You'd have to have been living under a stone for the last decade or so to have missed public health messages on smoking, drinking, healthy eating and exercise. Whether we act on them or not, many of us can reel off figures on weekly exercise and alcohol limits – as well as the exact contribution to our five-a-day half a kumquat provides – like well-loved stanzas of poetry.

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

For instance, 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". And Dr Alpa Patel, of the American Cancer Society, thinks it is only a matter of time. "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 – or more accurately, 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 comprehensive recent studies are more nuanced – and perhaps more worrying – than that. Because what they suggest is that prolonged periods of sitting are bad for you 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 sat at a desk or on a sofa you are putting yourself at increased risk from an array of dangerous ailments.

The most recent research, published in July in the American Journal of Epidemiology, 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 37 per cent more likely to die during the period than those who sat fewer than three hours a day, while men were 18 per cent 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.

It is not an isolated finding. After studying the relationship between sitting in cars and sitting in front of TVs and cardiovascular disease in men, researchers from the University of South Carolina recommended in May that reducing sedentary time, as well as increasing physical activity, should be at the heart of health-promotion efforts.

Another study, published in 2009 in the journal Medicine & Science in Sports & Exercise, again found that prolonged sitting increased mortality from all causes, regardless of exercise at other times.

Professor Peter Katzmarzyk of the Pennington Biomedical Research Centre in Baton Rouge, Louisiana, has researched the effects of sedentary behaviour on health and mortality, and 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 in more vigorous activities? It's fair to say that nobody is 100 per cent sure, but scientists are inching towards an answer. It seems that sitting for a long time causes some pretty fundamental changes in metabolism. "The biological mechanisms have yet to be fully worked out, but 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," says Professor Katzmarzyk.

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.

The evidence looks pretty convincing. One study found that men who cut down on the amount they walk for two weeks become worse at metabolising sugars and fats. Yet another – among people who sit for long periods – found that subjects who took frequent short breaks were better at metabolising sugars and fats than those who didn't.

And therein lies an answer. Thanks to office-bound careers, cars and sedentary, sofa-based entertainment, most of us sit for long periods. But the simplest things can help to 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".

Tips for sitting less

* Deliver messages to colleagues personally rather than by email.

* Use your lunch break. Walk around town rather than munching a sandwich at your desk.

* Conduct meetings and make calls standing up.

* Buy a height-adjustable desk and combine periods of sitting and standing (www.heightadjustabledesks.com).

* Stand up and stretch or walk around for 60 seconds every 15 minutes or so.