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.
The meta-analysis comes just two years after Australian researchers found that people who said they watched TV for more than four hours a day were 46% more likely to die of any cause than people who said they spent less than two hours a day watching TV. Those watching TV more than four hours a day were also 80% more likely to die of cardiovascular disease.
"Sedentary behavior is something we need to take note of beyond telling people to get 30 minutes of activity a day," said Peter T. Katzmarzyk, one of the lead researchers for the study and a professor of population science at the Pennington Biomedical Research Center in Baton Rouge, La.
"We have people who can meet that guideline," explained Dr. Katzmarzyk. "However, if you're sedentary or sitting the other 20 hours a day, you're still going to be at risk for that."
But while the evidence linking sedentary behavior to various illnesses is mounting, it remains difficult for many people to find time to get on their feet, especially if they work desk jobs.
"Try to stand as much as you can," Dr. Katzmarzyk said. "Typically when you're on the telephone you can stand with speaker phone. Instead of emailing someone in the office, just get up and go talk to them."
However, Dr. Katzmarzyk added, standing shouldn't be an alternative to exercising, but an alternative to sitting. "Several studies show that when you're sitting, your leg muscles are completely inactive," he said. "When you're sitting and completely inactive, this is when you run into trouble managing blood glucose."
Researchers determined the prevalence of sedentary behavior by using the National Health and Nutrition Examination Survey, which provided data on sitting habits between 2009 and 2010. The most recent available data on TV viewing was between 2005 and 2006.
When looking at the five underlying studies, the researchers eliminated data on patients with existing diseases and were therefore more likely to be sitting a lot.
One of the analysis's limitations was that it relied on self-reporting, as people tend to underestimate how much sitting they do, Dr. Katzmarzyk said.
The study bolsters an emerging body of research that points to a number of dangers associated with leading a sedentary lifestyle.
Last year, scientists found that people who worked 10 years in sedentary jobs, or jobs that don't require a lot of energy expenditure, had twice the risk of colon cancer and a 44% increased risk of rectal cancer, compared with people who had never worked sedentary jobs.
And in March, scientists found that the rate of cancers linked to obesity and lack of physical activity, such as cancers of the kidney, pancreas, lower esophagus and uterus, rose every year from 1999 through 2008.