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The question

Can sitting too long really hurt my health - even if I exercise regularly?

The answer

The numerous studies linking excessive time in front of TV and computer screens with such health problems as heart disease and diabetes may sound like they belong in the Journal of Really Obvious Research. After all, you're not getting any exercise when you're lying on the couch eating chips.

But there's a subtle point whose implications are only now being appreciated: sitting (or lying down) too much is not the same as exercising too little.

Two new studies highlight the growing consensus that long bouts of uninterrupted sedentary behaviour carry health risks that can't be erased even if you're getting plenty of exercise at other times during the day. Researchers are now rushing to determine exactly what counts as "sedentary," and how people whose jobs require them to sit at a desk for the majority of their waking hours can mitigate some of these risks.

In a study reported in January in the Journal of the American College of Cardiology, British researchers monitored 4,512 adults for 4.3 years, and found that those who accumulated more than four hours of screen time daily were twice as likely to be hospitalized or die from a "major cardiac event" compared with those who got two hours or less.

Crucially, this relationship held true no matter how much exercise the subjects got, which suggests that the problem with too much screen time isn't simply that you don't burn as many calories as you consume. Instead, studies with rats and mice show that any muscle that doesn't contract for several hours starts to undergo harmful metabolic changes.

Over the past decade, scientists have observed that levels of an enzyme called lipoprotein lipase drop in muscles left idle for too long. This enzyme is responsible for drawing fat from the bloodstream into muscles, where it's burned as fuel. When the muscles don't need any fuel, the fat remains in the bloodstream and wreaks havoc elsewhere in the body.

"The animal research and the few physiological studies that have been done suggest that as long as a muscle is contracting, regardless of how low the intensity is, that seems to prevent you from experiencing some of the metabolic adaptations that happen when you're sedentary," says Travis Saunders, a researcher at the Children's Hospital of Eastern Ontario in Ottawa who co-authored a recent review paper on this emerging field of study.

That means that household chores and simple tasks such as walking to a bathroom on another floor are enough to qualify as non-sedentary. Whether simply standing up counts - and if so, for how long - will only be settled by further research. "It's a grey zone," Mr. Saunders says.

There's more encouraging news for committed desk jockeys in a forthcoming study in the European Heart Journal. Genevieve Healy of the University of Queensland in Australia drew on data from 4,757 people who wore tiny accelerometers to measure their movement patterns for seven consecutive days. Once again, greater amounts of sedentary time were linked to a wide range of blood markers for heart disease and metabolic disorders.

But the accelerometers were also able to record breaks as short as a minute that interrupted sedentary time. The number of breaks taken by the subjects throughout the week ranged from 99 to 1,258, and the researchers found that those who took the most breaks were significantly healthier (for example, their waistlines were 4.1 centimetres smaller) than those who took the fewest breaks - independent of the total amount of sedentary time they accumulated.

While the general message from this research is clear - take frequent, short breaks from desks and couches - more specific guidance will have to wait for the results of new studies being planned by Mr. Saunders and others to directly measure how many hours at a time you can stay seated before metabolic changes begin.

Until then, we're left with interim solutions - such as the $30 foot-pedal device that Mr. Saunders invested in last fall while preparing for his doctoral exams, when he realized he was spending 14 hours a day sitting at a desk reading about the dangers of sitting at a desk.

"It just sits under my desk and I pedal, I'd say about half [an]hour out of every hour. Very low intensity, but it's engaging the muscles in my legs and the muscles in my lower back," he says.

"I have no idea whether or not this is making a difference, but it's plausible … and in the absence of any other options, I'm going to keep doing it."

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