All runners have heard about the tragedies. The marathoner Alberto Salazar, at the age of forty-eight, suffered a heart attack and lay dead for fourteen minutes before a stent opened up a blocked artery and saved his life. Micah True, the ultra-marathoner at the center of the best-selling book “Born to Run,” went for a twelve-mile run in the New Mexico wilderness and was later found dead. Ryan Shay dropped dead in the middle of the 2008 United States Olympic marathon trials. There is also the story of the first marathon itself, run by Pheidippides, who, after completing the 26.2 miles, collapsed and died. These deaths are all the more shocking because the peak physical condition of the deceased would seem to protect them from heart disease. Hundreds of studies, as well as our own intuition, associate exercise with cardiac health. But, in recent years, a small group of cardiologists have advanced a hypothesis that suggests these tragedies may not be so shocking, after all: they believe that an excess of exercise actually damages the heart.

The most vocal proponent of the too-much-exercise theory is James O’Keefe, a cardiologist and the director of preventive cardiology at the Mid America Heart Institute, in Kansas City, Missouri. In a TEDx talk in 2012, as well as in a series of editorials and reviews, O’Keefe has argued that exercise beyond a certain threshold can lead to heart disease, and possibly decrease the benefits of moderate exercise. In a video accompanying a review he co-authored titled “Potential Adverse Cardiovascular Events from Excessive Exercise,” O’Keefe suggests that extreme exercise is “not conducive to great long-term cardiovascular health,” and cautions against the assumption that, if moderate exercise is good, more must be better. “Darwin was wrong about one thing,” O’Keefe says. “It’s not survival of the fittest but survival of the moderately fit.”

For those of us who believe that the “everything in moderation” rule applies to, well, everything, this argument makes sense. Exercise remains one of the best things you can do to improve your cardiovascular health, but you certainly do not need to run marathons to achieve the benefits. Moderate amounts of exercise throughout life are perfectly adequate. Athletes who exercise in extremes generally do so for reasons other than their health—competitiveness, professional requirement, compulsion. But recognizing that exercising more than a certain amount reaps no greater cardiovascular benefits is quite different than suggesting that this level of exercise causes cardiovascular harm.

O’Keefe argues that exercise beyond a certain threshold increases cardiovascular risk. Given the complexity of the heart, the argument is tough to unpack. For the heart to do its job, the coronary arteries must be open, the electrical impulses need to be coördinated and rhythmic, and the muscle itself must be able to relax and fill. Exercise affects these systems both directly and indirectly, by mitigating the many risk factors, like obesity and high blood pressure, that cause heart disease in the first place. Because exercise affects all these systems, O’Keefe can be both right and wrong at the same time.

First, the bad news for marathoners and other extreme athletes. Excessive exercise has been consistently associated with atrial fibrillation, a rhythm disturbance that increases the risk of stroke and leaves some people feeling weak and breathless. One study looked at the rates of atrial fibrillation in over fifty thousand Swedish men who had participated in the Vasaloppet, a ninety-kilometre cross-country ski event, over a ten-year period. Those who completed the most races or who had the fastest times seemed to have a higher risk of atrial fibrillation.

So how high is this risk? The magnitude varies, but some reviews suggest that the risk of atrial fibrillation for extreme athletes may be increased five fold when compared to sedentary people. While this sounds drastic, Brian Olshansky, a heart-rhythm specialist in Iowa and an avid runner (every cardiologist I spoke to who studies this issue is a current or former endurance athlete), helped put the risk in context: “Let’s say one’s lifetime risk of atrial fibrillation is 0.3 per cent,” he said. (The risk varies depending on several factors, like age and obesity.) “A five-fold increase still leaves your lifetime risk of atrial fibrillation at only 1.5 per cent.”

Risk estimates can be hard to wrap your head around, though, because no matter how well we can predict the likelihood of something happening in the future, once it happens it hardly matters what the likelihood was in the first place. This may be why John Mandrola, a heart-rhythm doctor at Baptist Medical Associates, in Louisville, Kentucky, increasingly cautions against extreme exercise. Mandrola was an élite cyclist for decades, and a few years ago he crashed his bike. Despite painful rib fractures, he quickly resumed riding; twenty miles into his first ride after the crash, Mandrola became acutely short of breath and dizzy, and he lost his muscle strength. The butterfly sensation in his throat had been described to him several times by his own patients: he was in atrial fibrillation.

Once his immediate fear of stroke passed, Mandrola’s fears became existential: Had his life changed irreparably? For years, he had been a cyclist who also happened to be a cardiologist. Now he was just a cardiologist. Some scientists postulate that inflammation may play a role in atrial fibrillation, and Mandrola began to see his earlier hard-driving life-style choices as “the inflammation of excess.” He believes that an endlessly striving, never-enough mind-set begets ill effects far beyond those we can attribute to excess physical activity. “It’s not just being on that edge in a race,” he told me. “It’s being there in training, at home, at work, and for decades. Always on the gas—yes, this is the problem.”

* * *

Like Mandrola, O’Keefe was once an über-exerciser. He won Kansas City’s sprint triathlon five years in a row. Then, in midlife, he decided to change his ways. At the beginning of his TEDx talk, reflecting on his previous exercise habits, he says, “I’m worried I may have made a lethal mistake.” In the editorial, which largely mirrors the TEDx talk, O’Keefe and his colleague Carl Lavie suggest that vigorous exercise should be limited to “thirty to fifty minutes per day.” They conclude that “running too far, too fast, and for too many years, may speed one’s progress toward the finish line of life.” The editorial received widespread and mostly alarmist media attention, including an article in the Wall Street Journal titled “One Running Shoe in the Grave.”

Is there any reason to believe that O’Keefe’s years of endurance competition will shorten his life, or worsen his cardiovascular health? At this point, no. First of all, other studies, most of which come from Paul T. Williams’s National Runners’ Health Study, suggest that risk factors for cardiovascular disease continue to improve with increasing amounts of exercise. When it comes to the specific association between extreme exercise and mortality, statistically rigorous associations are hard to come by, because in any given population relatively few people exercise in an extreme way, and even fewer still die during the study.

For example, one study cited by O’Keefe to suggest that mortality benefits are lost with increased exercise looked at mortality rates among nearly eighteen thousand runners in Copenhagen. The authors of the study, published in the American Journal of Epidemiology, confidently concluded that, when compared to non-joggers, runners lived, on average, five to six years longer. But, in their discussion of whether more intense exercise was associated with harm, the authors were far more circumspect: “We don’t have evidence to support faster or more frequent jogging, nor do our limited data rule out the possibility.”

Another study cited in O’Keefe and Lavie’s editorial found that, in over four hundred thousand people in Taiwan, forty-five minutes of vigorous exercise daily was associated with a forty-per-cent decrease in risk of death. In describing this study, O’Keefe and Lavie note that “at about forty five minutes, a point of diminishing returns is reached whereby longer exercise efforts do not appear to translate into lower death risk.” They go on to compare exercise to a drug, an analogy O’Keefe frequently invokes. “As can be expected with any potent drug, an insufficient dose will not confer the optimal benefits, while an excessive dose can cause harm, and even death in extreme overdoses.”