Running is supposed to delay death, not hasten it. Though stories of athletes dying mid-stride are as old as Phiddipides, the courier who (as myth has it) ran from Marathon to Athens in 490 BC bearing news of a Greek victory over the Persians, only to expire once he reached the agora, running deaths are exceedingly rare. The tragic exceptions make headlines, most recently the disappearance of ultrarunner Micah True in New Mexico’s Gila Wilderness. His body was discovered in March in a canyon, legs resting serenely in a stream. An autopsy later revealed an enlarged heart, but offered few clues as to why it had stopped beating.

True was 58 and a veteran ultrarunner, a man who, in his prime, logged 170 miles a week on the trail. What fells such an elite athlete?

A review in this month’s Mayo Clinic Proceedings presents evidence from several recent studies that “excessive endurance exercise”—the kind of training required for ultra marathons, Iron Man competitions, and long-distance bike races—may do lasting damage to the heart. While the results are far from damning—the data are often mixed—they suggest that, at a minimum, lacing up the running shoes and going gonzo does little to improve one’s health.

Chronic over-exercisers, writes Dr. James O’Keefe, lead author of the MCP review and a cardiologist at St. Luke’s Hospital, in Kansas City, Missouri, may develop scarring and calcification inside their ventricles and arteries. The medical term for this is “structural and electrical remodeling,” and it’s just what it sounds like: the slow hardening and thickening of the heart’s plumbing, the fraying of biological circuitry, due to years of strain.

Under such a theory, it wasn’t a 12-mile run that killed Micah True, but a lifetime of training and racing that may have permanently “remodeled” his heart in dangerous way, predisposing him to an arrhythmia, or wild, irregular heartbeat.

O’Keefe takes great pains to emphasize that running itself is not the culprit. “This in no way detracts from the importance of exercise,” he says. “Physically active people are much healthier than their sedentary counterparts. So much so that they, on average, live seven years longer than someone that doesn’t exercise at all.”

The problem patients are those who fail to see exercise as a game of diminishing returns. Just as drinking 10 beers doesn’t make me five times happier than drinking two, running ultra marathons doesn’t make me exponentially healthier than my friends running 5Ks. It just makes me sweatier, and insufferable at dinner parties.

O’Keefe points to a 15-year observational study of 52,000 adults, which found that runners had a 19 percent lower risk of “all-cause mortality” than non-runners. Good news, makes sense. Among the runners, though, those who logged big miles and high intensity workouts faired no better than those who ran less than 20 miles a week at sane paces. “These data suggest not only that more is not better, but in fact, more may be worse,” says Dr. Carl Lavie, a cardiologist at the Ochsner Heart and Vascular Institute, in New Orleans, who co-authored the study.

Physical activity is like a drug, and a powerful one at that. It’s known to combat our worst diseases, including hypertension, depression, diabetes, and heart failure. Doctors prescribe it as such, sometimes going so far as to write it out on a script: 60 minutes vigorous exercise, three to five times per week. But, O’Keefe warns, “as with any pharmacological agent, a safe upper-dose limit potentially exists, beyond which the adverse effects” of musculoskeletal damage and cardiovascular stress outweigh its benefits.

He puts the “upper-dose limit” at an hour a day, beyond which the protections gained through exercise begin to diminish.

O’Keefe isn’t out to scaremonger. The incidence of “sudden cardiac death” among marathoners, he points out, is very low—one in 100,000. Just driving to the Boston marathon is a hell of a lot more dangerous than running in it. And there’s no evidence that recreational marathoners, even the sinewy master blasters you see zipping through the park on Saturday mornings, are at risk of heart damage.

Instead, the Proceedings review concerns itself with career super-athletes, men and women for whom endurance sports are less recreation than therapy. During intense workouts, O’Keefe explains, the heart pumps five times more blood than when at rest. The atria and right ventricle expand; the vessels swell. Over time, this continual stretching and contracting of the heart’s architecture may lead to permanent structural changes, including enlarged chambers, scarred muscle, and stiffened arteries. “In the long run,” O’Keefe concludes, “it not only reduces cardiovascular health but might even shorten longevity.”

Most of the recent studies are retrospective, however, and so causation and causality are impossible to parse. “Just because you notice an association between a group of people and an abnormality doesn’t mean that one caused the other,” says Dr. Aaron L. Baggish, a cardiologist at Mass General with 30-plus marathons under his laces. He points out that marathoners often contract illnesses in the two to four weeks after a big race, owing to a compromised immune system. “Those types of viruses can, in certain situations, infect the heart and cause some of the scarring patterns that we see.” The running and the scarring are related, but their relationship is counterintuitive.

Baggish calls O’Keefe’s review “fair,” but adds, “high-intensity, high-volume exercise is really, at this point, a black box in terms it what it does for you over the long haul.”

Of course, there are plenty of reasons to run 'til it hurts and they have nothing to do with hoping to see 90. It’s just the opposite—nothing quiets the dysphoria of modern living like breathing so loudly that you can’t hear yourself think.

Micah True lived like an outlaw, ran like an animal, and died cooling his heels in a shadowy canyon of a landscape that he loved. What other race is there to win?