In the last 10 years, a handful of studies have gained attention for finding that long term, high-volume endurance exercise could harm your heart as much as you're trying to help it. A study from the Journal of Applied Physiology found that compared to their more sedentary counterparts, longtime marathoners had a higher risk for myocardial fibrosis, a scarring build up of the heart's lining, which can lead to loss of flexibility over time.
Research in the Journal of the American College of Cardiology showed that men who trained for and completed marathons for 25 consecutive years were more likely to have a build-up of coronary plaque in the arteries, the result of which can lead to arrhythmia and other heart-pumping problems. Even former professional cyclists are five times more likely to have ventricular tachycardia, a disorder that disrupts the heart's normal function and increases heart rate, according to another study.
Does that mean you'll keel over from a heart attack during your next long run? "No," says Paul Thompson, Ph.D, medical director of cardiology at Hartford Hospital and The Athletes' Heart Program. "You can't do a lot of exercise unless you already have a good engine. There's not a lot of evidence saying that that much exercise is good for you, but there's no direct evidence yet that says it's harmful. These studies are just a starting point."
Thompson also points out that less than one percent of the U.S. population works out two or three hours a day, so the average gym-goer needn't be concerned—pushing yourself through one more set of burpees won't make you keel over. For the same reason, a cyclist who rides for an hour-and-a-half at a moderate speed every day won't be facing the same potential risks of a cyclist who puts in three hours of intense riding for 10 years. But even if you do work out like an Olympian, if you feel healthy, you probably are. In fact, Thompson recommends athletes skip screening if you don't have a family history of heart disease.
"When a person exercises that much, the heart enlarges and atrial chambers change," says Thompson. "An athlete's heart can mimic an abnormal EKG reading to a doctor who doesn't work with people that fit, and it could result in multiple, expensive tests. Wilt Chamberlain's EKG would have looked like he had a heart attack."
Thompson said the only time you should worry is when there's a sudden change to how you feel. If you're suddenly robbed of energy and can no longer do workouts, or if you pass out during exertion, see a doctor. In those cases, it could be an underlying medical problem, oftentimes heart-related. As for what happens down the road, researchers are looking into it. For now, the good still outweighs the bad when it comes to cardio.