Does endurance training boost your chance of heart arrhythmias?

I’m starting to really dislike these jerks who keep studying Swedish cross-country skiers and producing findings that conflict with my worldview… First it was arthritis; now, the researchers studied 47,000 people who participated in the 90-km Vasaloppet ski race in Sweden between 1989 and 1998, looking for associations between the number of times they participated in the race and the odds that they were subsequently diagnosed with arrhythmias (a task made possible by Sweden’s comprehensive national health records). The result (according to a press release describing a conference presentation; the findings haven’t yet appeared in a peer-reviewed journal):

Compared to those who had completed one single race, those who had completed 7 or more races had 29% higher risk of a subsequent arrhythmia. Further, elite athletes finishing at 100-160% of the winning time had 37% higher risk of arrhythmias than recreational athletes finishing at more than 241% of the winning time.

Leaving aside the quibble that “elite” is a bit generous for someone finishing at 160% of the winning time, the findings seem to suggest pretty clearly that extensive endurance training increases the chances of arrhythmia. The biggest differences were found in subjects under 45, and were exclusively associated with atrial fibrillation and bradyarrhythmias, which are considered less serious than the “potentially lethal” ventricular arrhythmias, according to the researchers:

Dr. Andersen summarizes: “Basically, this study shows, that even though physical activity is generally healthy, athletes committed to endurance sports at elite level have higher risk of suffering from a heart rhythm disorder… We emphasize that we do not find any increased incidence of potential lethal heart rhythm disorders. However, this study only compares athletes at different levels and a future large scale study comparing athletes against the normal population would be very interesting.”

The last point is interesting. It does seem increasingly clear that training as an elite endurance athlete is more likely to have an impact on the heart than training at a recreational level — but what about compared to sedentary life? Is this a linear relationship, or a “U-curve” where moderate training produces the best results?

Bottom line: although the press release skips some relevant details (like how common were these arrhythmias in absolute terms?!), I don’t think this changes my risk-benefit assessment. It’s like the well-known trade-off for exercise of any sort: your chance of a heart attack rises temporarily during extreme exertion, but your overall odds of heart attack decline with exercise. In this case, it’s worth bearing in mind the findings from previous studies of the same race: the more Vasaloppets you do, the longer you live. So whatever the downsides of arrhythmias, they’re evidently outweighed by other benefits.