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Here's a warning from health experts: Sitting is deadly. Scientists are increasingly warningthat sitting for prolonged periods--even if you also exercise regularly--could be bad for yourhealth. And it doesn't matter where the sitting takes place at the office, at school, in the car orbefore a computer or TV--just the overall number of hours it occurs.
Research is preliminary, but several studies suggest people who spend most of their dayssitting are more likely to be fat, have a heart attack or even die.
In an editorial published this week in the British Journal of Sports Medicine, Elin Ekblom-Bakof the Swedish School of Sport and Health Sciences suggested that authorities rethink how theydefine physical activity to highlight the dangers of sitting.
While health officials have issued guidelines recommending ~nini~num amounts of physicalactivity, they haven't suggested people try to limit how much time they spend in a seated position.
"After four hours of sitting, the body starts to send harmful signals," Ekblom-Bak said. Sheexplained that genes regulating the amount of glucose and fat in the body start to shut down.
Even for people who exercise, spending long stretches of time sitting at a desk is still harmful.Tim Armstrong, a physical activity expert at the World Health Organization, said people whoexercise every day--but still spend a lot of time sitting--might get more benefit if that exercisewere spread across the day, rather than in a single bout.
That wasn't welcome news for Aytekin Can,31, who works at a London financial company,and spends most of his days sitting in front of a computer. Several evenings a week, Can alsoteaches jiujitsu, a Japanese martial art involving wrestling, and also does Thai boxing.
"I'm sure there are some detrimental effects of staying still for too long, but I hope that beingactive when I can helps," he said. "I wouldn't want to think the sitting could be that dangerous."
Still, in a study published last year that tracked more than 17,000 Canadians for about adozen years, researchers found people who sat more had a higher death risk, independently ofwhether or not they exercised.
Figures from a US survey in 2003-2004 found Americans spend more than half their timesitting, from working at their desks to sitting in cars.
Experts said more research is needed to figure out just how much sitting is dangerous, andwhat might be possible to offset those effects.
"People should keep exercising because that has a lot of benefits," Ekblom-Bak said. "Butwhen they're in the office, they should try to interrupt sitting as often as possible," she said."Don't just send your colleague an e-mail. Walk over and talk to him. Stand up."

What is the main idea of this passage?
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A A．People should keep exercising.

B B．Exercising may not reduce the harm of sitting.

C C．Sitting for long time is deadly.

D D．How to eliminate the harm of sitting.