Too much exercise may damage your heart, doctors have warned, suggesting that rather than adding years to their lives, fitness freaks could be working themselves into an early grave.

Experts warn that exercising intensely for more than an hour or two can damage the heart, causing its tissue to stretch, tear and scar and raising the odds of dangerous changes in heart rhythm.

"A routine of moderate physical activity will add life to your years, as well as years to your life. In contrast, running too fast, too far and for too many years may speed one's progress towards the finish line of life," doctors said.

In the study published in journal Heart, US cardiologists James O`Keefe and Carl Lavie St Luke`s Mid America Heart Institute, also advise that those who want to exercise at full pelt should limit themselves to 30 to 50 minutes a day.

They said marathons should be viewed as something to do occasionally or once in a lifetime, rather than a regular challenge, the `Daily Mail` reported.

Two studies recently presented at major medical conferences back up their argument that exercise, like many other things in life, is best done in moderation.

The first, tracked the health of more than 50,000 people for up to 30 years. The 14,000 runners in the study were 19 per cent less likely to have died than the others during this time.

However, closer analysis of the results revealed the longevity benefits were limited to those who ran between eight and 32 km a week.

In contrast, those who ran more than that did not seem to fare any better than non-runners.

The same study found that running quickly was of little benefit, with those who fared best doing a `comfortable jog` of 9-10 kmph.

Similarly, running between two and five days a week was better than pounding the pavements daily.

The second study, of 20,000 Danes, found jogging slowly adds most to life expectancy.