Cardiac function differs between elite marathon runners and runners less aerobically fit, a Canadian doctor has found. (Bizuayehu Tesfaye/Associated Press)

Marathon runners who lack proper training can temporarily damage their hearts, a Canadian doctor and runner has found.

Cardiologist Dr. Eric Larose's interest in the problem was sparked when he saw someone collapse and die at a marathon in Quebec City in 2001. Larose presented the research on Monday at the Canadian Cardiovascular Congress 2010 in Montreal, and the findings offer a clue to sudden deaths among marathoners.

Lack of aerobic fitness may impair how the heart copes with the stress of running a marathon, Larose and his colleagues said.

"This is not a permanent injury that will leave any type of scar," said Larose, a professor of medicine at Laval University in Quebec City who runs shorter distances than marathons, which are 42.2 kilometres.

The temporary nature of the damage was a relief to Larose and his colleagues, who looked at 20 marathon runners. Those who were less physically fit were compared to those who were fitter.

Those in the less fit group showed abnormal functioning in more than half the 17 segments in the heart's left ventricle, unlike the fitter competitors, the team found. Other areas of the heart had to work harder to compensate, Larose said.

All of the runners in the study took a test of aerobic endurance known as VO2 max — a stress test that also measures the body's oxygen consumption.

MRI revelations

The researchers studied the effects using MRI measurements, which allowed them to detect small changes that older techniques could not show.

Specifically, the MRI showed inflammation and edema — swelling in some sections of the heart caused by accumulation of fluid — similar to the damage that occurs when someone hits a knee against the side of a table. In both cases, the injuries hurt, but the damage is temporary.

The runners were all tested before the marathon, within 48 hours of the end of the race and three months afterward.

By three months, the damage was no longer visible.

The researchers also found when runners were dehydrated, their cardiac function also went down. It is a sign that less experienced runners are pushing too hard, Larose said.

Larose continues to work up a sweat on the track but said he's careful to not overexert himself.

"This is really Canadian science leading the way, showing why there may be problems when people exercise too vigorously without supervision," Dr. Beth Abramson, a cardiologist and Heart and Stroke Foundation spokeswoman, said from the conference in Montreal on Monday.

Physical activity is key to heart health, but it is important for people to train and get medical advice, Abramson said.

Larose also recommended that novice marathon runners visit a health-care professional and, when appropriate, get the V02 test.

Susan Easterbrook, 51, from Windsor, Ont., who trained for this past weekend's race in Niagara Falls, Ont., said she has already taken the advice to heart.

"I do this for pleasure, for stress relief," said Easterbrook. "I know my limitations and I don't push beyond them."

The congress is co-hosted by the Heart and Stroke Foundation and the Canadian Cardiovascular Society. It runs until Wednesday.