Long-term, high-intensity athletic training might raise the risk for irregular heart rhythms, called arrhythmias.
Exercise remains essential for overall heart health.
Those who train for long periods also face an increased risk.
Endurance athletes who train and race frequently may experience a high rate of unusual heart rhythms called arrhythmia, found a new study on cross-country skiers. Arrhythmia, which are often harmless, can sometimes lead to strokes and other serious problems.
Experts remain unsure what to make of the results. Exercise is known to prolong lifespan and to improve all sorts of measures of health, including the heart. And certain kinds of arrhythmia, a type of flutter called atrial fibrillation, are very common.
Still, the study suggests there may be a point at which a lot of training becomes too much. At the very least, serious athletes should be aware of the potential for their hearts to behave strangely.
"I would be worried if people thought cross-country skiing, for example, to be dangerous. That would be an unlucky headline," he said. "We're looking at extreme amounts of training. Most people need to get off the couch and exercise a little bit."
Exercise has long been associated with a boost in health, including lower risks of heart disease. Heavy training can change the structure of the heart, making it bigger and adding extra connective tissue.
Those adaptations help the heart pump blood more efficiently. But some small studies have also shown evidence of higher rates of arrhythmia in serious athletes, opening up lots of questions.
Does exercise, for example, cause the heart to develop irregular rhythms, or do people who are prone to arrhythmia end up becoming athletes? And are there certain kinds of sports or types of training that have the biggest effects?
Previous research has shown that people who ski the Vasaloppet are healthier than the general population and have a mortality rate that is half that of less athletic people. Still, because the race includes such a wide range of athletes, it also offered a good opportunity to study the effects of extreme athleticism on arrhythmias.
To investigate, the researchers looked at data from a decade's worth of races. They rated the skiers' fitness level based on how close they finished to the winning time and how many times they had previously competed in the race. Then they looked at national health records to see how often participants ended up in the hospital for arrhythmia problems.
People who had done seven or more Vasaloppets were nearly 30 percent more likely to end up seeking help for arrhythmias compared to people who had only done the race once, the researchers reported last week at a meeting of the European Society of Cardiology in Paris.
And skiers who finished close to the winning time were 37 percent more likely to be hospitalized for arrhythmias than were people who finished in more than double the winning time. The researchers made sure to take into account age, income and other factors so that they were looking only at speed and experience.
The link between fitness and arrhythmias were strongest for athletes who were younger than 45. And the vast majority of arrhythmias fell into two groups: Atrial fibrillation -- a fast, irregular rhythm, and bradyarrhythmias, a particularly slow rhythm. Neither type is among the most dangerous.
Despite the new evidence, plenty of research shows that even extreme exercise offers far more benefits than risks to the heart, said Benjamin Levine, a cardiovascular physiologist at the University of Texas Southwest Medical Center and Texas Health Presbyterian in Dallas.
In his research, he said, he has found little difference between the hearts of elite Master's athletes, who have spent a lifetime training intensely, and healthy people who are decades younger.
"There are dramatic positive changes, both cellularly and structurally," Levine said. "High-intensity exercise is good, and we are even starting to use it for patients with heart disease."
For staying healthy, Levine recommended working out four or five days a week, with one interval session a week and a couple days of strength training.
"That's sufficient to preserve a lot of youthful cardiovascular function, and it keeps the risk of dying from cardiovascular disease as low as you can get from that strategy," he said. "That's a prescription for life."