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Professional athletic trainer and sports journalist Ian MacMahan asked us that question as part of an article he published in The Atlantic Monthly magazine.

McMahan reported on recent research that raises concerns about ultra-endurance sports carried to an extreme level for many years.  Such an exercise regimen increases one’s chance of having arrhythmia, such as atrial fibrillation, and may also cause scarring in the ventricle of the heart.

Some cardiologists have sounded alarm bells, comparing such exercise regimens to a “toxin.”  In my opinion, it is not yet clear if long-term, high-intensity exercise alone is the cause of the problem, or if those affected have other factors that come into play.  Nevertheless, the research should not be dismissed.   Here are some highlights:

• In repeated studies, about 5-8% of veteran competitive endurance athletes develop atrial fibrillation.  The sports best studied have included cross-country skiing, orienteering, and cycling, but there is a correlation between exercise and atrial fibrillation in many different sports.
• At the end of a marathon or high-intensity endurance event, one can detect in the blood stream of the athlete abnormally high levels of a heart protein called “troponin.”  The more intense the event, the more common the elevation in troponin.  In some very high intensity events, virtually all the athletes demonstrate this abnormality.
• In addition to high levels of troponin in the blood stream, ultrasound images of the heart taken immediately after the race can sometime show that the right ventricle of the heart is weak.  In some cases, the weakness can persist for several days after the event.
• One study performed in Europe used MRI technology to take pictures of the hearts of ultra-endurance athletes.  A very high rate of unsuspected scar tissue was discovered, and the frequency and degree of scarring seemed to correlate with the intensity of effort in the prior 10 years.

No one has identified a limit over which exercise is “too much,” And while there is reason for concern, there is also data showing that veteran athletes from events like the Tour de France have excellent longevity and health,  so we shouldn’t be too quick to discourage people from their favorite mode of exercise.

What should we do about this possible problem among ultra-endurance athletes?  There is no consensus among doctors on how to approach this situation.  In my opinion, athletes who fit in that category would do well do get a periodic assessment of their heart rhythm to see if there are tell-tale signs of bigger problems to come.

We should also not lose sight of the fact that coronary artery disease is a far more common cause of sudden cardiac arrest in adult athletes, and this condition can be easily detected with non-invasive means.

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Dr. Accad