The idea that long-term endurance exercise increases the risk of arrhythmia should no longer be considered counterintuitive. The list of published studies confirming this association is long, and this week, it got a little longer.

In a study published in the European Heart Journal, researchers from Sweden report a cohort study of more than 52 000 cross-country skiers followed for decades. These were no ordinary weekend athletes; the analyzed group included finishers of the Vasaloppet, a grueling 90-km (55-mile) cross-country ski race. Reliable sources tell me that cross-country skiing over that distance is the Nordic equivalent of an Ironman or double marathon. Yikes.

The null hypothesis of the study held that both the number of races completed (exercise dosage) and finishing time (exercise intensity) associate with arrhythmia. (I would have bet my new mountain bike on that one.)

The results

The average age of athletes at study entry was 38, while the average age of first arrhythmia was 57. Of the 52 000 athletes studied, there were 919 inpatient visits for any arrhythmia during a mean follow-up of 9.7 years.

The most common diagnosis was atrial fibrillation (n=681), followed by bradyarrhythmia (n=119), including 34 athletes with complete AV block. Typical supraventricular tachycardia (SVT) occurred in 105 athletes, and premature ventricular contractions (PVCs)/ventricular tachycardia (VT) in 90. Only patients with symptoms were counted.

Athletes who completed the highest number of races had the highest risk of arrhythmia. Arrhythmia risk increased on a continuum by races completed, up to 30% higher for five-time finishers. Exercise intensity mattered too: Those who had the fastest finishing times had the higher risk of arrhythmia.

Three features of this report stand out

The study group included mostly high-level endurance athletes. The aerobic capacity required to finish such an event selects a narrow group. For instance, nearly 80% of Vasaloppet finishers participate in intense training all year round. These are not moderate exercisers dabbling in weekend 5Ks or spin classes.

The second finding was the strong correlation with dosage of exercise. The more races completed, the higher the risk of arrhythmia. The increase in risk was linear, with a 10% increase per race completed. And do not be fooled by the seemingly low overall incidence of arrhythmia (1.97%). That’s more than double the rate one would expect in an age-matched group.

The third, and perhaps most striking, finding was the association with finishing time. The fastest finishers had the highest risk of arrhythmia. The "strollers," or those who finished in more than double the fastest finishing time, had the lowest risk of arrhythmia.

Summary and parting shots

It’s pretty simple: extreme endurance exercise, done over the long term and with great intensity, increases the risk of arrhythmia. There’s no refuting this strong association. These observations are both plausible and consistent with prior studies.

There should be no surprise when an endurance athlete shows up with atrial fibrillation or some other arrhythmia. We are not surprised when masters-aged athletes suffer from other inflammation-induced maladies, like overuse injuries, heart attacks, infections, and even divorce; why are we surprised they get AF?

But context is important. Previous studies have shown Vasaloppet finishers enjoy lower overall mortality. They smoke less, carry less body fat, and report better eating habits. This bolsters the idea that the lifestyle of endurance racing confers good overall health to most participants. Exercise is good. That observation remains unchanged and unchallenged. In the US, we would do better with an epidemic of over- rather than underexercise.

It’s also important to emphasize that association is not causation. We don’t know whether excessive exercise alone caused the arrhythmia episodes. There are too many possible confounding variables to make a causation link.

And . . . just because intense and long-term endurance exercise increases the risk of arrhythmia does not mean athletes should avoid a sport they love. These studies don’t tell us to recommend against endurance exercise. They simply inform both doctor and athlete of possible consequences. There are always trade-offs.

As physicians and teachers, knowledge of the association between chronic inflammation and disease might help us give better advice to our athletic patients. My guess, and it is just a guess, I am no coach, is that the same things that help an athlete avoid AF might also make them faster. Do you think getting adequate rest and recovery improves VO₂ max? Do you think being content with something less extreme than an Ironman or cross-country ski marathon might be antiarrhythmic? What’s wrong with a fast 10K?

On a personal note, I admit to being drawn to these findings. It’s normal to like science that validates one’s beliefs: in this case, the dose-response relationship of exercise with arrhythmia only strengthens my theory that excess inflammation is the connector.

Moderator note 2024-09-25: basenote date revised as a side effect of getting this "UnKnown" TYP-vintage thread cross-assigned to a suitable Inner Circle forum.