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Exercise is important for health and wellbeing. But past studies suggest high-intensity exercise may be a risk factor for an irregular heart rhythm called atrial fibrillation that sometimes leads to stroke. So, are athletes who develop AFib at higher risk for stroke?

Researchers in Sweden looked at men and women who completed one or more races in the 30- to 90-kilometer cross-country skiing event called Vasaloppet from 1989 through 2011, comparing them to a group of non-skiers. The investigators hypothesized high-intensity exercise would increase the risk of AFib, thus leading to more strokes in the ski group. But they discovered the opposite may be true.

"Although the group who had the best performance in a ski race had a higher risk of atrial fibrillation, they didn't have a higher risk of having a stroke," said Dr. Kasper Andersen, senior author of the study published Monday in the journal .

Overall, male skiers had a lower risk of stroke after an AFib diagnosis compared to non-skiers with AFib, and their risk of dying from stroke was lower as well. Among women, the news was even better for skiers: They were less likely to develop AFib and less likely to have a stroke compared with women who didn't ski.

"Women (skiers) really didn't have an increase in atrial fibrillation, and that hasn't been seen before," said Dr. Kevin Monahan, director of the Electrophysiology and Arrhythmia Center at Boston Medical Center, who was not involved in the new study.

If AFib is a risk factor for stroke, why don't high-intensity athletes who develop it have a higher incidence of stroke?

According to Andersen, AFib is only one of the many risk factors for stroke, and exercise is one of the best ways to reduce the risk of those other factors, such as high blood pressure and diabetes.

According to , 2.7 million to 6.1 million people in the United States have AFib. Risk factors for AFib include older age, high blood pressure, diabetes and underlying heart disease. The most common symptom is a fluttering or quivering heartbeat.

The risk of developing AFib because of high-intensity exercise is low, Monahan said, noting the findings should not cause alarm for the average person.

"At the very extremes, for people who are training hours and hours per day, maybe there's a small risk of atrial fibrillation," he said. "It's a very small segment of the population that would be affected."

Andersen advised people with AFib to follow their doctor's recommended treatment to lower stroke risk because "well-trained skiers who had developed atrial fibrillation still had a higher risk of stroke compared to both skiers and non-skiers without atrial fibrillation."

Both Andersen and Monahan also encourage people in general to exercise more.

"In the overall context, exercise is very good for you," Monahan said. "I think people should go out there and achieve the best level of fitness that they can."

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