Endurance athletes like marathon runners may suffer damage to their hearts but it doesn't mean that exercise is unhealthy, a researcher says.

In a small study published online in Wednesday's issue of the European Heart Journal, Australian researchers said they found that some athletes who participate in events such as marathons, endurance triathlons and alpine cycling showed reversible damage to the right ventricle of the heart — one of the organ’s four chambers involved in pumping blood.

Of the 40 elite athletes studied, five showed evidence of more permanent damage. The scarring or fibrosis of the heart muscle showed up on MRIs.

"It is most important that our findings are not over-extrapolated to infer that endurance exercise is unhealthy," said Dr. André La Gerche, a postdoctoral researcher at St. Vincent's Hospital in Melbourne.

"Virtually all of the changes in the athletes' hearts had resolved one week after having taken part in a competitive event."

The unresolved question is whether extreme exercise may cause injury to the heart that does not heal completely in some athletes, he added in a release.

In the study, all of the athletes trained for more than 10 hours a week and completed in the top 25 per cent of the field in a recent event, with no known heart problems.

The researchers studied the athletes using echocardiography, MRI, and blood tests two the three weeks before the race, within one hour of the event and about a week afterwards.

None of the athletes showed any changes in their left ventricle.

The British Heart Foundation called the findings interesting but noted it is too early to draw any firm conclusions.

Explosion in popularity

"It is important to remember that the health benefits of physical activity are well established," Doireann Maddock, a senior cardiac nurse at the foundation said on the group's website.

"Any endurance athletes who are concerned should discuss the matter with their GP."

While the study was small, the findings offer "food for thought," a journal editorial accompanying the study said.

"The data should be embraced to galvanize more detailed and longitudinal assessment of large groups of endurance athletes," Dr. Sanjay Sharma and Dr. Abbas Zaidi of St. George's University of London wrote.

"The potential for such projects is enormous considering the colossal increase in participation rates in endurance events such as the marathon."

The study was funded by the National Health and Medical Research Council in Australia and a Cardiovascular Lipid Grant from Pfizer Australia.