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Phys Ed Gretchen Reynolds on the science of fitness.

Just as we were all settling in front of the television to watch the baseball playoffs, two new studies about the perils of sitting have spoiled our viewing pleasure.

The research, published in separate medical journals this month, adds to a growing scientific consensus that the more time someone spends sitting, especially in front of the television, the shorter and less robust his or her life may be.

To reach that conclusion, the authors of one of the studies, published in the October issue of The British Journal of Sports Medicine, turned to data from the Australian Diabetes, Obesity and Lifestyle Study, a large, continuing survey of the health habits of almost 12,000 Australian adults.

Along with questions about general health, disease status, exercise regimens, smoking, diet and so on, the survey asked respondents how many hours per day in the previous week they had spent sitting in front of the television.

Watching television is not, of course, in and of itself hazardous, unless you doze off and accidentally slip from the couch onto a hard floor. But television viewing time is a useful, if somewhat imprecise, marker of how much someone is engaging in so-called sedentary behavior.

“People can answer a question like, ‘How much time did you spend watching TV yesterday?’ much better than a question like ‘How much time did you spend sitting yesterday?’ ” says Dr. J. Lennert Veerman, a senior research fellow at the University of Queensland, who led the new study.

Australians, as it turns out, watch lots of telly. According to the survey data, in 2008, the year that the researchers chose as their benchmark, Australian adults viewed a collective 9.8 billion hours of television.

Using complex actuarial tables and adjusting for smoking, waist circumference, dietary quality, exercise habits and other variables, the scientists were next able to isolate the specific effect that the hours of sitting seemed to be having on people’s life spans.

And the findings were sobering: Every single hour of television watched after the age of 25 reduces the viewer’s life expectancy by 21.8 minutes.

By comparison, smoking a single cigarette reduces life expectancy by about 11 minutes, the authors said.

Looking more broadly, they concluded that an adult who spends an average of six hours a day watching TV over the course of a lifetime can expect to live 4.8 years fewer than a person who does not watch TV.

Those results hold true, the authors point out, even for people who exercise regularly. It appears, Dr. Veerman says, that “a person who does a lot of exercise but watches six hours of TV” every night “might have a similar mortality risk as someone who does not exercise and watches no TV.”

These rather unnerving results jibe with those of another new study of sitting. Published on Monday in the journal Diabetologia, its authors reviewed data from 18 studies involving 794,577 people. Many of the studies measured full-day sitting time, covering not only hours whiled away in front of the television, but also time spent in a chair at work.

Together, those hours consumed a majority of a person’s life. “The average adult spends 50 to 70 percent of their time sitting,” the authors report.

The researchers then cross-referenced sitting time with health outcomes, and found that those people with the “highest sedentary behavior,” meaning those who sat the most, had a 112 percent increase in their relative risk of developing diabetes; a 147 percent increase in their risk for cardiovascular disease; and a 49 percent greater risk of dying prematurely — even if they regularly exercised.

“Many of us in modern society have jobs which involve sitting at a computer all day,” says Dr. Emma Wilmot, a research fellow at the University of Leicester in England, who led the study. “We might convince ourselves that we are not at risk of disease because we manage the recommended 30 minutes of exercise a day.”

But, she says, we “are still at risk if we sit all day.”

Why a seemingly blameless activity like sitting should be detrimental to health, even for those of us who work out, is not fully understood, although it is assiduously being studied at many labs.

One partial explanation, however, is obvious. “The most striking feature of prolonged sitting is the absence of skeletal muscle contractions, particularly in the very large muscles of the lower limbs,” says David W. Dunstan, a professor at the Baker IDI Heart and Diabetes Institute in Australia, senior author of the Australian study, and a pioneer in the study of sedentary behavior.

When muscles don’t contract, they require less fuel, and the surplus, in the form of blood sugar, accumulates in the bloodstream, contributing to diabetes risk and other health concerns.

Thankfully, excessive sitting is theoretically easy to combat. First, cut TV time. “The evidence indicates that four hours per day is in the ‘risky’ category,” Dr. Dunstan says, “while less than two hours per day is in the lower-risk group.”

Then look to the rest of your day. When Dr. Wilmot asked a group of volunteers recently to reduce their daily sitting time by an hour, “they came up with lots of ideas,” she says, including “putting the garbage bin on the other side of the office, standing during coffee breaks and telephone calls, having standing meetings, standing on the bus.”

But don’t, she emphasizes, cease exercising. “There is absolutely no doubt that exercise is beneficial for health,” she says. It just may not, by itself, be sufficient for health.

If you exercise for 30 minutes a day, she says, “take time to reflect on your activity levels for the remaining 23.5 hours,” and aim to “be active, sit less.”