In 2015, more than 509,000 Americans crossed marathon finish lines arounds the country — nearly double the amount of runners recorded 20 years ago. With intensive training and dogged persistence, endurance athletes put their bodies to the test, which has led sports medicine experts to wonder what toll, if any, does it take on the heart? A study, published in the journal Circulation, debunked one of the greatest health misconceptions about endurance athletes: they’re prone to heart complications. A team of researchers from the University Saarland in Germany has revealed, through scans of the heart, just how healthy an undertaking a marathon or triathlon can be.

Previously, a body of research published by a team of Belgian scientists in 2012 found repeated bouts of intensive endurance exercise increased the risk of life-threatening health problems, including sudden cardiac death. But when the German researchers re-tested the theory, they found no such link — in fact, just the opposite. Endurance athletes who were still engaged in elite level physical activities had notably larger and stronger hearts compared to those who had no history of endurance training.

But despite the enlarged heart, which can serve as a sign for a heart condition, researchers found "no evidence of lasting damage, pathological enlargement or functional impairment of either the right or left ventricle in the athletes who had been doing long-term intensive elite-level endurance exercise,” said the study’s co-author Philipp Bohm, who is now working at the Cardiology Centre at the University Hospital Zürich, in a statement.

For the study, a team of sports physicians recruited 33 male elite master athletes, some of whom were Ironman participants and former Olympians. Each athlete had 30 years of extreme endurance training under their belt, which they still continued with for about 17 hours a week at the time of the study. Researchers compared them to another group of 33 men of similar age, weight, and height, but who had no experience with endurance training or competition.

Athletes who were still engaged in endurance exercise had hearts that were visibly larger and stronger. When researchers conducted cardiovascular magnetic resonance imaging (cardiac MRI) on the participants, they didn’t find any adverse health effects that could put them at risk for a heart complication.

“Our cohort of elite master athletes therefore represents our best means of investigating the long-term impact of years of competition-level endurance sport,” said the study’s lead author Dr. Jürgen Scharhag, a researcher at the Institute for Sports and Preventive Medicine, in a statement.

Those who advocate against endurance training may be basing their misconceptions on certain findings from the 2012 study. However, even those researchers highlighted the importance regular exercise has “for producing optimal cardiovascular and overall health.” Like all muscles of the body, the heart can become stronger as a result of exercise, which helps it to pump more blood efficiently through the body at maximum levels but with less strain.

Endurance training may not be for everyone, but according to the American Heart Association, incorporating physical activity into your routine is important to prevent heart disease and stroke. At least 30 minutes of moderate to intense aerobic exercise, like walking, running, or biking should be integrated into five out of the seven days of the week, along with 25 minutes of vigorous aerobic activity three times a week and moderate-to high-intensity muscle-strengthening activity at least twice a week.

Source: Bohm P, Scharhag J, Schneider G, et al. Right and Left Ventricular Function and Mass in Male Elite Master Athletes: A Controlled Contrast-Enhanced Cardiovascular Magnetic Resonance Study. Circulation. 2016.