While untold numbers of studies have shown that exercise is good for the heart, growing evidence shows that high levels of intense exercise, such as distance swimming and marathon running, can be harmful.

It's important to get your heart rate up with regular exercise for many reasons, including helping the heart pump blood more effectively and activating the immune system, and many studies have shown that regular exercise reduces the risk of heart disease.

However, one study found that running a marathon increased cardio risk by sevenfold. Another study found that those who had run a minimum of 100 marathons showed scarring of their heart muscle.

Still another study found that athletes who participated in endurance training had decreased function in the right ventricle of the heart after a competitive event. They also had increased blood levels of heart enzymes that indicates heart injury. In addition, the study, which was published in European Heart Journal, found scar tissue on the heart muscle of 12 percent of athletes one week after the race.

Some experts believe that heart problems in athletes who participate in endurance sports are the result of underlying abnormalities.

Sports cardiologist Dr. André La Gerche is trying to sift through the many studies to discover if intense exercise is truly dangerous, noting that large population studies support the benefits of exercise, although in amounts far less than those of endurance athletes, and the studies indicating harm in intense exercise are relatively small.

La Gerche points out that all therapies, whether drugs or other types, have a dose-response relationship where benefits diminish at high doses and the risk of adverse events increases. He says this may also be the case for exercise.

"The answers regarding the healthfulness of 'extreme' exercise are not complete and there are valid questions being raised," continues La Gerche.

A new review by La Gerche published in the Canadian Journal of Cardiology explores the links between intense exercise and heart damage, and concludes that large studies which examine the effects of intense exercise on heart structure and function should be undertaken.