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Why strenuous runs may not be so bad for you after all

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.