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When word circulates that a runner has died of a heart attack, as the inexhaustible ultramarathoner Micah True did last month during a solo wilderness trail run, many people begin to wonder about the healthiness of prolonged strenuous activity. Could marathon training and racing perhaps have damaged the heart muscle of the 58-year-old Mr. True, a lead character in the book “Born to Run”? And, conversely, shouldn’t marathon training have made him — and, by extension, all runners — immune to heart disease?

Those questions, familiar to any scientist or physician who works with endurance athletes, inspired several recently published studies of the relative risks of marathon running. The science suggests that, over all, distance running and racing are extremely unlikely to kill you — except when, in rare instances, they do.

Phys Ed Gretchen Reynolds on the science of fitness.

The newest of the studies, published this month in The American Journal of Sports Medicine, gathered publicly available data on participation in and deaths during or immediately after every known marathon race in the United States from 2000 to 2009.

The totals were, of necessity, approximate. “Marathon-related deaths are not reportable,” says Dr. Julius Cuong Pham, an associate professor of emergency medicine at Johns Hopkins University School of Medicine and lead author of the study. Physicians aren’t required by law to report information about deaths during marathons to the local health authorities.

So the Johns Hopkins researchers turned to news reports, which are actually a very reliable guide to such fatalities. “It’s sensational news when someone dies during a marathon,” Dr. Pham says. “It makes headlines,” and the coverage skews public opinions about the safety of the event. “Tens of thousands of people finish a marathon, but people hear mostly about the one who dies,” he says.

“We did not set out to in any way minimize the tragedy of a single death,” he says. “But we did want to determine what the record really shows.”

What the researchers found was that, even as participation in marathon racing almost doubled during the past decade, to more than 473,000 finishers in 2009 from about 299,000 in 2000, the death rate remained unchanged, and vanishingly small. A total of 28 people died during or in the 24 hours immediately after a marathon, most of them men, and primarily from heart problems. (A few of the deaths were due to hyponatremia, or low blood sodium, in those who drank excessive amounts of fluid.) Those numbers translate into less than one death per 100,000 racers.

“Our data shows, quite strongly, that marathon running is safe for the vast majority of runners,” Dr. Pham says, “and I suspect that, for many of the runners,” the activity saved them from suffering a heart attack that might otherwise have been brought on by a sedentary, unhealthy lifestyle.

A similar epidemiological study, published in January in The New England Journal of Medicine, reached the same conclusion as Dr. Pham’s report, even as its authors looked more widely at data involving fatal and nonfatal cardiac arrests in half and full marathons over the past decade. The researchers found 59 cases of cardiac arrest during a half or full marathon, 51 of them in men, and 42 of them fatal. The average age of the affected racers was 42, and an overwhelming majority of them were approaching the finish line — within the last six miles for the marathon and the final three for the half — when they fell.

“The findings reinforce what we really already knew,” says Dr. Paul Thompson, the chief of cardiology at Hartford Hospital in Connecticut, an author of the study and a longtime marathon runner, “which is that you are at slightly higher risk of suffering a heart attack during a marathon” than if you were merely sitting or walking sedately during those same hours. “But over all, running decreases the risk of heart disease” and therefore the likelihood of your suffering cardiac arrest at all.

But, Dr. Thompson continues, running does not absolutely inoculate anyone against heart disease. “Genetics, viruses, bad habits from the past, bad diet or plain bad luck” can contribute to the development of plaques within the arteries or of heart damage like cardiomyopathy, an unnatural enlargement of the heart muscle, which running simply cannot prevent.

Micah True was found during autopsy to have suffered from cardiomyopathy, the origins of which are unknown, according to a medical examiner in New Mexico, where Mr. True died.

Whether his years of strenuous ultramarathon training and running in any way contributed to the damage to his heart is impossible to know at this point, says Dr. Thompson, who has not seen the autopsy report and never examined Mr. True. Several provocative studies in recent years have found some signs of scarring or unusual plaque development in the hearts of older male longtime marathon runners and former Olympians, he says. But the studies were small, and deaths during running, as his and Dr. Pham’s studies underscore, are rare.

If you have any symptoms of heart problems, like chest pain, dizziness or unusual fatigue, you should, obviously, see a doctor, no matter how fit you believe yourself to be, Dr. Pham says.

Dr. Thompson agrees. “My opinion is that even decades of hard running aren’t likely to damage” the heart in most people, he says. “On the other hand, I wouldn’t tell people to run dozens of marathons for good health, either. You can get healthy from far less activity.”

But there is a pull, an imperative to running. Forcibly retired from the activity by a severe hip injury, Dr. Thompson says: “I ran marathons because I loved them, not because I expected them to help me live forever. I don’t know if it’s the healthiest way to spend years of your life. But it was enjoyable. I will miss running very, very much.”

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Gretchen Reynolds is the author of “The First 20 Minutes: Surprising Science Reveals How We Can Exercise Better, Train Smarter, Live Longer” (Hudson Street Press, 2012).