Sitting down for more than three hours a day can shave a person's life expectancy by two years, even if he or she is physically active and refrains from dangerous habits like smoking, according to a study to be published on Tuesday in the online journal BMJ Open. 
 
 Watching TV for more than two hours a day can exacerbate that problem, decreasing life expectancy by another 1.4 years, said the report, which analyzed five underlying studies of nearly 167,000 people over a range of four to 14 years. 
 
 ... ||||| Article summary Article focus This paper presents the results of an analysis aimed at determining the effects of sedentary behaviour on life expectancy in the USA. Key messages The analyses indicate that population life expectancy in the USA would be 2.00 years higher if adults reduced their time spent sitting to <3 h/day and 1.38 years higher if they reduced television viewing to <2 h/day. Strengths and limitations of this study The use of the well-accepted prevalence-based methodology to estimate the public health burden of sedentary behaviour is a marked strength, which allows for comparability with the effects of other established risk factors. 
 
 This study relied on self-reported engagement in sedentary behaviours rather than an objective measurement, which is a limitation. 
 
 An important limitation of this study is the lack of adjustment for confounding beyond age and sex in the main analysis. 
 
 Over 60 years of research on physical activity and health culminated in the release of the 2008 Physical Activity Guidelines for Americans, which recommend that adults should accumulate at least 150 min of moderate-to-vigorous physical activity per week to attain the health benefits associated with physical activity.1 Recently, there has been considerable interest in understanding the role of sedentary behaviours on health, independent of overall physical activity levels,2–4 since one can be both sedentary and physically active (eg, an office worker who sits most of his work hours, but who also jogs regularly). 
 
 Sedentary behaviours that involve sitting for extended periods are ubiquitous in modern society. Based on self-reports, a recent survey of 20 countries documented a median of 300 min/day spent sitting, ranging from ≤180 min/day in Portugal, Brazil and Colombia to ≥360 min/day in Taiwan, Norway, Hong Kong, Saudi Arabia and Japan.5 Several studies have demonstrated positive associations between sedentary behaviours including sitting and television viewing and health outcomes such as type 2 diabetes, cardiovascular disease mortality and all-cause mortality.6 Thus, excessive time spent in sedentary behaviour is undoubtedly having an impact on public health. A recent study from Australia estimated that television viewing reduced life expectancy at birth by 1.8 years in men and 1.5 years in women.7 The purpose of this study was to determine the impact of sedentary behaviours on life expectancy in the USA. 
 
 Methods A prevalence-based approach was used to estimate the impact of sedentary behaviour on life expectancy. The population-attributable fraction (PAF) was computed from the prevalences of sedentary behaviours (defined here as sitting and television viewing) and the RR of all-cause mortality associated with these behaviours. The PAF equation used was ∑ P i ( RR i − 1 / RR i ) , where P is the prevalence of the risk factor among cases in stratum i (i=1–2 in this study; details are provided below). This equation produces internally valid estimates when confounding exists and adjusted RRs must be used.8 RRs associated with sedentary behaviours Sedentary behaviour can be captured globally using questionnaires that address total daily sitting time or time spent in specific sedentary behaviours like television viewing, reading or computer use. Published studies on the associations between sitting or television viewing and all-cause mortality were identified through a systematic literature search of MEDLINE (up to end of 2011) using the following search terms: (physical inactivity OR sedentary OR television OR sitting) AND (cohort study) AND (mortality OR death). Studies that reported on the relationship between sedentary behaviours and outcomes other than all-cause mortality, or those that did not use a prospective observational cohort design, were excluded from consideration. Abstracts and papers were reviewed by both authors and any discrepancies were resolved by consensus. Studies were included if they provided RR estimates along with 95% CIs. We pooled RR estimates from each study separately for sitting or television viewing and all-cause mortality using a random-effects meta-analysis. Pooled RR estimates were obtained for two levels of sitting and two levels of television viewing relative to a referent group in each case. In order to maintain consistency across studies, the age- and sex-adjusted RR estimates were used from each study. In cases where the authors presented only multivariable-adjusted RR estimates or used different exposure categories, we contacted them and asked them to provide this information. MIX V.2.0 software was used to conduct the meta-analysis.9 Prevalence of sedentary behaviour The prevalences of time spent sitting and television viewing were obtained from the National Health and Nutrition Examination Survey (NHANES). The NHANES uses a complex, multistage, probability sampling design to select participants who are representative of the civilian non-institutionalised US population.10 The prevalences were obtained from the most recent NHANES data available for adults (2009–2010 for sitting; 2005–2006 for television viewing). The prevalence of sitting (<3 h, 3–5.9 h and ≥6 h/day) among non-pregnant adults aged 18 years and older was determined from responses to the following question in the 2009–2010 NHANES: ‘How much time do you usually spend sitting on a typical day?’ The prevalence of television viewing (<2 h, 2–3.9 h and ≥4 h/day) among non-pregnant adults aged 18 years and older was determined from responses to the following question in the 2005–2006 NHANES: ‘Over the past 30 days, on average how many hours per day did you sit and watch TV or videos?’ Data analysis followed the guidelines of the National Center for Health Statistics for analysis of NHANES data due to the complex sampling design and methods.11 The PAF equation used in this study requires the prevalence of sedentary behaviour among cases (ie, decedents) rather than from the source population (ie, NHANES prevalence). Therefore, each population prevalence obtained from the NHANES was adjusted by using the weighted average case:source prevalence ratio (ie, the prevalences among cases divided by the prevalences among the baseline source population from the prospective cohort studies). Gains in life expectancy The PAFs for all-cause mortality associated with sitting and television viewing were computed using the summary RR estimates obtained from the meta-analyses and the adjusted prevalences from the NHANES, as described above. Potential gains in life expectancy attributable to reducing sedentary behaviours were estimated using a cause-deleted life table analysis, which estimates years of life gained at birth if deaths from a specific cause are eliminated from the current death rates.12 ,13 The most current abridged life table for the USA (2009) was downloaded from WHO website.14 The PAFs computed for sedentary behaviour were used to reduce the mortality rates in the life table for adults aged 40–79 years, and the life expectancy at birth was recalculated from the new mortality rates. The difference between the current life expectancy and the cause-deleted life expectancy represents the estimated gain in life expectancy from reducing the prevalence of sedentary behaviour. Sensitivity analyses A sensitivity analysis was used to estimate the effects of simultaneously varying the estimates of RR (using the upper and lower bounds of the 95% CI) and the prevalences of sitting and television viewing (±20%). Furthermore, given that we relied on age- and sex-adjusted RR estimates for the computation of the PAFs, we estimated the potential bias in using this approach by computing PAFs after adjusting the RR estimates for differences observed between multivariable-adjusted RRs and age- and sex-adjusted RRs in the available studies. 
 
 Results The systematic literature search yielded a total of 460 abstracts for review. From these, 455 were excluded, yielding five full articles, which were retrieved. The reasons for abstract exclusion included non-human studies (n=4), reviews, commentaries or methods papers (n=51) and not studying either sitting or television viewing as an exposure and all-cause mortality as the outcome (n=400). All retrieved articles met the inclusion criteria and were included in the meta-analysis. The characteristics and main results of the prospective cohort studies included in the meta-analyses of sedentary behaviour and mortality are presented in table 1. Two studies were included that investigated the association between sitting and all-cause mortality.15 ,16 The exposure levels of sitting in the first study were (1) none/¼ of the time, (2) ½ of the time and (3) ¾ of the time/all of the time based on activities that participants did most days of the week,15 whereas the levels in the second study were (1) <3 h, (2) 3–5 h and (3) ≥6 h of leisure-time sitting.16 Thus, three categories of exposure were used in each cohort study, and the prevalence of sitting categories from the NHANES was obtained for three groups (<3 h, 3–5.9 h and ≥6 h/day). However, given that the exposure categories from the Canadian cohort study were not quantifiable in terms of absolute hours per day, some misclassification may have occurred when combining the results. Table 1 Characteristics and main results of the prospective cohort studies included in the meta-analysis Three studies were included that studied the association between television viewing and all-cause mortality.17–19 The levels of television viewing in two of the studies were (1) <2 h, (2) 2–3.9 h and (3) ≥4 h,17 ,18 whereas the levels in one study were (1) <2.5 h, (2) 2.5–3.6 h and (3) >3.6 h.19 The prevalence of television viewing obtained from NHANES (<2 h, 2–3.9 h and ≥4 h/day) match quite well with the exposure categories from the cohort studies. Figure 1 presents the results of the meta-analysis for sitting and all-cause mortality. The pooled RRs were 1.18 (95% CI 1.14 to 1.21) and 1.45 (95% CI 1.39 to 1.51) for levels 2 and 3 versus level 1, respectively. Figure 2 presents the results for the meta-analysis for 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 levels 2 and 3 versus level 1, respectively. Figure 1 Risk of all-cause mortality associated with sitting. Level 3 corresponds to ¾/all of the time for Katzmarzyk et al15 and ≥6 h for Patel et al.16 Level 2 corresponds to ½ of the time for Katzmarzyk et al15 and 3–5 h for Patel et al.16 Level 1 corresponds to none/¼ of the time for Katzmarzyk et al15 and <3 h for Patel et al.16 Figure 2 Risk of all-cause mortality associated with television viewing. Level 3 corresponds to ≥4 h for Dunstan et al17 and Stamatakis et al18 and >3.6 h for Wijndaele et al.19 Level 2 corresponds to 2–3.9 h for Dunstan et al17 and Stamatakis et al18 and 2.5–3.6 h for Wijndaele et al.19 Level 1 corresponds to <2 h for Dunstan et al17 and Stamatakis et al18 and <2.4 h for Wijndaele et al.19 The prevalences of sitting and television viewing in cases (decedents) and in the source population at baseline in the prospective studies are presented in table 2, along with the average case:source prevalence ratio. These ratios were applied to the population prevalences obtained from the NHANES in order to estimate the prevalences among cases in the population, which are required for inclusion in the PAF calculations. Figure 3 presents the weighted population prevalences of sitting and television viewing in the NHANES, along with the prevalences after adjustment for the average case:source prevalence ratios from table 2. Table 2 Prevalences of sitting and TV viewing at baseline in the prospective cohort studies included in the meta-analysis Figure 3 Prevalences of (A) sitting and (B) television viewing in the US National Health and Nutrition Examination Survey (NHANES). *Adjusted prevalences of sitting and television viewing using the weighted average case:source prevalence ratio obtained from cohort studies of sedentary behaviour and all-cause mortality. The PAFs for all-cause mortality associated with sitting and television viewing were 27% and 19%, respectively. The results of the life table analyses indicate a gain in life expectancy from reducing the prevalence of sedentary behaviour from level 2 or 3 to level 1 results in a gain of 2.00 years for reducing sitting prevalence and a gain of 1.38 years from reducing television viewing prevalence. 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. Data from two studies of television viewing17 ,18 and one study of sitting15 were available, where both age- and sex-adjusted as well as multivariable-adjusted RR estimates for all-cause mortality were presented. For sitting, the RR for level 2 versus level 1 was the same and the RR for level 3 versus level 1 was 6% lower for the multivariable compared with the age- and sex-adjusted RR. For television viewing, the RR for level 2 versus level 1 was 9.6% lower, and for level 3 versus level 1, the RR was 16.4% lower for the multivariable compared with the age- and sex-adjusted RR. After reducing the RR estimates obtained from the meta-analysis by these percentages, the estimates of attributable life expectancy were 1.76 years for sitting and 0.93 years for television viewing.
– It's a wonder anyone dares sit anymore, considering the avalanche of recent studies that have been telling us that doing so ups our risk of cancer and heart disease, makes us fatter, and turns meetings into awful experiences. And here's a new one to add to the pile: Sitting for three hours a day can take two years off of your life, reports the Wall Street Journal—even if you exercise and eschew smoking, according to a new "meta-analysis" of five studies of 167,000 people. Couch potatoes have it even worse: Watch more than two hours of television a day and subtract another 1.4 years. "Sedentary behavior is something we need to take note of beyond telling people to get 30 minutes of activity a day," says one of the study's authors. "Several studies show that when you're sitting, your leg muscles are completely inactive. When you're sitting and completely inactive, this is when you run into trouble managing blood glucose." You can find the full study at BMJ Open.