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Does spending years running marathons or cycling for long distances potentially strain someone’s heart?

Two major new studies of athletes and their coronary arteries suggest that the answer may be a qualified yes. Both studies find that endurance athletes, especially men, who spend years training and competing show a surprisingly high incidence of plaques in their arteries, which can be a hallmark of cardiovascular disease.

But the studies also find that these plaques seem to differ somewhat in their makeup from the kinds of plaques found in less active people’s hearts and so may not be a cause for much concern.

Probably at least since Pheidippides ran the purported first marathon thousands of years ago in Greece and then promptly collapsed and died, people have wondered whether strenuous exercise is dangerous for the heart.

There have been indications, both anecdotal and scientific, that it might be. One study from 2011 of long-time, elite, male endurance athletes found that a disproportionate number had scarring within their heart muscles.

Other studies since have indicated that marathon runners, particularly men, seem to have a greater risk of developing plaques inside their coronary arteries than people who exercise less or not at all. Such plaques are worrisome, since if they break free from the artery walls, they can block blood flow, causing a heart attack.

But most of these past studies have been small, often involving fewer than a dozen participants.

So for the new studies, which were published simultaneously last week in Circulation, scientists set out to examine far more hearts than in earlier experiments. For one of the studies, researchers at Radboud University in the Netherlands and elsewhere recruited 284 men who had exercised throughout their adult lives. For the other, cardiologists from St. George’s University in London and other institutions gathered almost 300 men and women, about half of whom were masters runners and cyclists with long histories of training and competing, while the other volunteers were mostly sedentary. None had any history of heart disease.

The volunteers in both studies completed extensive questionnaires about their lifelong exercise histories, detailing the time, if any, that they had spent training for and competing in endurance sports since adolescence.

The researchers in each of the studies then scanned their volunteers’ hearts, using a variety of techniques. While most earlier studies of athletes’ hearts had relied primarily on basic CT scans of the heart and blood vessels that reveal how much plaque exists in someone’s arteries, the new studies also deployed additional techniques that pinpoint the composition of those plaques.

And the makeup of plaque tissue matters. Cardiologists know that if plaques are dense and heavily calcified, they tend also to be stable and unlikely to break free from artery walls. If, on the other hand, the plaques are fatty and somewhat loose, they can more easily rupture from the wall and initiate a heart attack. 

In both studies, a long history of heavy exercise was linked to having arterial plaques. In the Dutch study, the men who had exercised — mostly by running — for more than about four hours per week throughout their adult lives were far more likely to have plaques in their arteries than the men who had run for less than about an hour per week during that time. The correlation was strongest among the men who had run the most intensely, according to their training and race times.

Similarly, in the British study, while a majority of the participants had clear arteries, those masters athletes whose scans did show plaques tended to have far more of them than the sedentary volunteers did.

But in both studies, the more active someone was, the more likely that his (and in rare instances, her) plaques were calcified and dense. Less-active people had fattier, more-problematic plaques.

Together, these studies suggest that “there may be an association between high volumes of exercise and coronary calcification,” says Dr. Benjamin Levine, a professor of cardiology at the University of Texas Southwestern Medical Center and director of the Institute for Exercise and Environmental Medicine at Texas Health Presbyterian in Dallas. He was a co-author of an editorial in Circulation last week that accompanied the studies.

“But if you dig into the morphology of the plaques,” he continues, “they appear to be more benign” than in people who exercise less.

Of course, these studies cannot tell us whether people’s exercise habits directly cause plaques of any kind to develop in their hearts, only that the two are related. They also cannot explain why exercise might contribute to plaques, or whether, over time, the athletes with plaques are at any greater risk than other people of experiencing a heart attack.

Dr. Levine and his colleagues have just begun a long-term study, he says, that will follow masters athletes for years, tracking changes within their arteries and medical outcomes.

But for now, he says, the available data, including these new studies, suggest that prolonged, intense endurance exercise may alter your arteries, but does not seem likely to harm them.

If, however, you are concerned about your cardiac health, obviously consult a doctor, he says, and do not hesitate to err on the side of caution. “If you want to run a marathon, fine, run a marathon,” he says. “But if your goal from exercise is simply to be healthy, a half-hour of jogging will do.”

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