Marathon runners, triathletes and long-distance cyclists could be doing harm to their hearts in the long term, cardiologists say.

The June issue of the Mayo Clinic Proceedings includes a review of how excessive endurance exercise is thought to cause damage to the heart.

The study's authors say that based on both animal and human data, the cardiovascular benefits of vigorous aerobic exercise build up over about one hour, "beyond which further exertion produces diminishing returns and may even cause adverse cardiovascular event effects in some individuals."

Physical exercise is similar to a drug and is important for preventing and treating coronary heart disease, hypertension, heart failure and obesity.

Rick Rayman ran four marathons over the last five weekends and doesn't plan to quit. (David MacIntosh/CBC)

The findings don't detract from the importance of exercise, stressed the study's lead author, Dr. James O'Keefe of Saint Luke's Hospital in Kansas City, Mo.

"When people come to me as a cardiologist and say they want to run a marathon I say, 'OK, do one and cross it off your bucket list and then let's focus on an exercise pattern that's more ideal to producing long-term health benefits and improving your longevity,'" O’Keefe said.

People who exercise regularly have lower rates of disability and a life expectancy seven years longer on average than couch potatoes, the researchers noted.

O'Keefe wants people to understand that the lion's share of benefits come at a relatively modest level. No further benefits are obtained beyond 30 to 60 minutes a day of vigorous activity.

The researchers said elite-level athletes commonly develop abnormal electrocardiograms and their hearts adapt in ways that traditionally weren't thought to be harmful. Now it seems the cardiac remodelling from excessive exercise may increase their risk of heart rhythm problems like atrial fibrillation.

After people reach their mid-40s, long and intense exercise can cause scarring and fibrosis in the heart, O'Keefe said.

Pushing exercise limits

Running probably ought not to be done more than four or five times per week and ideally more like two or three times, O'Keefe said.

On other days, people can do cross-training, weightlifting, stretching, yoga, walking and swimming.

For people who are overweight and obese, 60 to 90 minutes a day of less intense exercise may be advisable. There's no limit on walking or light bicycling, a level of activity we're genetically designed for, O'Keefe said.

Dr. Davinder Jassal, a principal investigator of the cardiovascular imaging laboratory at St. Boniface Hospital and Research Centre and associate professor of Cardiology at the University of Manitoba, is also studying the hearts of marathon runners.

"Some studies have been showing scar, other studies like ours have shown no permanent damage to the heart, so the jury is still out there in terms of whether there's conclusive evidence that scar occurs in marathon runners," Jassal said.

Marathoner Rick Rayman, 65, of Toronto has run 268 marathons and hasn't missed a day of running in more than 33 years.

"As long as I can do what I doing, I'm going to continue," Rayman said.

Despite some running-induced back pain that has slowed him down, Rayman said he feels great.

With files from CBC's Kelly Crowe