In the long run, your heart will thank you for running a marathon. In the short term, not so much. 

A new study finds that marathon runners experience temporary heart damage during the 26.2 mile (42.2 kilometer) races. The damage is reversible, but it may be one reason that seemingly healthy people sometimes die during long races.

"We know that regular exercise reduces cardiovascular risk by a factor of two or three in the long run, but while we're doing vigorous exercise such as marathon running, our cardiac risk increases by seven," study researcher Eric Larose of the Institut Universitaire de Cardiologie et de Pneumologie de Quebec and Universite Laval in Quebec City told LiveScience.

Larose presented the study results on Oct. 25 at the Canadian Cardiovascular Congress 2010 in Montreal.

Why fit runners die

The risk of dying while running a marathon is very low. One 2007 study, published in the British Medical Journal, looked at 30 years of marathon data and put the risk of dying at 0.8 per 100,000 runners. According to the researchers on that study, running a marathon is half as risky as driving the same route on a normal day.

Nonetheless, there are a handful of race-related deaths each year. Last October, a 23-year-old graduate student collapsed and died during the Baltimore marathon, and this March, a 32-year-old man died shortly after crossing the finish line during a half marathon in Dallas. That death echoed the events of 2009's Detroit Marathon, during which three half-marathoners died.

Larose experienced the situation firsthand while running the 1999 Quebec City Marathon, in which a young runner close to him in the pack collapsed and died near the finish line.

"The question I asked myself was, 'Why is this happening to otherwise healthy people?" Larose said.

To find out, he and his colleagues recruited 20 runners in the 2008 Quebec City Marathon. The runners ranged in age from 21 to 55. Seventy percent were men.

Six to eight weeks before the race, each runner got an extensive checkup, including blood work and a stress test to measure their VO2max, or aerobic capacity. VO2max is a measure of how much oxygen your body can consume during maximum intensity exercise.

The runners also underwent magnetic resonance imaging (MRI) of their hearts to measure overall heart function as well as the cardiac function in the 17 segments of muscle that make up the left ventricle. The MRI also provided a measure of edema, or water content in the heart. Edema is a sign of potentially dangerous inflammation in the muscle.

Immediately after crossing the finish line, the runners arrived at a medical tent for a second round of blood work. They repeated all of the tests 48 hours after the race and again three months later.

Train and hydrate

The results showed that during a marathon, over half of the segments of the heart lose function, Larose said. The reason turns out to be an increase in inflammation and a decrease in blood flow through the muscle.

"This is the first potential explanation as to why a runner has increased risk while he's running or she's running," Larose said.

The good news is that the damage isn't permanent. By the time of their three-month checkups, the runners' hearts were back in tip-top shape.

A second bit of good news is that better training and hydration can decrease the damage. A higher VO2max, indicating better fitness, was linked with less temporary damage. Testing runners' VO2max before a race may be a good way to assess their cardiac risk, Larose said.

All but the fittest runner in the study had at least some damage. But the study group was too small to determine whether there is a universal level of fitness that protects runners from heart damage, Larose said.

Because dehydration was associated with greater damage, runners also must focus on staying hydrated during races, Heart and Stroke Foundation of Canada spokesperson Dr. Beth Abramson said in a statement. Abramson, a professor of medicine at the University of Toronto, was not involved in the study, though the Heart and Stroke Foundation partially funded the research.

Larose doesn't intend to dissuade wannabe marathoners, he said, but he does want to keep them from pushing themselves beyond what they're prepared for.

"People should not enter a marathon lightly," he said. "Training needs to be well done... You can't cheat the marathon."

This article was reprinted with permission from LiveScience.

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