Anyone who’s finished a marathon or Ironman wouldn’t be shocked to find that the effort caused damage to their body and heart. Traditionally, though, that damage has been thought to be only temporary, subsiding after a few weeks.

But, a newly published report in Mayo Clinic Proceedings suggests that the damage endurance athletes do to their hearts actually adds up over time. Repeated extreme exercise or long-distance racing can cause a buildup of scar tissue on the heart, which can lead to the development of patchy myocardial fibrosis in up to 12% of marathon runners. The effects of “chronic exercise” can also include premature aging of the heart, stiffening of the heart muscles, and an increase in arrhythmias and atrial fibrillation.

“It’s a cumulative thing,” said Dr. James O’Keefe, of the Mid-Atlantic Heart Institute and one of the authors on the study.

“More [exercise] is certainly not better,” said Dr. Chip Lavie, another author on the study and a cardiologist at the Ochsner Medical Center.

In fact, in the release announcing the study, the recent death of ultra-marathoner Micah True–who frequently ran distances in the range of 50-100 miles–during a training run is called out as likely being connected to the long-term effects of excessive endurance exercise. An autopsy of his heart found it enlarged and scarred and suggested that he died of a lethal arrhythmia, or irregular heartbeat.

Multiple studies have shown that immediately after a marathon, 30 to 50% of runners show increased levels of enzymes and biomarkers that are typically released during heart attacks and associated with heart failure. Originally, it appeared the race-related damage was less severe in people who trained over 45 miles per week, but O’Keefe says that doesn’t prove to always be true.
In fact, elite athletes often suffer from an enlarging of the heart and thickening of the heart muscle known as “athlete’s heart.”
Much of the damage seen immediately after the race goes away within the month. It is only when the heart is consistently and repeatedly damaged that the scarring builds up. If you’re going to continuously compete in long-distance running, cycling or triathlon events, there are a few precautions you could take, O’Keefe says.
Break up your exercise to give your heart a rest, recommends O’Keefe. Sitting at your desk, your heat pumps about five liters per minute, but during exercise it can pump up to 25 liters a minute. “That’s a lot of cardiac work to do for four hours at a time,” he said.
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