alarming, it would not be appropriate to attempt to apply them to individual risk; life expectancy is a population statistic that does not readily translate to individuals, and potential confounders are numerous.
We are left to ponder a troubling notion — that the effects of sedentary behavior on health may be independent of the effects of physical activity. A cottage industry has recently sprung up around treadmill desks to lessen time spent sitting at work — aside from concerns over the potential for traumatic injury (coordination being a challenge for this reviewer), it could be a good idea. At the very least, it seems we should be asking people to get up and move about at regular intervals. The idea is not new but the data are increasingly convincing. Piling on the miles walking or running while wearing the latest in footwear, among other activities, is clearly good for our health but may not be enough to fully counter the effects of the largely sedentary style of living our society promotes. It appears we need not only figure out how to get people to move to promote optimal health, but also how to simply get them out of their chairs.
References
1. Patel AV, et al. Leisure time spent sitting in relation to total mortality in a prospective cohort of US adults. Am J Epidemiol 2010;172:419-429.
Findings from this unique investigation add a new wrinkle to concerns over the increasingly sedentary nature of American society. They also suggest that years could be added to general life expectancy simply by limiting time spent sitting and TV viewing.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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