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.”

O’Keefe reviewed studies of people exercised at extreme levels for marathons, triathalons, ultramarathons or long bike races. 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, 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.”

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.

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