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鈥檚 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鈥檚 presents evidence from several recent studies that 鈥渆xcessive endurance exercise鈥濃�攖he kind of training required for ultra marathons, Iron Man competitions, and long-distance bike races鈥攎ay do lasting damage to the heart. While the results are far from damning鈥攖he data are often mixed鈥攖hey suggest that, at a minimum, lacing up the running shoes and going gonzo does little to improve one鈥檚 health.

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

Under such a theory, it wasn鈥檛 a 12-mile run that killed Micah True, but a lifetime of training and racing that may have permanently 鈥渞emodeled鈥� his heart in dangerous way, predisposing him to an arrhythmia, or wild, irregular heartbeat.

O鈥橩eefe takes great pains to emphasize that running itself is not the culprit. 鈥淭his in no way detracts from the importance of exercise,鈥� he says. 鈥淧hysically active people are much healthier than their sedentary counterparts. So much so that they, on average, live seven years longer than someone that doesn鈥檛 exercise at all.鈥�

The problem patients are those who fail to see exercise as a game of diminishing returns. Just as drinking 10 beers doesn鈥檛 make me five times happier than drinking two, running ultra marathons doesn鈥檛 make me exponentially healthier than my friends running 5Ks. It just makes me sweatier, and insufferable at dinner parties.

O鈥橩eefe points to a 15-year observational study of 52,000 adults, which found that runners had a 19 percent lower risk of 鈥渁ll-cause mortality鈥� 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. 鈥淭hese data suggest not only that more is not better, but in fact, more may be worse,鈥� says Dr. Carl Lavie, a cardiologist at the , in New Orleans, who co-authored the study.

Physical activity is like a drug, and a powerful one at that. It鈥檚 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鈥橩eefe warns, 鈥渁s with any pharmacological agent, a safe upper-dose limit potentially exists, beyond which the adverse effects鈥� of musculoskeletal damage and cardiovascular stress outweigh its benefits.

He puts the 鈥渦pper-dose limit鈥� at an hour a day, beyond which the protections gained through exercise begin to diminish.

O鈥橩eefe isn鈥檛 out to scaremonger. The incidence of 鈥渟udden cardiac death鈥� among marathoners, he points out, is very low鈥攐ne in 100,000. Just driving to the Boston marathon is a hell of a lot more dangerous than running in it. And there鈥檚 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鈥橩eefe 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鈥檚 architecture may lead to permanent structural changes, including enlarged chambers, scarred muscle, and stiffened arteries. 鈥淚n the long run,鈥� O鈥橩eefe concludes, 鈥渋t not only reduces cardiovascular health but might even shorten longevity.鈥�

Most of the recent studies are retrospective, however, and so causation and causality are impossible to parse. 鈥淛ust because you notice an association between a group of people and an abnormality doesn鈥檛 mean that one caused the other,鈥� says , 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. 鈥淭hose types of viruses can, in certain situations, infect the heart and cause some of the scarring patterns that we see.鈥� The running and the scarring are related, but their relationship is counterintuitive.

Baggish calls O鈥橩eefe鈥檚 review 鈥渇air,鈥� but adds, 鈥渉igh-intensity, high-volume exercise is really, at this point, a black box in terms it what it does for you over the long haul.鈥�

Of course, there are plenty of reasons to run ’til it hurts and they have nothing to do with hoping to see 90. It鈥檚 just the opposite鈥攏othing quiets the dysphoria of modern living like breathing so loudly that you can鈥檛 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?

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