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Health

How much exercise is too much?

Growing evidence shows that overdoing endurance training may damage your heart and shorten life expectancy. But don’t hang up your running shoes and wetsuits just yet.

Runners compete in the 2012 Boston Marathon.

Charles Krupa/AP/file

Runners compete in the 2012 Boston Marathon.

Dr. Paul Thompson has run 29 Boston Marathons over the past four decades — he finished 16th in 1976 — but has also spent a good part of his career as a cardiologist researching the detrimental effects that high-endurance training has on the heart.

Some of his recently published papers reviewing the latest research suggest that regular marathon running increases the risks of an abnormal heartbeat, damage to heart tissue, and hardening of the arteries. Other research suggests that those who train hard every day don’t live as long as those who run at a more moderate pace a few days a week.

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In a February study, Danish researchers followed nearly 1,900 runners for three decades and found that those who jogged slowly for up to 2½ hours a week lived about six years longer on average than those who ran longer and faster. Swedish researchers reported in June that elite cross-country skiers who had the fastest times in a 56-mile ski marathon or those who competed in the greatest number of those marathons were also 30 percent more likely than their fellow competitors to be hospitalized for an irregular heartbeat.

“When there’s enough smoke, there’s usually some fire,” Thompson said. “This may be a small fire, but I think most of us believe there’s cause for some concern.”

Thompson, 65, said he never ran marathons for health reasons. “I did them because I enjoyed racing and competing — to see how good I could get.” A recent hip injury has forced him to quit running; he now regularly bikes 32 miles round-trip from his Connecticut home to Hartford Hospital, where he’s chief of cardiology.

He and other cardiologists involved in this area of research agree that those who enjoy competing shouldn’t curtail their workouts solely because of the recent findings. For those trying to maintain weight loss or improve their fitness for a competition, a harder or longer workout may be beneficial. But for moderate exercisers thinking about extending their typical 3-mile jogs to 6 or 8 miles solely to improve their heart health or live longer, the doctors’ advice is: don’t.

More than a million Americans compete in high-endurance events. Above: participants at a recent triathlon in Lowell.

Mark Lorenz for the Boston Globe

More than a million Americans compete in high-endurance events. Above: participants at a recent triathlon in Lowell.

“It appears that most people can get maximum health benefits with relatively low amounts of exercise, and that’s comforting,” said exercise researcher Dr. Carl Lavie, medical director of cardiac rehabilitation and prevention at the John Ochsner Heart and Vascular Institute in New Orleans. “You don’t need to push it longer and harder if you’re trying to make yourself as healthy as you can be.”

Of course, the majority of Americans aren’t pushing themselves nearly hard enough, failing to meet the government’s exercise recommendations: 150 minutes a week of moderate activity (a 30-minute stroll five days a week) or 75 minutes of vigorous exercise (such as jogging briskly for 25 minutes, three days a week). Getting that amount of exercise has clear health benefits such as reducing the risk of heart disease, diabetes, and a variety of cancers, and helping people maintain their weight and muscle mass as they age.

But more than a million Americans fall into the extreme end of the exercise spectrum, competing in marathons or ultra-marathons of 40 or 50 miles in length or participating in other high-endurance events such as Olympic-length triathlons that include a 1-mile swim, 25-mile bike ride, and 6-mile run.

“I personally think that making a habit of running marathons is just not a good idea, which is supported by the medical literature,” said Dr. James O’Keefe, a cardiologist at the Mid America Heart Institute of St. Luke’s Hospital in Kansas City, Mo. “When you go out and exercise hard, a healthy heart will pump four to five times as much blood as when you’re sitting. Doing this for hours at a time is asking the heart to do something it wasn’t designed to do.”

While exercise researchers haven’t determined the exact toll this takes on the heart muscle, evidence suggests that too much prolonged exertion over time can do irreversible damage. In a 2012 review of more than 50 studies published in the journal Mayo Clinic Proceedings, O’Keefe and his colleagues concluded that endurance athletes who participate in marathon-style running, biking, and swimming races have five times the risk of developing an irregular heartbeat called atrial fibrillation because of an enlargement of their heart muscle. Some also have more scarring on heart tissue and higher levels of a protein called troponin, which is involved in inflammation and atherosclerosis, a chronic condition that damages blood vessels.

“Survival of the fittest doesn’t really hold true today,” O’Keefe said. “Survival of the fit will do just nicely.”

There still isn’t enough evidence, however, to determine a safe upper limit for exercise, and it likely varies from person to person. Even researchers who have collaborated on studies don’t agree on how much is too much.

O’Keefe advises those older than 40 to avoid marathon-length races because the training causes microtears in heart muscle that don’t heal as well as we age. Thompson, on the other hand, said participating in a long endurance race once or twice a year is fine at any age, barring any orthopedic issues in the hips, legs, or feet, or established heart disease that would require a doctor’s clearance.

Lavie agrees but pointed out the dangers of entering such races too often. “There’s a small group of Americans who do some sort of triathlon every weekend or a marathon once a month — which no one was doing 20 or 30 years ago — and that I think is concerning because the heart never has a chance to recover.”

While the risk of dying during a marathon is remote — 1 in 200,000 — the heart does need time to repair itself after competition. Studies suggest about one-third of marathon runners develop signs of heart stress immediately after a race, such as elevated troponin levels or enlarged heart chambers on imaging tests, but Lavie said these abnormalities usually vanish within a week.

Whether elite endurance athletes live shorter lives than those who run, swim, or bike shorter distances in slower times remains unknown, though population studies indicate this could be the case.

“This research doesn’t show a U-shaped curve where all the benefits of exercise disappear at the extreme end,” said Dr. Timothy Church, director of preventive medicine research at Pennington Biomedical Research Center in Baton Rouge, La. “There is probably an optimal level of exercise, but I think everyone's optimal level will be different based on who you are and your genes.”

Until further studies can help physicians personalize their advice on exercise, researchers agree that the safest bet is to listen to your body and cut back if you experience pain or excessive fatigue between workouts. Many people check off marathons and triathlons from their bucket lists and then ease back into shorter workouts.

Resistance training with weights, balance exercises, and stretching also become more important as the body ages, to combat the loss of muscle mass, balance, and flexibility. “Optimal aging includes not just cardiovascular fitness but retention of overall muscle strength,” Church said.

Deborah Kotz can be reached at kotz@globe.com. Follow her on Twitter @debkotz2.
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