If you’re the type who looks for any excuse to avoid taking up jogging or cycling, you may have a new argument in your favour.

Research by a team at the Mayo Clinic in Rochester, N.Y., found that excessive exercise can lead to permanent scarring of the heart in some people. The studies found that extreme athletes, which the research team describes as those who participate regularly in marathons and other demanding, high-cardio activities such as long-distance bicycling, have enlarged hearts and arteries as well as actual scarring.

The researchers believe the scarring is related to the enzyme troponin, which is known to signal heart distress when evident in the blood. People suffering a heart attack tend to score high on troponin.

Marathon participants have more of this enzyme in their blood after a race. Although most people see their levels bounce back to normal over the course of a few days, some runners’ hearts seem to lack that resiliency and, instead, suffer serious long-term damage in the form of scarring. In fact, the Mayo Clinic researchers warn that these individuals run a greater risk of heart attack in the two years after each race.

But don’t put down your running shoes just yet. The “right” amount of vigorous, heart-pumping exercise is still remarkably good for you, says Jack Goodman, professor with the faculty of kinesiology and physical education at the University of Toronto.

“We do know that the recommended exercise is very beneficial and heart-healthy,” Goodman says, “lowering the risk for heart disease, certain types of cancer and all kinds of mortality.”

Regularly active people reduce their risk of having a heart attack or getting something like colon cancer by anywhere from 20% to 40%.

Researchers have long known about the so-called “athletic heart syndrome” – in which one endurance athlete’s super-efficient heart can turn out to be bigger and have thicker walls than those of his or her peers, Goodman says. But recent research indicates that these elite-level athletes also are more likely to develop scarring of the heart and cardiac arrhythmia, the irregular heartbeat that can predispose individuals to sudden cardiac arrest and death if not treated quickly. However, Goodman points out, many Canadians, particularly those who don’t exercise, can develop changes to their heart rhythms, such as atrial fibrillation, as they age, which can increase the incidence of stroke. Some people might not even know they have it, he adds. To further complicate matters, the condition also can be benign.

Marathons and half-marathons are increasing in popularity – the number of participants has increased tenfold since the 1980s, with more than 700,000 people running in marathons across North America last year. The fastest-growing group of marathoners, Goodman adds, is people over age 40. And that group is facing this Goldilocks-style conundrum: while a little cardio exercise is good and a little more is better, you can have too much of a good thing.

But science hasn’t yet revealed how much is “just right,” or whether all of us should aim toward the same goal. “This is the big question,” Goodman says. “If you take two people and train them like crazy and they accumulate 4,000 hours of vigorous exercise over their lifetimes, one might develop atrial fibrillation and one may not.”

A safe rule of thumb, Goodman advises, is to bank between 1,000 and 2,000 kilocalories of energy expenditure each week. And that translates into the oft-recommended 30 to 60 minutes of heart-pumping exercise each day. The more you do – within that range – the better off you’ll be when it comes to disease prevention and good overall health. And there’s no evidence, Goodman adds, that running 10 kilometres or a marathon now and again is dangerous. But if you regularly go over that limit, you’re not gleaning any benefits. Rather, you’re taking some risks with your health.

Goodman offers advice for those who love the runner’s high but don’t want to risk heart damage: listen to your body. You’re more likely to suffer from musculoskeletal problems before any damage to your heart occurs. Avoiding strenuous exercise when you’re sick (especially if you have a fever) is essential in ensuring you’re getting the benefits of cardio without the serious risks.

Cardio is all about raising your heart rate, which is something that happens when you’re having any number of exciting episodes in life, good or bad. It’s still better to choose heart-healthy fitness over sitting in front of the TV. And, according to Goodman, 40-plus fitness buffs have to weigh the small risk against the very real, achievable benefits: “Every time you exercise, you’re at a greater risk of having a heart attack than you would be at rest. But the more you exercise over the long run, the lower that risk is. And you’re still reaping significant health benefits.”

© 2012 Investment Executive. All rights reserved.