Running marathons could cause permanent heart damage, say scientists.
A study found that high-endurance activities can lead to scarring of the right ventricle, increasing the risk of health complications.
Researchers hope the discovery will prompt the development of suitable treatments and preventative measures.
In the meantime they are urging elite athletes to adopt sensible training techniques and allow adequate recovery time after events.
Lead researcher Dr Andre La Gerche, from the University of Melbourne, Australia, said: 'Our study identifies the right ventricle as being most susceptible to exercise-induced injury and suggests that the right ventricle should be a focus of attention as we try to determine the clinical significance of these results.
'Affected athletes may be at risk of reduced performance - a cardiac 'over-training' syndrome - or it may cause arrhythmia (erratic heart beats).'
Scientists assessed 40 elite athletes with no history of heart problems who were planning to compete in one of four endurance events.
Test results showed that immediately after racing the athletes' hearts had changed shape, growing in volume, while right ventricle function decreased.
After a week right ventricle function recovered in most, but in five there was evidence of potentially permanent scarring.
Dr La Gerche added: '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.'
In five of the athletes there was evidence of potentially permanent scarring
The right ventricle is one of the heart's four chambers and pumps blood to the lungs.
Scar tissue can weaken over time and can form an abnormal bulge of tissue known as an aneurysm.
In conjunction with other heart problems this mass can cause the heart to enlarge, reducing its ability to pump blood effectively, resulting in heart failure.
Professor Sanjay Sharma, of St George's University London and medical director of the London Marathon, called for more research looking at larger groups of endurance athletes.
He said of the findings, published in the European Heart Journal: '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.'
A spokesperson from the British Heart Foundation agreed that further long-term research is needed and urged athletes to consult their GP with any concerns.