Slow down, marathon and long-distance runners: You might be racing toward developing atrial fibrillation (AF), an abnormal heart rhythm common in older people.

While such irregular heart rhythms won’t give you a heart attack, they can eventually lead to cardiac problems and strokes, according to health experts.

Dr. Gregory Flaker, a cardiologist and professor of medicine at the University of Missouri, warned that AF sometimes develops in people with slow heart rate, a condition common among marathon runners.

“It seems that marathoners have a higher degree of vagal tone. Their heart rate is slower [and their] vagus nerve is very active,” Flaker told reporters in an interview at the launch of Pfizer’s anticoagulant, Apixaban.

Vagal tone is the degree of activity occurring within the parasympathetic nervous system, resulting in changes to the heart rate and other key functions.

Good news

Flaker said he had to advise some patients in the United States to cut back on their running to prevent themselves from developing AF, which increases the risk of a stroke five-fold among older people who have other risk factors, such as high blood pressure and diabetes.

The good news is that while young marathon runners are just as predisposed to developing AF, they are not at risk for stroke and don’t need an anticoagulant, said Flaker.

“That person who has this condition might not be able to run as far during the marathon but is not at risk for a stroke like a 75-year-old or an 80-year-old who has high blood pressure, diabetes and other risk factors,” he said.

While there have been contradicting literature on the long-term effects of long-distance running and endurance sports to the heart, Mayo Clinic Proceedings published a study in 2012 showing that extreme endurance events could have adverse cardiovascular effects.

Get cardiac clearance

According to the study, repeated endurance exercise could cause cardiovascular problems, including atrial and ventricular arrhythmias, and diastolic dysfunction.

In a separate interview, Dr. Anthony Leachon, a cardiologist at Manila Doctors Hospital and president of the Philippine College of Physicians, advised Filipinos who love to join marathons to first get cardiac clearance from their doctors.

The safest running time is from 30 minutes to an hour, Leachon said, adding that marathon runners should avoid drinking energy sports drinks, alcoholic beverages and caffeinated drinks prior to joining competitions, as these could trigger arrhythmias.

“Avoid taking diuretics that can cause hypokalemia or decreased potassium serum levels, which can lead to cardiac arrhythmias,” the doctor said. Marathon runners must instead take a light meal and fruit drinks prior to the event to prevent decreased blood sugar that could potentially cause a spike in heart rate, Leachon said.