It’s one of the more puzzling ironies of exercise — tragic news of people dying during or after completing a marathon. In 2009, four runners died during half-marathons in San Jose, Cal. and Detroit, and last year, two runners died at the Philadelphia Marathon, one at the finish line and another about a quarter-mile from completing the race, all from apparent sudden heart attacks.
Isn’t running supposed to improve your fitness and lower your risk of succumbing to cardiac arrest?
In a new study published in the New England Journal of Medicine, researchers explain the paradox of why a seemingly high number of marathoners have heart attacks. They analyzed heart attacks among 10.9 million racers running either half-marathons or marathons between 2000 and 2010, and found that such long-distance races actually did not lead to an increased risk of heart attack. In fact, the rate was relatively low, and even lower than the risk of having a heart attack and dying while participating in college sports, a triathlon, or even jogging. Of the 10.9 million marathon racers, 59 had cardiac arrest either during or in the hour after their long-distance run and 42 died. That’s a death rate of 1 per 259,000 participants, compared to 1 death per 52,630 participants in triathlons.
When the scientists, led by Dr. Aaron Baggish, associate director of the Cardiovascular Performance Program at Massachusetts General Hospital, looked more closely at the marathoners’ health records, they found that those who had a heart attack or died of a heart attack were also more likely to have had underlying heart disease. Many had thickened heart walls, or hypertrophic cardiomyopathy, in which the heart muscle becomes enlarged and less flexible, making it harder to pump blood efficiently. And because marathon runners tend to be older, many also had atherosclerosis, or buildup of plaques within their heart vessel walls, which can also impede blood flow.
Either of these conditions, says Baggish, can worsen when patients take up vigorous exercise, and put additional strain on the heart to pump blood quickly and efficiently. That explains why in the study, most of the marathoners who had heart attacks were not first-time runners, but accomplished racers who presumably had trained well and prepared their bodies for the grueling long-distance run. “Training for something like a marathon is certainly important, but as important for cardiac arrest risk is having undiagnosed or underlying heart problems,” says Baggish.
Previous studies have found that marathons can lead to a increase in the enzyme troponin, which rises when the heart is in distress and can’t get enough blood. In one study, troponin levels of runners immediately after their race were alarmingly high, but the boost was temporary, and the enzyme went back to normal levels three months later.
Whether or not those participants had some underlying heart weaknesses is not clear, but Baggish notes that these heart conditions are relatively rare, and that for most people, the benefits of exercise, including marathons, outweigh the risks. Determining whether you have a potentially hazardous heart condition should become an important part of preparing for long-distance running, he says, and anyone considering running in a marathon should start with a visit to his doctor. “Your individual risks depends on how many risk factors for heart disease you may have, including hypertension, high cholesterol, whether you smoke, and if you have a family history of heart disease,” he says. “A person who wants to start running should have a conversation with his doctor to understand the physiology of what running does to the body, how at-risk they are, and what tests can be done to keep the risk minimized.”
There was other good news from the study as well. Of the people who did have heart attacks, the fatality rate was 71%, which is lower than the 92% average death rate for heart attacks that occur outside of the hospital setting. More runners might have survived their attacks because bystanders or medical personnel were on hand to perform CPR to save their lives, says Baggish.
Baggish says he is also planning on parsing the data from the study even further, to better understand why some runners with risk factors had heart attacks, while others with similar profiles didn’t, as well as why some of those who had cardiac arrest survived, while others did not.
In the meantime, the results shouldn’t be seen as a deterrent to running, or exercising, for that matter. They simply clarify that it’s not the exercise per se that may be putting marathoners at risk of dying from heart attacks. If anything, the findings highlight how much undetected heart disease lurks among us, and how important it is to improve our fitness so these weaknesses don’t emerge when we put our bodies to the test.
Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny . You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.