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Getting to the real seat of health issues

And if a growing band of doctors and medical researchers have their way, health advice will include another bullet point. Experts in cancer, heart disease and obesity are all calling for advice on "sensible sitting" to become part and parcel of public health drives.

The authors of one recent study on the dangers of sitting for prolonged periods conclude that "public-health messages and guidelines should be refined to include reducing time spent sitting in addition to promoting physical activity".

Dr Alpa Patel of the American Cancer Society says: "I think the research community is building a strong evidence base [on the dangers of sitting] that will likely influence public-health guidelines in the future," she says.

So why the sudden interest in directing how much we sit? One reason is obvious. If we sit down for eight hours a day, we are not spending any of that time walking, running or swimming. Sitting implies an absence of exercise. In fact, as Dr Patel says, "sitting is one of the most passive things you can do".

Pretty much anything burns more calories than just sitting down. Standing up, even without moving around, burns about twice as many.

But the results from a number of recent studies are more nuanced -- and perhaps more worrying. They suggest prolonged periods of sitting are bad regardless of what you do at other times. You can run for an hour each morning, but if you spend the rest of the day at a desk or on a sofa, you are putting yourself at increased risk from an array of dangerous ailments.

Research published in July analysed survey responses from 123,000 people with no history of cancer, heart disease, stroke or lung disease.

The study followed them for 13 years, up to 2006. Researchers found that women who reported more than six hours per day of sitting were 37pc more likely to die during the period than those who sat fewer than three hours a day, while men were 18pc more likely to die. It didn't matter how much physical activity the subjects took at other times.

Which means, quite simply, that you can be lean and fit and still increase your chances of an early death from all causes by sitting down too much.

Professor Peter Katzmarzyk of the Pennington Biomedical Research Centre in Baton Rouge, Louisiana says that while there are huge health benefits from exercise, the benefits of not sitting down are different and unique.

"We are finding that the effects of sitting are independent of other risk factors," he says. "This is not to say that being physically active and fit doesn't have benefits. But these benefits are separate from sitting. Just as smoking and obesity are both bad for you, they are independent -- smoking is bad if you are obese and also if you are normal weight."

So why is sitting down uniquely unhealthy? What is it doing beyond stopping us burning extra calories? Nobody is 100pc sure, but scientists are inching toward an answer. It seems sitting for a long time causes some pretty fundamental changes in metabolism.

"Our basic science colleagues are finding several metabolic disturbances at the level of the muscle --like changes in hormones and blood lipid fractions that are unhealthy -- even after just one day of extended sitting," he says.

When we sit down, we're not tensing muscles. Studies on rats have shown that muscles only produce the lipoprotein lipase -- a molecule that helps the body process fats -- when muscles are flexed.

Thanks to office-bound careers, cars and sofa-based entertainment, most of us sit for long periods. But the simplest things can help mitigate the worst effects. "The simple take-home message here is to try and sit less," says Dr Patel. Standing up, stretching and walking around at lunchtime all "keep your muscles engaged and keep you from being sedentary for long periods of time".