While health officials have issued guidelines recommending minimum amounts of physical activity, 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. She explained that genes regulating（调节） the amount of glucose（葡萄糖） 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 who exercise every day—but still spend a lot of time sitting—might get more benefit if that exercise were spread across the day, rather than in a single bout(一回).

　　Still, in a study published in 2009 that tracked more than 17,000 Canadians for about a dozen years, researchers found people who sat more had a higher death risk, whether or not they exercised.

　　"We don't have enough evidence yet to say how much sitting is bad," said Peter Katzmarzyk of the Pennington Biomedical Research Center in Baton Rouge, who led the Canadian study. "But it seems the more you can get up and interrupt this sedentary behaviour, the better."

　　Figures from a U.S. survey in 2003-2004 found Americans spend more than half their time sitting, from working at their desks to sitting in cars.

　　Experts said more research is needed to figure out just how much sitting is dangerous, and what might be possible to offset those effects.

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