The results fall in line with other studies showing that moderate running is good but excessive running may be bad. One long-term study of 54,000 Americans, published last year, found the lowest rates of death among people who ran 5 to 20 miles a week. People who ran more than 25 to 30 weekly miles, on the other hand, lived no longer than people who were inactive.
It's not yet clear why ultra-athletes might be at greater risk for artery blockages, but one theory is that the extra twisting and pumping motions induced by repetitive exertion might put too much wear and tear on the arteries, leading to a type of scarring called fibrosis.
It's also possible that over-exercised hearts suffer from chronic stress and the need to constantly repair damage induced by excessive free-radical production.
About half a million people finished marathons in the United States last year and someone dies in virtually every major long-distance race, McCullough said. The new findings are a wake-up call for people who think they're doing their bodies a favor by pushing themselves to their limits.
"I was personally a big-time marathoner. I ran a marathon in every state in the U.S.," McCullough said. "I retired based on our research findings. I said, 'This is not doing me any good and could be doing me harm.'"
For people who maintain a more moderate level of exercise up to two or three hours a week, it's also important to remember that exercise is not the only factor that influences heart health, said Norman Lepor, a cardiologist at the Cedars-Sinai Heart Institute and the University of California, Los Angeles' David Geffen School of Medicine.
Smoking, eating a diet high in salt and trans fats, and having a genetic predisposition to heart disease are just as important.
"Focusing on athletics only affects what's really a modest part of the equation for the development of cardiovascular disease," Lepor said. "Someone can be in shape and still at risk of having a heart attack."
"I don't want in any shape or form to slam exercise," Lepor said. But, he added, "it can really be too much of a good thing for some people."
Emotions ran high at the American Heart Association meeting last year when researchers held a seminar to discuss the new findings, McCullough said. Many doctors feared that the public would get the wrong message about exercise, which remains essential for combatting rising rates of obesity and related health problems.