Running marathons could permanently damage the hearts of some endurance athletes, scientists believe.
New research suggests extreme physical activity can scar the right ventricle of the heart in susceptible individuals. Athletes taking part in high-endurance activities, such as marathons or triathlons, could be at risk without knowing it.
Scientists in Australia and Belgium studied 40 elite athletes who were planning to compete in one of four endurance events. All were highly trained with no known heart problems.
Test results showed that immediately after racing the athletes' hearts had changed shape and right-ventricle function decreased.
The right ventricle, one of the heart's four chambers, pumps blood to the lungs.
Right-ventricle function recovered in most athletes after a week, but in five there was evidence of potentially permanent scarring.
Study leader Dr Andre La Gerche, from the University of Melbourne, Australia, said: "Virtually all of the changes in the athletes' hearts had resolved one week after having taken part in a competitive event. In most athletes, a combination of sensible training and adequate recovery should cause an improvement in heart-muscle function.
"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 or it may cause arrhythmias (erratic heart beats)."