Distance runners, cross-country skiers, cyclists and triathletes have long been thought of as the fittest of fit. After all, you have to be in pretty good shape to run, bike, swim or ski for hours on end, not to mention the stamina it takes to ramp up intensity during competitive endurance events like a marathon, ultramarathon, century cycle, half iron man or iron man.

But these long-heralded athletes are now being warned that pushing their physical limits may be doing their body more harm than good.

"Chronic extreme exercise appears to cause excessive wear and tear on the heart, inducing adverse structural and electrical remodelling, which offsets some of the cardiovascular benefits and longevity improvements conferred by moderate physical activity," said James O'Keefe, a cardiologist at the Mid America Heart Institute of St. Luke's Hospital in Kansas City and author of a 2012 article titled "Run for your life .... at a comfortable speed and not too far." O'Keefe's paper in the British Medical Journal sparked significant debate within the exercise community and resulted in several more researchers following up with more data on the physiological effects of repeated bouts of exercise longer than 60 to 90 minutes.

What happens to the heart of endurance exercisers? It adapts to the demands of pumping greater volumes of blood to the working muscles by making itself bigger and stronger.

Known as an "athlete's heart," the thicker walls and enlarged chambers of endurance athletes' hearts have long been considered an adaptation similar to that which occurs when muscles are overloaded. But with the benefit of new imaging technology, it looks as though the stress on the heart is more serious than previously believed both in the short and long term.

Fortunately, most of the immediate damage to the heart repairs itself within about a week of the latest bout of endurance exercise and is rarely cause for concern. But for athletes who spend years training for and competing in endurance events, the repeated acute damage can take its toll, leaving scarring that can lead to irregular heart rhythms (arterial fibrillation) and premature aging of the heart.

Some studies suggest that middle-aged endurance athletes have a fivefold increase in the risk of atrial fibrillation when compared with non-athletes.

O'Keefe says this type of longterm damage isn't a guaranteed death sentence, but it may erode many of the health benefits that moderate exercise confers. Several studies have reported that runners who adopt a training regimen of less than 32 kilometres per week benefit from a three-to six-year increase in life expectancy. Any additional miles over that threshold, however, seem to result in no greater life expectancy and may indeed result in fewer good years.

Larry Creswell is a triathlete who also happens to be a cardiac surgeon and associate professor at the University of Mississippi Medical Center. He's done his share of ultra-endurance events and agrees in principle that it's possible to exercise too much. But he's less clear on where the line is between doing not enough and doing too much exercise.