Adults over the age of 40 are the fastest growing group participating in marathons and triathlons.

The endurance sports are associated with many cardiovascular benefits, but what many middle-aged athletes don't know is that their sport could actually damage their heart.

Jack Goodman, U of T researcher

Jack Goodman is conducting a study with 150 athletes at the University of Toronto to find out how high levels of exercise training can change heart structure, function and rhythm. (CBC)

There is growing evidence that intensive endurance activity for an extended period of time can be dangerous.

Jack Goodman is conducting an athletic heart and exercise study at the University of Toronto.

He says for about five per cent of middle-aged athletes, going way over the recommended 150 minutes a week of exercise can cause an irregular heart rhythm — which could lead to blood clots and stroke.

Goodman's study aims to find out how high levels of exercise training can change heart structure, function and rhythm. The goal is to understand potential risk factors for the development of arrhythmias.

The study compares adults with a history of high-volume training — or "excessive" exercise — to adults who have maintained light-to-moderate exercise history over a similar time-frame.

"Our advice is you're probably doing something that's very healthy, but you've got to listen to your body and proceed with caution."

Gruelling sports can take a toll 

Renato Alessandrini's last race was the equivalent of biking up Mount Everest — seven times.

"If you don't like to suffer, you don't do this long distance, ultra stuff," he said. "It hurts."

Renato Alessandrini

Renato Alessandrini exercises a lot, but his heart is now enlarged and has a small blockage. (CBC)

As a cardiologist, Alessandrini can read his own chart. Because of all this exercise, his heart is now enlarged and has a small blockage.

He doesn't recommend this gruelling sport to his patients.

Dr. Mark Tarnopolsky, another athlete in the medical field, has races that can last four days.

A couple of years ago, during one of those long races, he noticed his heart was racing abnormally. So the doctor hooked himself up to one of his own electrocardiogram machines.

"I looked it up and sure enough, this atrial fibrillation and atrial flutter were much more common in top endurance sport athletes," he said.

Five times more common — especially in men. 

But after two surgeries to repair the damage in his heart, Tarnopolsky is back in his running shoes.

Start slowly, says endurance athlete

Running has become a huge part of Martin Parnell's life. He started at the age of 47 before completing 250 marathons in 2010.

It was to raise money for the charity Right to Play, and spurred several other quests of extreme endurance.

"There's been a big increase in runners in triathlon between the age of 40 and 60. A lot of folks have come into it," said the 58-year-old from Cochrane, Alta.

Martin Parnell

Martin Parnell has completed nine quests for charity that require some extreme endurance, including completing 250 marathons in 2010. (Rachel Maclean/CBC)

But Parnell is aware there can be complications from extended endurance activities, so a medical team has been tracking his health for more than four years.

"I didn't want to injure myself. I didn't want to end up having a heart issue, or whatever, so we kept pretty close track on it during that year and since then," he said.

He has seen improvement in bone density, blood work and heart health but still cautions others to take it slow when starting out.

"My recommendation is it takes time for the body to change," he said. "And if you just step into something and haven't done these sports you're going to pay a price. So take the time, get prepared, enjoy the event and don't push it to the point where you're going to injure your yourself and I think you will be OK." 

With files from CBC's Rachel Maclean and Kim Brunhuber