Sit less than 3 hours a day, live 2 years longer?

In yet another study on the risks associated with sedentary behaviour, a new U.S. analysis suggests that limiting the amount of time spent sitting to less than three hours a day might boost life expectancy by two years at least in theory.

Moreover, restricting television viewing to less than two hours per day could extend life by almost 1.4 years, according to findings published Monday in the online British Medical Journal.

The authors at Louisiana State University and Brigham and Womens Hospital and Harvard Medical School emphasize that their analysis assumes a theoretical association rather than proving that there is one.

They caution that their findings should not be interpreted as meaning that someone who leads a more sedentary lifestyle can expect to live two or 1.4 years less than someone who is more active.

The research provides an estimate of life expectancy based on a survey of self-reported sitting time (at work and watching television) applied against the known risks of sedentary behaviour.

For their analysis, the Louisiana and Boston researchers used data collected for the National Health and Nutrition Examination Survey (NHANES), which regularly surveys a large representative sample of the U.S. population on various aspects of their health and lifestyle.

The researchers used self-reported data for 2005-06 and 2009-10 to determine that U.S. adults spend 55 per cent of their day engaged in sedentary pursuits such as watching TV and sitting down on a daily basis.

They then trawled the research database Medline and pooled the relative risk data from five relevant studies on sitting time and deaths from all causes, involving almost 167,000 adults. The database was then reanalyzed, taking age and sex into account.

They combined these data and the NHANES figures to come up with an estimate of the theoretical effects of a risk factor on a population, rather than an individual level, to calculate the number of deaths associated with time spent sitting down.

"The results of this study indicate that extended sitting time and TV viewing may have the potential to reduce life expectancy," the researchers wrote.

"Given that the results from objective monitoring of sedentary time in NHANES has indicated that adults spend an average of 55 per cent of their day engaged in sedentary pursuits, a significant shift in behaviour change at the population level is required to make demonstrable improvements in life expectancy."

Further research is needed to determine safe levels of sedentary behaviour, they said.

Several previous studies have linked extended periods spent sitting down and/or watching TV to poor health, such as diabetes and death from heart disease and stroke.