Over the past few years, health experts have warned that spending an excessive amount of time sitting—regardless of whether one gets sufficient exercise or not—can kill. Now, a study released Monday puts a number on it: two years.
An analysis of five large-scale studies following about 2 million people in several different countries found that the life expectancies of people who said they spent more than three hours a day sitting were two years less than people who spent less than three hours sitting daily.
"It's right in the same ballpark as smoking and obesity—sedentary behavior is in the same category," says Peter Katzmarzyk of Louisiana State University's Pennington Biomedical Research Center and lead author of the paper. "Smoking is still the Number One risk factor [for early death], but sitting is catching up."
Previous studies have suggested that people who spend six or more hours a day sitting have a 20 percent higher rate of early death than people who sat for three hours or less each day.
Katzmarzyk's study found that the life expectancy was reduced regardless of whether a person got sufficient exercise or not. His analysis also found that people who spend less than two hours daily watching television live about 1.4 years longer than people who spend more than two hours a day watching TV.
Previous studies have found that prolonged sitting increases the rate of cardiovascular disease, decreases insulin effectiveness, and slows metabolism.
"We have the epidemiology about why sitting a lot can harm you, but this takes a look at what the population-level effects are," he says.
Katzmarzyk says previous studies have helped usher in the popularity of standing and treadmill desks, but that more needs to be done.
"The workplace is a good place to look at interventions. Many people have active jobs and are on their feet all day, but there's a large fraction stuck behind their desk all day," he says. "The government can promote guidelines on sedentary behavior, just like they have recommendations for daily exercise. And workplaces can provide opportunities for people to stand—you might not have a treadmill desk for everyone, but they could have four or five per floor."
The next step, Katzmarzyk says, is to determine how long it takes for prolonged sedentary behavior to have an effect so that he can determine the answers to questions such as whether spending a year or two at an office job severely impacts life expectancy or whether such impacts take place over the course of a career.
"We need to look at what the optimal levels of sitting are, how much sitting we can get away with," he says. "We have 50 years of exercise data to go on, but we need more research to be able to say how much people should be sitting."
Jason Koebler is a science and technology reporter for U.S. News & World Report. You can follow him on Twitter or reach him at email@example.com.