Running Marathons Can Be Terrible For Your Heart

On Sunday, 47,000 runners will race through the five boroughs for the New York City Marathon.

Although most participants have spent a year training for the grueling 26.2-mile run and are in superb health, there's growing evidence that endurance running can hurt your heart in the long run.

A report from the Canadian Journal of Cardiology earlier this year found that half of runners studied suffered decreased right and left ventricle function (which work together to pump blood to the body) in the 48 hours after completing the Quebec City Marathon. Researchers also found general swelling in the heart, which caused reduced blood flow. But the changes didn't last.

The damage each time you run, however, can add up over the years. Last year, a Mayo Clinic study led by James O'Keefe, head of preventative cardiology at St. Luke's Health System, found the elevated levels of certain substances in the body can lead to arrhythmia, or an irregular heartbeat.

In 2012, 58-year-old ultra-runner Micah True (of "Born To Run" book fame) died while on a a 12-mile trail run - a short jaunt for a man accustomed to running three to four times the distance of a marathon. According to USA Today, an autopsy report showed that True died from cardiomyopathy, a disease in which the heart muscle becomes enlarged and thick and can lead to irregular heartbeats. Other marathoners, like Jim Fixx and Ryan Shaw, also died while running - most likely from heart conditions caused by the activity.

To drive the danger home, O'Keefe wrote an editorial in the British journal Heart about his worries for multi-marathoners. Requiring the heart to pump "massive" quantities of blood at once year-after-year can do serious damage, he wrote according to The Telegraph.

"Up to 30 percent of those who finish marathons have elevated troponin levels, which is a marker for heart damage. That's the marker we look for to see if someone's having a heart attack - it's irrefutable evidence of heart damage," O'Keefe told NBC News.

At this point, multiples studies have proven elevated troponin levels, according to The New York Times' health blog. Still, one doctor points out that many of these studies don't indicate whether people already had heart issues before they started running, according to NBC News.

All of this doesn't mean running is bad for us. The benefits of regular, vigorous exercise are well-established. The answer may be moderation.

Take the advice from the title of O'Keefe's editorial: "Run for your life ... at a comfortable speed and not too far."