"Researchers found elevated levels of troponin, a protein used to help diagnose heart attacks, in some runners after races. (Walter Bieri / EPA)"

" In the marathon's early days, runners who churned through its 26.2-mile course were thought to be putting their lives on the line. "People used to watch these races to see if anything bad would happen," said Dr. Paul Thompson, director of cardiology at Hartford Hospital in Connecticut. "

" As runners start the 111th Boston marathon this morning, these suspicions have gotten a second wind."

"In 2006, a year with more fatalities than usual in U.S. marathons, two men died during the Los Angeles Marathon, and at least four more died in other big-city marathons. "

"These deaths have been coupled with recent studies of runners that have revealed signs of possible heart damage immediately following a marathon."

" In a study published in November in the journal Circulation, researchers reported elevated levels of a protein called troponin in a group of runners 20 minutes after finishing the Boston Marathon. When heart muscle cells die or are damaged, their membranes leak, and troponin is one of the things that spills out. Emergency-room doctors check troponin levels in the blood as one way of diagnosing heart attacks. "

" Forty percent of the runners had levels high enough to suggest heart damage. Runners who had trained 35 miles or fewer each week produced more troponin than those who ran more than 45 miles a week."

" Stanford University School of Medicine researchers also found high troponin levels in a quarter of the 48 runners they tested after the Silicon Valley Marathon in October. "If I saw them in the emergency department, I would tell them they had a heart attack," says Dr. Suzanne Miller, a resident in emergency medicine. "

"But none of the runners had chest pains or other medical problems. Training didn't seem to affect how much troponin they released. The next day, their troponin levels were back to normal. "

""I don't want anyone to be afraid of running a marathon because of my study," says Miller, who is submitting a manuscript for publication. "

""But we do need people to do more studies to determine what this means, and we certainly need people to be in good shape when they're [running a marathon].""

"These changes in the heart seem transient, researchers say. "I don't think you should worry excessively about damaging your heart, because there's not any evidence that you are," says Thompson, who finished 16th in the Boston Marathon in 1976. "

"That doesn't mean marathon running is risk-free, he says. For most, the boost to heart health that comes from running outweighs the chance of having a heart attack while exercising. But for people with known heart problems, and those who unknowingly harbor ailments under a healthy facade, the risk of marathon running may trump benefits, he says. "

"A German study of more than 100 experienced marathon runners without known cardiac risks, ages 50 and older, found they
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