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A recent study reported that joggers who exercise strenuously have the same life expectancy as people who do barely any exercise at all. But the author has now admitted he hasn't actually proved this.

"Training very hard 'as bad as no exercise at all,'" reported the BBC. "Fast running is as deadly as sitting on couch," agreed the Daily Telegraph and countless other newspapers around the world, striking fear into the hearts of hardcore runners.

The headlines were based on the results of a large Danish study - more than 1,000 healthy joggers and almost 4,000 healthy non-joggers were followed up over 12 years, as part of the Copenhagen City Heart Study.

The results were published in the prestigious Journal of the American College of Cardiology. The headline conclusion was that light and moderate joggers live longer on average than people who have sedentary lifestyles and don't exercise at all, whereas - and this was the surprise - strenuous joggers don't.

Strenuous joggers were defined as people who run at a fast pace - more than 7mph - for more than four hours per week, or more than three times per week for at least two-and-a-half hours in total.

But, although the overall number of people studied was large, the number of strenuous joggers was not. Only 36 people fitted the strenuous jogging category - two of them had died.

Critics say these numbers are too small to be statistically significant.

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And the researchers don't know how the two strenuous joggers died - whether they had succumbed to an illness such as cardiovascular disease or whether they had been involved in some sort of accident. They could have been knocked down by a bus.

The lead author of the study, clinical cardiologist Dr Peter Schnohr, now concedes that he didn't have the evidence to say that strenuous jogging is bad for you.

"We should have said we suspect that it is so, but we can't say for sure. Everybody makes some mistakes in papers," he says.

But Schnohr thinks experienced readers of research papers would have realised this - he says it was obvious from the statistical analysis that you couldn't have confidence in the claim that strenuous joggers have the same average life expectancy as those with a sedentary lifestyle.

"It shouldn't have been misunderstood," he says, because if you go into the statistics the limits of the research are clear. "If you normally read papers you could say 'Ah! This is not good statistically - this is too thin.'"

But most journalists around the world weren't going to be delving into the statistical tables - they take their lead from the researcher's conclusions, according to Alicia White, who analysed the study for the UK's National Health Service website.

"In today's world, research findings can rapidly be disseminated across the globe - particularly when they capture the public imagination due to a shocking finding," she says.

"It's not realistic to assume that everyone who hears a headline will track down the research paper, read and critically appraise it before they decide to act or share it with their friends. That's why it is so important that research findings, their implications and limitations are communicated in a clear way right from the start."

Image copyright AFP Image caption 924,741 runners have completed the London Marathon since it was set up 1981

The Journal of the American College of Cardiology, which published the research, stands by the paper.

"Some news articles appear to have misinterpreted and exaggerated" the story leading to "misleading headlines," says editor-in-chief Valentin Fuster.

But the fact remains that the paper's headline conclusion included the statistically insignificant finding about strenuous jogging - something Peter Schnohr admits shouldn't have been highlighted.

Even so, he doesn't regret the shock-horror headlines, and isn't worried that they might have put people off jogging.

"I don't think so… you always have deaths in marathons and so on," he says - and suggests that with regular check-ups, cases of heart disease may have been detected and some of these fatalities "could have been prevented".

Schnohr remains convinced that although he hasn't proved it this time, strenuous jogging might be bad for you.

"We're not saying that you should not do the marathon, but we're saying that maybe every other year you should do an investigation - echocardiography, and so on - to look at your heart."

He says that his findings that light jogging is associated with significantly lower mortality rates is robust though, and that this important health message was picked up by a number of media outlets.

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