THE GOSPEL OF CARDIOVASCULAR HEALTH HAS BEEN THE SAME FOR DECADES: an active lifestyle helps prevent heart disease. Moderate exercise, such as brisk walking, gives the cardiovascular system a real boost, with benefits that max out at about 150 minutes per week. Beyond that, however, the picture has started to become more complicated.

A string of recent studies raises the possibility that very intense workout routines may in fact cause long-term damage to the heart. This news comes as endurance athletics, such as marathons and long-distance cycling, have become increasingly popular. Older athletes may be at especially high risk.

Researchers have long known that sustained, serious training can gradually reshape the heart—a fact discovered in the late 19th century, when medical teams detected increases in the size of cardiac muscle in elite Nordic skiers. An athlete’s heart not only enlarges, but its four chambers are also able to relax and dilate more easily, allowing blood to be pumped out to hard-working limbs with greater efficiency, explains Aaron Baggish, director of the Cardiovascular Performance Program at the Massachusetts General Hospital Heart Center. But, asks Baggish, “do these adaptations confer an advantage later in life, or do they come at a cost?”

Elite athletes live longer than their peers, on average, suggesting that their training is beneficial to their overall health. Yet some physical changes that accompany extreme fitness have left researchers concerned. For instance, some long-term endurance athletes develop scarring, or fibrosis, at the juncture of the right and left lower chambers of the heart. “It’s the same pattern seen in people with congenital heart disease,” notes Paul Thompson, chief of cardiology and director of The Athletes’ Heart Program at Hartford Hospital in Connecticut. The scarring may occur because competing in marathons or other endurance events raises pulmonary blood pressure, and that increased pressure could cause the right ventricle to stretch. Whether this scarring in otherwise healthy athletes poses a threat, though, is still under investigation.

The arteries of athletes can also accumulate large calcium deposits. Calcification signals an increased risk of coronary artery disease in nonathletes, but Baggish says deposits of calcium in the arteries of athletes may be more benign, perhaps helping repair microscopic injuries to vessel walls caused by the forceful pumping of blood during exercise.

The biggest worry comes from studies showing that endurance athletes have up to five times the risk of developing a disturbance in heart rhythm known as atrial fibrillation (AFib), which can cause the most common form of stroke. Athletes’ hearts tend to have large left atriums, which Thompson notes have been linked to AFib. Baggish coauthored a 2016 study of 591 elite athletes age 35 and older, which found another factor. The risk for having disturbed heart rhythm rose with the number of years a person had exercised, but primarily occurred in men and women with hypertension, whether it was controlled or not.

Endurance athletes can also put themselves at risk of a heart attack through extreme exertion, which is especially dangerous if they have hypertension or other common risk factors for cardiovascular disease. Baggish is quick to note that cardiac arrest remains rare among endurance athletes, occurring in roughly one of every 100,000 male long-distance runners. But when they do strike, “it’s almost always within sight of the finish line.” Runners who have maintained an even pace for 26 miles suddenly “drop the hammer and go for the gold” over the final one-fifth of a mile. A sprint to the end can cause the demands of cardiac muscle to outstrip blood supply, resulting in potentially fatal fibrillation. Slow and steady is a mantra that can keep elite athletes safer, he says.

The research is decidedly personal for both Thompson—who placed 16th in the 1976 Boston Marathon and, at age 71, takes 75-mile bicycle rides for fun—and for Baggish, a long-distance runner himself. While health has always been the goal of the athlete, they say, it’s important to investigate any downsides. Where the heart is concerned, moderation may be the key.