Marathon runners risk "scarring their hearts" after new research suggests that extreme physical activity could have damaging effects on the right ventricle.
The study is likely to worry the millions of amateur athletes who casually take part in marathons, cycling and other high-cardio events.
Scientists in Australia and Belgium studied 40 athletes who were competing in one of four endurance events. The test results showed that, immediately after exerting themselves, the athletes' hearts had changed shape, growing in volume, while right ventricle function decreased.
Study leader Dr Andre La Gerche, of the University of Melbourne, said: "Virtually all of the changes in the athletes' hearts had resolved one week after having taken part in a competitive event.
"The question from our research is whether there are some athletes in whom extreme exercise may cause injury from which the heart does not recover completely. If this occurs, affected athletes may be at risk of reduced performance - a cardiac 'over-training' syndrome - or it may cause arrhythmias (erratic heart beats). If this occurs, it is likely to affect only a minority of athletes, particularly those in whom more intense training fails to result in further improvements in their performance."
The findings are published online in the European Heart Journal.
Professor Sanjay Sharma, of St George's, University of London, who is also medical director of the London Marathon, called for more research on larger groups of endurance athletes.
He said: "My personal feeling is that extreme endurance exercise probably does cause damage to the heart in some athletes. I don't believe that the human body is designed to exercise at full stretch for as long as 11 hours a day, so damage to the heart is not implausible. It is too early to say that taking part in endurance sports causes long-term damage to the right ventricle, but this study is an indication that it might cause a problem in some endurance athletes with a predisposition and, therefore, it should be studied further."
Doireann Maddock, senior cardiac nurse at the British Heart Foundation, said: "The findings from this very small study are interesting. However, it is much too early to draw any firm conclusions from it.
"It is important to remember that the health benefits of physical activity are well established. The highly trained athletes involved in this study were competing in long distance events and trained for more than 10 hours a week.
"Further long-term research will be necessary in order to determine if extreme endurance exercise can cause damage to the right ventricle of the heart in some athletes. Any endurance athletes who are concerned should discuss the matter with their GP."