Running a marathon seems like a healthy thing to do, but it can actually hurt your heart—and not just if you’re out of shape. A Canadian cardiologist and marathon runner studied 20 other runners, some novice and some seasoned, six to eight weeks before their marathon, immediately after the race, and then three months later. He found that many distress signs were present before and immediately after the race, similar to what you might see during a heart attack, Time reports.
Before the race, the runners' hearts were functioning below normal capacity; right after crossing the finish line, the runners showed their highest levels of troponin, an enzyme that rises when blood flow is reduced—factors that damage heart muscle. But three months after the race, the damage appeared to have been reversed, and the heart functioned normally again. Though all runners were affected, fitness does matter: “People who were less ready or less fit pushed the envelope more and induced more reversible injury to the heart,” says the doctor.