Yikes! What if endurance training were not good for your heart? That’s the suspicion of some cardiologists at Uppsala University Hospital, where researchers tracked the heart health of 52,755 cross-country skiers who competed in the 90 kilometer cross-country ski race known as the Vasaloppet between 1989-1998. Checking in on the racers in 2005, the researchers found that those who completed five or more races in ten years had a 30 percent higher risk of developing any arrhythmia than those who did one race only. They also found that skiers who had the fastest finishing times relative to the other participants had a 30 percent higher risk of developing any arrhythmia in subsequent years. Wait, there’s more, but it’s more complicated. When the researchers looked at the different types of arrhythmia, they found that the most frequent was atrial fibrillation, with a 29 percent increased risk of AF among skiers completing five or more races compared to those completing only one, and a 20 percent higher incidence of AF for those with the fastest finishing times compared to the slowest. The second most common arrhythmia was bradyarrhythmia, whose risk more than doubled among those who completed five or more races compared with those who completed one. The researchers also found a tendency for the risk to increase with faster finishing times. Read more in a news release from the European Society of Cardiology.