If a little bit of daily exercise is good for your heart, then more must be better, right? Not so fast.

New research suggests long-term, intensive exercise can slowly lead to serious heart damage.

The study, which appears in the June issue of Mayo Clinic Proceedings, finds that training and competing in extreme endurance sports, such as ironman triathlons and long-distance bicycle races, can lead to permanent scarring of the heart and large arteries.

Dr. James O'Keefe, a cardiologist at the Mid America Heart Institute of St. Luke's Hospital in Kansas City, Mo., led a team that reviewed studies of people who trained and exercised regularly as well as those who participated in marathons, triathlons, ultra-marathons or long bike races.

They found that people who exercised regularly saw significant health benefits and tended to live seven years longer than those who were physically inactive. But when it came to "extreme athletes," those regularly engaging in marathons and the like, there came a point of diminishing returns.

The studies showed that immediately following a marathon, runners showed up to a 50 per cent increase in levels of an enzyme called troponin -- the same enzyme that shoots up in patients having heart attacks when the heart muscle is in distress.

Spikes in troponin and other heart enzymes can last for days after an event. In most cases, the heart recovers within a week.

But for some athletes engaged in repetitive training, the heart and its main arteries can develop scarring, say scientists. One study showed that around 12 per cent of apparently healthy marathon runners showed evidence of patchy myocardial scarring.

All that exercise can also lead to enlarged ventricles -- which can in turn lead to irregular heartbeats, called arrhythmias, and sudden cardiac arrest.

The authors note the case of Micah True, an ultra-marathoner known as Caballo Blanco, who died suddenly on March 27 during a 19-kilometre run.

Though 19 kilometres isn't considered an extreme distance for an experienced runner, it's thought that True would run as much as 270 kilometres a week.

An autopsy of his heart showed it was enlarged and scarred, which the study's authors said may have been caused by "Phidippides cardiomyopathy," a condition caused by chronic excessive endurance exercise.