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Last year American cardiologist James O’Keefe made headlines (including this one in �����ԹϺ���) when he argued that exercising for more than seven hours or running more than 20 miles per week can damage the heart, causing potentially deadly arrhythmias—irregular heartbeats—or sudden cardiac arrest.

Many experts considered his claim a publicity stunt, based more on conjecture than hard science. So do the results of a —the first large-scale study to link endurance exercise with a higher risk of arrhythmias—mean O’Keefe is right?

Swedish researchers evaluated 52,755 cross-country skiers over a 10-year period. The skiers had all participated at least once in the Vasaloppet, a 56-mile race held annually in central Sweden, and none of them had any known cardiovascular disease prior to racing.

The researchers found that the fastest finishers and the skiers who had completed the most races had a higher risk of two types of arrhythmias: bradycardia, a low resting heartbeat, and , a rapid heartbeat that hinders blood flow to the body.

But this increased risk, the study’s lead author says, shouldn’t be cause for alarm. “My advice to endurance athletes is to carry on as usual,” Kasper Andersen, M.D., says. The benefits of endurance exercise—including lower incidence of physical and mental illness, and lower general mortality—outweigh the risks, he says.

Andersen isn't alone in his opinion. , a cardiologist and researcher specializing in the says “There’s a lot of mounting evidence that extreme exercise is associated with some problems, but these things are rare.”

Indeed, of the skiers Andersen studied, just 919, or 1.7 percent, experienced a disturbance in heart rhythm. And while going faster or racing frequently did increase the risk of arrhythmias, those factors did not increase the risk of sudden cardiac death.

Researchers don’t currently know whether training-induced abnormalities, such as arrhythmias, lead to increased mortality as O’Keefe suggests. Without clear evidence to support that connection, they cannot prescribe a maximum amount of exercise. “We’re miles from saying that doing more than 20 miles is ridiculous,” La Gerche says. Right now, “there’s not strong enough evidence for people to change the way they do things.”

His advice: Be sensible. “Consider your heart like every other part of your body,” he says. “You wouldn’t run a marathon every day, that’s just asking for problems.”