Ryan Shay was at the peak of fitness when he lined up at the start of the New York City Marathon in November 2007 with the hopes of qualifying for his first Olympic team. Less than six miles later, the 28 year old collapsed. Soon after, he was pronounced dead.

When Shay's autopsy revealed scarring in the healthy, young runner's heart, scientists finally woke up to the possibility that exercise might not be the panacea for health that it has long been touted to be, said Peter McCullough, a cardiologist at Baylor University Medical Center in Dallas.

Since then, a growing, still controversial body of research has begun to show that all the exercise in the world won't protect your heart if you don't also eat well and for some people, take drugs to overcome cardiovascular risk factors. Exercising too much may even raise the risk of developing clogged arteries.

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The message is not that exercise is pointless, experts caution. Instead, it's that you can't eat with abandon just because you're fit and slim. It may also be unwise to exercise with abandon, no matter what you eat.

"There was this false belief that you could just eat junk food and be more liberal with your diet and then exercise and it would somehow prevent artery blockages to the heart," McCullough said. "Lo and behold, exercise doesn’t prevent these blockages from occurring. And some analyses show extreme endurance exercise may actually somewhat promote heart blockages."

"People say, 'I run, so I can eat whatever I want,'" he added. "That's not true. That needs to go away."

It's well established that moderate exercise has major benefits for heart health. When couch potatoes start moving, even by just walking three times a week, studies show that cardiovascular deaths drop by up to 25 percent and lifespans lengthen. Regular exercisers live an average of seven years longer than sedentary people.

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Exercise strengthens the heart, making it a more powerful pump and helping it become more efficient at turning oxygen into fuel. That kind of fitness boosts the chances of surviving heart attacks, illnesses and even car accidents.

But working out does nothing to prevent the artery-clogging effects of eating a diet high in saturated fat, new studies suggest. That contradicts a long-held belief that athletics worked like a drug to protect the heart against atherosclerosis -- a build-up of plaque inside the arteries that can constrict blood flow and lead to heart attacks and strokes.

Over-exercising may even increase the risk of developing atherosclerosis. In a study just published in The Journal of the Missouri State Medical Association, McCullough and colleagues used advanced CT imaging technology to compare the hearts of 23 sedentary men with 50 male marathoners who had completed a minimum of 25 races. Imaging revealed a greater volume of plaques in the distance runners.


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.

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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.

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"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.