Exercise is generally great for health, but extreme forms of it may be bad for the heart in some cases, experts say.

Recent evidence suggests training for and participating in extreme endurance exercises, such as marathons and triathlons, may cause heart problems in some, researchers say.

There's very little to gain from doing more than about an hour of exercise a day, said Dr. James O'Keefe, at cardiologist at Saint Luke's Hospital in Kansas City, Mo.

“A lot of people do not understand that the lion's share of health benefits accrue at a relatively modest level,” O'Keefe said in a statement. “Extreme exercise is not really conducive to great cardiovascular health,” he said.

However, experts emphasize that exercise is very important for health, and the proportion of endurance athletes at risk for exercise-related heart problems is quite small: The rate of sudden cardiac death among marathon participants is one in 100,000.

“I would never say don’t do it,” said Dr. Suzanne Steinbaum, a preventive cardiologist at Lenox Hill Hospital in New York City, referring to participation in extreme endurance sports. Instead, Steinbaum said she recommends that athletes undergo an examination from a heart doctor before participating in such activities. While tests cannot predict for sure whether an athlete will experience heart problems down the road, they can provide clues to how big a person's risk may be, Steinbaum said.

Exercise and the heart

Studies suggest extreme endurance training can cause temporary changes to the heart's structure, such as stretching of tissue, and increases in certain biomarkers known to be associated with heart injury, O'Keefe said. These factors have been shown to return to normal after one week, but over time, repeated bouts of extreme exercise may lead to more permanent damage, such as heart scaring, in some people.

In one study of about 100 apparently healthy marathon runners, 12 percent showed evidence of heart scaring — a rate three times higher than that of non-marathon runners.

Heart scaring can increase the susceptibility to heart rhythm problems, the researchers said.

Extreme exercise has also been associated with an increased risk of calcium build up in artery walls, leading to a narrowing of the arteries, the researchers said.

And studies of marathon runners show that up to 50 percent of them have increased levels of troponin, a marker of heart injury, and B-type natriuretic peptide, a marker of pressure on the heart, during and after a marathon, the researchers said.

How much is too much?

To exercise for health, the American College of Sports Medicine recommends 150 minutes of moderate exercise per week, or 75 minutes of vigorous exercise per week.

A recent study found exercise beyond this amount doesn't translate to extra health benefits. In the study, people who ran moderate distances at moderate speeds, and exercised a few times a week lived longer than those that ran longer distances at faster speeds (7.5 to 8 miles per hour) more than four times per week, O'Keefe said.

“When it comes to running, it helps to be a little less intense about it,” O'Keefe said.

Athletes interested in extreme endurance sports should have certain biomarkers checked, such as troponin levels, Steinbaum said. “If they're elevated, maybe it's not the right thing for you,” she said.

Pass it on: In a small percentage of endurance athletes, extreme exercise can lead to the development of heart problems.

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