Running, long considered a healthy hobby, may actually be dangerous for some. At least that’s the prevailing opinion of a number of the country’s top cardiologists and a new study due out next month from British journal Heart.

According to the editorial, endurance training and marathon running can literally push your heart to its limit, causing a variety of acute problems, such as arrhythmia or irregular heartbeat, and lasting damage, including calcification and scarring.

Some of these problems are impossible to predict, even if you’re an athlete who has been extensively pre-screened with cardiac tests prior to training. For older athletes, the toll that running takes may even outweigh the benefits gained from exercise, the study claims.

For many, the new information doesn’t add up. Why, for instance, would athletes who have been training for many years suddenly experience heart trouble associated with running?

The answer is simple. Intense physical exercise for long periods has the potential to take a toll on the body, in some cases aging it more quickly.

According to James H. O’Keefe, M.D. of the Mid-America Heart Institute of Kansas City, who co-authored an extensive 2012 study that examined the cardiac risks faced by athletes: “Physical exercise, though not a drug, possesses many traits of a powerful pharmacologic agent. A safe upper dose limit potentially exists, beyond which the adverse effects of physical exercise, such as musculoskeletal trauma and cardiovascular stress, may outweigh its benefits.”

In short, exercise is great in small doses, but too much physical exertion too quickly or for too long a period can actually put a person’s heart at risk, especially if he or she is over age 35.

Inside the Runner’s Heart

Watch any marathon runner and it’s obvious that his or her body is under a great deal of stress. Temperature increases ten-fold, causing sweating, fluid loss, potential dehydration, muscle weakness, and even disorientation. But now researchers also have an idea about how marathon racing and long-term endurance training affects the heart muscle, too.

In the aforementioned 2012 Mayo Clinic study, for instance, long-term endurance training was associated with “coronary artery calcification, diastolic dysfunction, and large-artery wall stiffening.”

In that study, researchers associated endurance training with temporary spikes in sustained inflammation that was, in some cases, injurious to the heart muscle. That impairment may lead to permanent scarring or ventricle damage, which was seen in 12 percent of cases, or more acute issues like arrhythmia or sudden death, which is what killed runner Micah True in 2012 while running a marathon in Mexico.

According to the Cleveland Clinic, sudden cardiac death occurs in about one of every 15,000 joggers and one in every 50,000 runners; a 2008 study in European Heart Journal equated the greatest risk of sudden cardiac arrest in marathon runners with those who had the least amount of training.

A 2010 study presented at the Canadian Cardiovascular Congress echoed those same findings. In that study, runners who were less fit (as assessed by V0₂ max testing) had signs of “inflammation, swelling, and decreased perfusion accessed by MRI over three months.” Those runners who were better trained over a longer period were less likely to experience the same level of heart damage

An Ounce of Prevention

According to Carl J. Levine, M.D., Medical Director of Cardiac Rehabilitation and Prevention at the Ochsner Medical Center in New Orleans, anyone interested in marathon running should undergo cardiac pre-screening tests to eliminate underlying heart conditions. As many as ten different pre-existing cardiac conditions can be detected.

Levine recommends that physicians also take detailed histories of their patients, especially those who are in training, to look for evidence of overexertion, such as tendonitis, stress fractures, and other overuse issues.

O’Keefe, a cardiologist and co-author of the Mayo Clinic study as well as a marathon runner himself, said it best: “Extreme exercise is not conducive to great cardiovascular health. Beyond 30 to 60 minutes a day, you reach a point of diminished returns.”