Can exercising too hard and too long cause heart problems?

In 2010, John Mandrola was 46 years old and training like a demon for the USA Cycling Masters Road National Championships when, out on a ride, his heart started beating erratically. He diagnosed it immediately, because, in addition to being an accomplished endurance athlete, Mandrola is also a cardiac electrophysiologist. What he was experiencing was a condition he treats every day at work: atrial fibrillation, or AFib, a heart-rhythm disturbance that can feel like you can't catch your breath or get your heart rate under control. (It also increases the risk of stroke.)

"I could barely get home," Mandrola recalls.

How could someone as fit and healthy as Mandrola develop heart trouble before age 50? We've all heard that exercise is good for the heart, and that's undeniably true. But researchers have begun to understand that some athletes who exercise to extremes — competing in endurance events for many hours at a time over multiple years — may be at increased risk of certain heart problems, in particular AFib.

It's a paradox that so enthralled Mandrola that he co-wrote a book about it, "The Haywire Heart: How Too Much Exercise Can Kill You, and What You Can Do to Protect Your Heart."

Studies have shown that endurance exercise reshapes the heart. When the heart is stressed with long bouts of endurance exercise, it responds by stretching and becoming bigger and stronger so that it can pump more blood, in much the same way that lifting a barbell causes biceps to strengthen and grow, says André La Gerche, a cardiologist at the University of Melbourne. He says the heart of an endurance athlete can be twice as big, or more, as a nonathlete's: "We don't have any medication or condition that causes as profound an effect on the heart's size and shape."

This increased size is generally a good thing, as it means that the heart can pump blood more efficiently. But in some cases, exercise may be associated with minor swelling or scarring where the heart is stretched. Some imaging studies of athlete hearts find scarring and fibrosis (a thickening of the cardiac tissue), but the practical significance of this micro damage isn't clear, says Benjamin Levine, director of the Institute for Exercise and Environmental Medicine at UT Southwestern Medical Center and Texas Health Presbyterian Hospital.

The jury is still out on whether exercise itself causes these changes and "the vast majority of the evidence is that it doesn't," Levine says. "Our data suggests that the hearts of these elite athletes are youthfully flexible and compliant and they function normally."

"We know for sure that chronic, extreme exercise increases atrial fibrillation risk by about 500%, maybe as much as 800%," says James O'Keefe, a cardiologist in Kansas City, Mo., who has sounded the alarm about the issue after experiencing it himself. "I've been an exercise addict my whole life," O'Keefe says. "There's a subconscious logic that says if some is good, more is better," but that is "absolutely wrong" when it comes to exercise.

In 2013, Swedish researchers published a study that looked for cases of arrhythmia among more than 52,000 skiers who had participated in the Vasaloppet, a 90-kilometer cross-country ski race in Sweden, between 1989 and 1998. Skiers who had finished the most races or who had the fastest times had the highest risk of arrhythmias.

In 2019, the same researchers published another study, this time looking at 208,654 Swedish skiers who had finished one or more cross-country skiing races of 30 kilometers or more between 1989 and 2011.

Again, they found that atrial fibrillation risk was higher in skiers with the most completed races and fastest finishing times, but this finding held for only men. Women in the study had a lower incidence of atrial fibrillation, compared with non-skiers, regardless of how many races they had completed or their finishing times. Previous studies have also found that female athletes do not have the increased risk of atrial fibrillation observed in male athletes, but there is no sure answer yet as to why.

But perhaps the study's most important finding was that the ski racers in the study who did develop AFib had a 27% lower risk of stroke and a 43 percent lower risk of dying compared with individuals from the general population who had the same diagnosis. The study implies that even when they do get AFib, athletes do better than nonathletes.

"There's no answer to that," Mandrola says. "What's going overboard for one person is not going overboard for another." What matters is probably not just the amount of exercise, but a combination of other things, such as genetics and environmental factors, he says.

La Gerche's group is using artificial intelligence and machine learning to look at data from sports watches and other trackers to see whether they can spot different training patterns in athletes who develop AFib. "There are probably little secrets in the data that people keep," La Gerche says. His group has begun a longitudinal study of endurance athletes to keep tabs on how their heart health changes over time and in relation to training.

To date, the only reliable risk factor researchers have identified for atrial fibrillation in athletes is being male.

"Virtually all heart arrhythmias are more common in males," La Gerche says, and this is also true for the ones associated with chronic high level exercise. It's not clear yet whether this difference is because historically fewer women have done extreme endurance exercise or whether there's some biological reason that women might be protected.

La Gerche's group is looking at training data from electronic tracking devices such as smartwatches to try to figure it out.

The heavy exercisers in his studies who have had high levels of coronary calcium also had a 25% lower risk of cardiovascular events and mortality, so this calcification doesn't seem to increase heart disease risk. A study published earlier this year found that training for and completing a marathon made arteries more supple - as if they were four years younger.

Given what we know now, experts say there's no reason to limit the amount of endurance exercise you do - unless you develop symptoms, such as an irregular heartbeat.

"I would never tell anybody don't do a marathon or don't train for a bike race because this 1/8atrial fibrillation3/8 could happen," Mandrola says. "The chance of a bad outcome is very, very low in absolute terms and very modifiable."

Athletes who take part in extreme events do not appear to be at increased risk of dying from heart disease. A research team headed by Laura F. DeFina used a database from the Cooper Center Longitudinal Study to identify 66 participants who reported getting the equivalent of about 35 hours or more of physical activity per week. DeFina's group found that these "extraordinary" exercisers had no increased risk of dying from heart disease or any other cause.

The heart and other benefits of physical activity — such as reduced blood pressure, improved blood sugar and lower risk of diabetes and obesity — continue to build up to about five or eight hours of exercise per week. But "once you get more than that, you're not training for health, you're training for performance," Levine says. That's not to say that exercising more than that will necessarily hurt you, but that range is where you hit the point of diminishing returns.

Although it can increase the risk of stroke, AFib is considered benign by the medical community. But it may not feel that way to athletes who find their training plans disrupted and their athletic aspirations crushed as they're forced to cut back on the volume and intensity of their exercise, Creswell says.

The good news is that even when athletes develop exercise-related AFib, it's usually highly treatable, often with a surgical procedure called an ablation, which destroys the tissue that's involved in the faulty electrical signaling. But ablation isn't always necessary.

"One of the best ways to make it go away is to back off the exercise," Mandrola says. (In some instances, blood thinning drugs, which raise bleeding risks, may also be necessary, and that can limit the activities in which falls or collisions can occur.)