Researchers say there's an optimal "dose" of exercise that benefits the heart and extends life. Beyond that point, you're not doing yourself any favors.

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Getting at least some moderate exercise is good for us, and more is even better, doctors and public health experts say. But is there such a thing as too much exercise?

Apparently, yes, according to researchers led by Dr. James O’Keefe, a cardiologist at the Mid America Heart Institute of St. Luke’s Hospital in Kansas City, Mo. Reporting in the journal Mayo Clinic Proceedings, O’Keefe and his colleagues found that physical activity, like any medical treatment, can be harmful if it’s overdone.

“As great as exercise is, it’s like a powerful drug,” he says. “More is better up to a certain dose, but after that there is a point of diminishing returns, and it may actually detract from [heart] health and even your longevity.”

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O’Keefe reviewed studies of people who trained and participated in marathons, triathalons, ultramarathons or long bike races — people who exercised at extreme levels. Overall, people who exercised regularly reaped significant benefits, tending to live seven years longer than those who are physically inactive. But when O’Keefe focused only on the extreme athletes themselves, he found that the healthy effect of all their activity tended not only to wane, but to actually reverse itself and turn toxic.

Studies showed that during and immediately following a marathon, runners showed up to a 50% increase in levels of an enzyme called troponin, which signals damage to the heart (it’s the same enzyme that shoots up in patients having heart attacks). Troponin is released when heart muscle is in distress, and in the case of lengthy extreme exercise sessions, it may start to climb as heart muscle fibers start to tear under the intense burden of pumping continuously at a high level.

“When you’re sitting around, you heart is pumping about five quarts of blood a minute, and if you run up the stairs or hard or push yourself physically, it can go up 35 or 40 quarts a minute,” says O’Keefe. “If you go and run for 26 miles, or do a full-distance triathalon, it completely overtaxes the heart. The heart is pumping 25 quarts a minute for hours and hours, and that starts to cause muscle fibers to tear, which leads to a bump in troponin and other enzymes associated with inflammation, and it causes the death of some muscle cells in the heart.”

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Over time, that damage can cause scar tissue on the heart, and a thickened and scarred heart is more vulnerable to abnormal heart rhythms, says O’Keefe. Indeed, people who chronically exercise at extreme levels tend to have thicker right atria (which receive deoxygenated blood from the veins) and larger right ventricles (which pump this blood out to the lungs to be oxygenated and circulated). Studies show that endurance athletes have a five times higher risk of atrial fibrillation, or fluctuations in the heartbeat that can trigger more serious heart problems.

In the new data, presented at the annual meeting of the American College of Sports Medicine, one of the study co-authors, Dr. Carl Lavie, medical director of cardiac rehabilitation and prevention at the John Ochsner Heart and Vascular Institute in New Orleans, reported on the optimal “dose” of running for increasing life expectancy. Among 14,000 runners, the optimal amount of exercise appeared to be about 10 to 15 miles per week. “We were thinking that we would see progressively more benefit the more you ran,” says Lavie. “We thought it would level off at some point. But not only did the runners not get more benefit, but the more they did, the faster they ran, the more frequently they ran, the more miles they ran, they actually seemed to lose any benefit to the heart.”

No matter how much they ran, however, they didn’t do worse than non-runners when it came to longevity, Lavie says. But there was a limit to how much exercise contributed to life span. Beyond that point, he says, physical activity started to have a negative, or harmful effect and cut away at any improvement the runners may have accumulated to that point.

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The damage doesn’t happen overnight; rather, it builds up over years of training. That’s why O’Keefe, an avid exerciser himself, doesn’t discourage patients who are interested in participating in marathons from trying them. But he counsels hopeful marathoners to consider their race experience a once-in-a-lifetime thing. “If they want to train for a marathon, to cross it off their bucket list, I tell them okay, but it’s not a healthy long term habit to get into,” he says.

Given the results from O’Keefe’s analysis, it makes more sense to exercise at moderate levels. “We have people who are more and more on the extremes. Over the last 35 years, obesity rates have tripled in America, and the number of people completing marathons has gone up 20-fold,” he says. “What we need are more people doing moderate exercise daily, and not running heroic distances. You can get 70% to 80% of the benefit of exercise from doing it 15 to 30 minutes a day.”

And that’s the real lesson from the study. It’s not that exercise is bad for the heart or that it’s better to sit on the couch than go for a run — it’s that the heart and longevity benefits of exercise don’t require extreme efforts. When it comes to exercise, a Goldilocks approach is probably best: too little isn’t good for heart, but neither is too much. Moderate physical activity — anywhere from 15 minutes to an hour a day, several days a week — is just right.

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Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.