Like smoking and overeating, sitting has joined the list of behaviors that seem likely to lead to an early death. In recent years, dozens of studies have linked prolonged sitting with higher risks for cardiovascular disease, cancer, type 2 diabetes and more.
A welcome dose of hope for office-workers and couch potatoes seemed to arrive last week, when a new study suggested that sitting might not be so bad, after all. Based on 16-years of data on more than 5,000 middle-aged adults, UK researchers found no link between prolonged sitting and the risk of death.
But, experts caution, the new findings don't necessarily contradict or debunk growing evidence for the harms of sedentary lifestyle. One major reason is that participants in the new study were especially active, reporting an average of 15 hours of physical activity per week.
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That's drastically more than the 2.5 weekly hours recommended by many major medical groups. And that suggests that there might be some still-unquantified protective effect of exercise that counteracts the dangers of sitting. The study also looked only at death rates, and not at rates of diseases or other health measures.
Based on the bulk of accumulating data, experts say, a sedentary lifestyle still seems to carry real risks.
"There is a lot of very good research that is ongoing in research groups around the world, which is examining associations between sitting and disease in different populations and most importantly, the underlying biology behind these associations," says Richard Pulsford, an exercise physiologist at the University of Exeter, and lead author of the new study in the International Journal of Epidemiology.
"When we understand what's going on under the surface," he says, "we will know far more about whether sitting or a lack of activity is the most important factor for promoting good health."
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Although scientists have long known that physical activity promotes health, the first clue that sitting could be detrimental for people came in 2009, when researchers looked at data on more than 17,000 Canadians and found that people who sat more were more likely to die from cardiovascular disease.
Since then, a number of large studies in a variety of countries have produced similar results, says Peter Katzmarzyk, an epidemiologist at the Pennington Biomedical Research Institute in Baton Rouge, Louisiana, and co-author of the 2009 study.
Accumulating findings have been both alarming and punctuated by unknowns. In general, studies have shown no "safe level" below which sitting is good and above which sitting becomes bad, Katzmarzyk says. Instead, it appears that progressively more sitting leads to progressively worse consequences. Even avid exercisers suffer more ill effects when they spend more of their non-workout time sitting.
But there is also evidence to suggest that sitters who exercise fare better than sitters who just sit. In a review of 47 studies published in the Annals of Internal Medicine earlier this year, researchers concluded that, among people who did not exercise, prolonged sitting raised risks for various diseases by about 40 percent, says lead author David Alter, a cardiologist at the University of Toronto. Among avid exercisers, on the other hand, sitting raised risk by just five or 10 percent.
The new UK study supports the idea that the benefits from lots of physical activity can counteract the harms from lots of sitting.
"The real health risks associated with sitting are among those folks who are both sitting for long periods of time and not exercising," Alter says. "The magnitude of risk really diminishes and is marginal among folks who exercise."
What science can't yet offer are clear guidelines on how much exercise is enough in order to justify a long day behind a computer or a relaxing night of binge-watching shows on Netflix. For most people, two hours a day of activity is an unrealistic goal. And while less might still offer protection, the only way to know for sure is with trials that assess the consequences of various amounts and types of movement. That kind of data is not yet available.
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Also still unclear is why exactly, sitting causes harm. One theory is that sitting inactivates major muscle groups, which in turn leads to metabolic changes in the body that can accumulate over the long-term. And while stand-up desks have become popular, standing brings health risks of its own, Katzmarzyk says, including circulation problems and back pain.
"There is still a lot of work to be done," Alter says. "We're just opening up the hood to this research."
For now, Katzmarzyk says, it can't hurt to aim to meet existing recommendations for physical activity, which many people still aren't managing to do. "There is no potential harm," he says, "in moving more."