Sitting Less Adds Years to Life

Peter Katzmarzyk, PhD, of Louisiana State University in Baton Rouge, and I-Min Lee, MD, of Brigham and Women's Hospital in Boston, reported online that, limiting time watching television to less than 2 hours a day added 1.38 years of life, and cutting total sitting time to less than 3 hours a day increased life expectancy by 2 full years.

The prevalence-based, cause-deleted analysis measured relative risks of all-cause mortality in association with sitting and television viewing derived from a meta-analysis of studies looking at that relationship and from National Health and Nutrition Examination Survey (NHANES) data from the 2009-2010 survey for sitting and the 2005-2006 study for TV viewing.

The cohorts included a combined 166,738 respondents followed for an average 9.28 years, and included mortality measures and breakdowns of time spent engaged in sedentary behavior. How time was broken down varied among the studies and included time as a fraction of a person's day or absolute hourly measures, although all studies stratified time by three levels (most, median, and least).

The NHANES prevalence data is representative of the non-institutionalized, nonpregnant U.S. adults 18 and older. The questionnaires divided sitting time into less than 3 hours, 3 to 5.9 hours, and 6 or more hours a day. The TV time survey used breakdowns of less than 2 hours, 2 to 3.9 hours, and 4 or more hours daily.

The pooled relative risk for sitting and all-cause mortality was 1.45 (95% CI 1.39 to 1.51) for those who spent the most time sitting versus those who spent the least, and 1.18 (95% CI 1.14 to 1.21) for those who spent an average time sitting versus those who spent the least.

For associations between television viewing and all-cause mortality, the pooled relative risk was 1.49 (95% CI 1.22 to 1.82) for those who watched the most television, and 1.17 (95% CI 1.04 to 1.32) for those who watched an average amount of television, versus those who watched the least, respectively.

Katzmarzyk and Lee noted that the gains in life expectancy for sitting (2 years) and television viewing (1.38 years) came from adjustment from the most or median amounts of time (half or more of a patient's day, or 3 or more hours) spent on the sedentary behaviors to the lowest range.

They noted, however, a number of mechanisms that could explain the association of sedentary behaviors and mortality, including increased risk of type 2 diabetes and cardiovascular disease.

The authors noted two limitations in their study: use of self-reported data that was subject to error and recall bias, and the assumption of a casual relationship between sedentary behavior and mortality.

Practice Pearls

• Note that the purpose of this study was to determine the impact of sedentary behaviors on life expectancy in the U.S.
• They noted that this effect on life expectancy is theoretical and that further research into health outcomes is required.
• Point out that the analyses indicate that population life expectancy in the U.S. would be 2 years higher if adults reduced their time spent sitting to less than 3 hours a day and 1.38 years higher if they reduced television viewing to less than 2 hours a day.

Katzmarzyk PT, Lee IM "Sedentary behaviour and life expectancy in the U.S.A.: a cause-deleted life table analysis" BMJ Open 2012; DOI: 10.1136/bmjopen-2012-000828.