Researchers in Australia and Belgium have uncovered evidence for the first time that endurance athletes run the risk of damaging their hearts through prolonged or extreme activities such as triathlons, alpine cycling and other ultra sports.

The research, published online Wednesday in the European Heart Journal, found that damage in the form of scarring to the heart muscle could occur in the right ventricle -- one of four chambers involved in pumping blood through the body -- during extreme exercise. But the researchers also noted that the damage could be reversed with proper rest over time.

The research, conducted under the lead of Dr. Andr' La Gerche of St. Vincent's Hospital at the University of Melbourne, followed 40 athletes over time as they competed in four increasingly intense events: a marathon, triathlon, alpine cycling race, and an ultra-triathlon. All of the participants in the study were extremely fit and did more than 10 hours of hard training every week. None had any symptoms or other problems going into their events that would indicate a risk of heart damage.

Most of them showed no signs of scarring or other problems within a week of competing in their events. But five of them showed evidence of permanent damage through scarring, known as fibrosis, after undergoing MRI scans and further examination. La Gerche noted that the five had had been competing at their respective endurance sports for longer than those who showed no heart damage.

Despite the findings, La Gerche stressed that it was important to understand that competing in endurance activities or extreme exercise is not unhealthy overall.

"Our data do not support this premise," he said in a statement about the research.

In fact, he added, "a combination of sensible training and adequate recovery should cause an improvement in heart muscle function" in most athletes.