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Many of us sit in front of a computer for eight hours a day, and then go home and head for the couch to surf the Web or watch television, exchanging one seat and screen for another. Even if we try to squeeze in an hour at the gym, is it enough to counteract all that motionless sitting?

A mounting body of evidence suggests not.

Increasingly, research is focusing not on how much exercise people get, but how much of their time is spent in sedentary activity, and the harm that does.

The latest findings, published this week in The Journal of the American College of Cardiology, indicate that the amount of leisure time spent sitting in front of a screen can have such an overwhelming, seemingly irreparable impact on one’s health that physical activity doesn’t produce much benefit.

The study followed 4,512 middle-aged Scottish men for a little more than four years on average. It found that those who said they spent two or more leisure hours a day sitting in front of a screen were at double the risk of a heart attack or other cardiac event compared with those who watched less. Those who spent four or more hours of recreational time in front of a screen were 50 percent more likely to die of any cause. It didn’t matter whether the men were physically active for several hours a week — exercise didn’t mitigate the risk associated with the high amount of sedentary screen time.

The study is not the first to suggest that sedentary activities like television viewing may be harmful. A study last year found that men who spent more than 23 hours a week watching TV and sitting in their cars were more likely to die of heart disease than those who sat for 11 hours a week or less, even if they exercised. And a 2009 study reported that young children who watch one and a half to five and a half hours of TV a day have higher blood pressure readings than those who watch less than half an hour, even if they are thin and physically active.

Another small study found that when overweight adults cut their TV time in half, they burned more calories than those who watched five hours or more a day. Children whose TV time is cut tended to eat less, but that wasn’t true for adults. And the light activities adults filled their time with, like reading and playing board games, actually burned more calories than watching TV.

In both the United States and Britain, people are spending three to four hours a day on average watching television, said the study’s author, Emmanuel Stamatakis, of the department of epidemiology and public health at University College London.

“This is excessive,” he said. “It is more than 20 percent of total waking time for most people.” And, he added, “it’s 100 percent discretionary.”

During the study’s follow-up period, from 2003 to 2007, 325 men died of various causes, and 215 suffered a heart attack or other cardiac event. Even after adjusting for differences in weight, smoking, occupational physical activity and risk factors like diabetes, high blood pressure and other longstanding illnesses, as well as marital status and social class, those who spent four hours or more of their leisure time in front of a screen each day were 50 percent more likely to have died. Those who spent two hours a day in front of a screen for entertainment were 2.2 times more likely to have had a cardiovascular event.

Recreational screen time has an “independent, deleterious relationship” with cardiovascular events and death of all causes, the paper concluded, possibly because it induces metabolic changes.

One possible mechanism, demonstrated in animal studies, is that being sedentary may affect lipid metabolism. Prolonged inactivity appears to sharply reduce the activity of an important enzyme called lipoprotein lipase, which is responsible for breaking down circulating blood lipids and making them available to muscles for energy, Dr. Stamatakis said. Lowered enzyme activity leads to higher levels of fats and triglycerides in the blood, and to a higher risk of cardiovascular disease. Exercise has very little impact on the enzyme’s activity, he said.

Extended sitting may also lead to high levels of low-grade inflammation, which can also lead to heart disease, Dr. Stamatakis said. A marker of low-grade inflammation called C reactive protein was about three times higher in the study participants who spent the most time slouched in front of a screen.

The study focused on recreational screen time because it’s the easiest to curtail, Dr. Stamatakis said. But he encouraged employees who work at computers all day to get up and take breaks and short walks periodically.