When cardiologist Dr. Eric Larose, a professor of medicine at Laval University, was running the Quebec City marathon in 2001, he was shocked to witness a 24-year-old man collapse near the finish line and die.

He says he had never finished a race to such utter silence.

But that experience prompted Larose to ask: If running a marathon is considered a peak of athletic achievement, why is it that some runners have heart attacks during a race or shortly after they cross the finish line?

Larose decided to investigate. What he and his research team found was that marathon running can actually temporarily damage the heart, especially among those with not enough aerobic training.

Larose, who presented the findings on Monday at the Canadian Cardiovascular Congress 2010 in Montreal, says it's no secret that exercise is a great way to reduce our lifetime risk of cardiovascular disease.

"However, we've also known for over 10 to 15 years now that while we're performing exercise, our risk temporarily rises. So we've been wondering how can we explain this?" Larose told CTV's Canada AM Monday.

So Larose and his colleagues recruited 20 marathon runners. Some were training for their first event, while others had done more than a dozen marathons.

They tested the heart function of the volunteers six to eight weeks before the marathon, immediately after the race, and then again three months later, to evaluate how their hearts were affected.

They performed a number of tests, including the V02 max, which directly measures body oxygen consumption while users run on a treadmill or on a stationary bike.

What they noticed in the testing before the race was that runners with less experience and preparation had lower V02 max, meaning they had lower exercise capacity.

The tests conducted immediately after the race offered a few other revelations. The first was that there was no permanent damage to the heart from running the marathon.

"That's an important finding," said Larose, who stresses that he doesn't want anyone to be scared out of exercise because of this research.

The second finding, which Larose says is new, is that there was temporary damage to the heart in some of the runners. The damage included:

•  increased edema -- or swelling and inflammation
• decreased perfusion -- meaning fewer nutrients and oxygen were going to certain areas of the heart
• decreased function in certain heart regions

When they divided the heart into 17 regions and examined each one in turn, they found that those runners who were less fit showed abnormal functioning in some of the segments.

"This is what MRI is bringing that is new to this area of research. MRI is exquisite in allowing us to see all the different regions of the heart," Larose said.

Some parts were having trouble functioning, so other regions were being asked to work extra hard to compensate. In some cases, more than 50 per cent of the regions had decreased function.

That finding told Larose's team that there's a lot going on in the heart that isn't always revealed at first glance.

"We realized that when you look at the heart as a whole, you're missing how different regions react to different situations. Some regions will be injured and functioning less and others will be functioning more," he said.

The good news is that as time passed following the marathon, the damage induced by the race dissipated, and the heart eventually resumed its normal functions. But in some of the less-fit runners, that took as long as three months.

Larose notes that the findings suggest that there may be a minimum fitness level needed beyond which the heart can bounce back from the strain of training and running a long race.

He says runners need to train properly over a long period, stay hydrated, and most importantly, speak to their physicians about what is right for them.

It's considered the best test to provide an accurate measure of a safe maximum heart rate (number of beats per minute) for runners.