Too much exercise might cause unusual heart rhythm problems called arrhythmia, finds Swedish study.
It found a higher incidence of arrhythmias in cross-country skiers with a long history of endurance training, the Discovery News reported.
Arrhythmia, which is often harmless, can sometimes lead to strokes and other serious problems.
"People who are exercising a lot should probably learn how to identify the symptoms of atrial fibrillation and seek counselling if they get it," said Johan Sundstrom, a cardiovascular epidemiologist at Uppsala University.
Sundstrom emphasized that exercise is essential for overall health and that the vast majority of people would do well to get more of it, not less.
"I would be worried if people thought cross-country skiing, for example, to be dangerous. That would be an unlucky headline," he said.
"We're looking at extreme amounts of training. Most people need to get off the couch and exercise a little bit," he added.
The study was presented at the ESC Congress 2011.