The Vasaloppet, a grueling 56-mile cross-country ski marathon held each March in northwestern Sweden, provides a convenient venue for studying the impacts of intense, prolonged aerobic exertion on the human body, and in recent years, several studies have reported on the generally enviable fitness and longevity of the racers.
But the newest Vasaloppet-related study, published in June in The European Heart Journal, is worrisome. For it, researchers from Uppsala University and other institutions examined the health records of almost 53,000 race participants and found that the more races that someone had completed between 1989 and 1999 or the faster they had finished, the more likely they were to require hospitalization in the next 10 years for an abnormal heart beat, a condition known as arrhythmia.
For some time, exercise scientists, as well as a few highly committed exercisers and their spouses, have wondered if there might be an upper limit to the amount of exertion that is healthy, especially for the human heart. While the evidence is overwhelming that exercise improves heart health in most people and reduces the risk of developing or dying of heart disease, there have been intimations that people can do too much. A 2011 study of male, lifelong, competitive endurance athletes aged 50 or older, for instance, found that they had more fibrosis — meaning scarring — in their heart muscle than men of the same age who were active but not competitive athletes.
Now the latest Vasaloppet study and a separate study of rats running the equivalent of several rodent marathons that was published this month in The Journal of the American College of Cardiology are likely to further the debate about possible upper limits to safe exercise. Providing some counterbalance, though, is another animal study, published this month in PLoS One, that suggests that even if strenuous prolonged exercise increases the chances of some arrhythmias, it may lessen the chance of suffering fatal heart problems.
Heart rhythm is, of course, a complicated affair. Arrhythmias include both abnormally rapid heart rates, a condition called atrial fibrillation if it involves contractions in the atria, or upper chambers of the heart; and ventricular fibrillation if there is disorganized fluttering and contractions in the ventricles, or lower chambers of the heart. Then there is bradycardia, or a heart rate that is abnormally slow.
In the Vasaloppet study, 1.7 percent of the racers suffered from some type of arrhythmia in the years after their race, a higher percentage than would be expected among such a fit population. But there was little consistency in the outcomes. Most of the affected racers developed atrial fibrillation, but some had bradycardia. The one constant was that those racers, male and female, who had completed the most races or finished the fastest were more likely than slower or more infrequent participants to develop an arrhythmia pronounced enough to require medical care.
Why training for and completing a Vasaloppet or two, or five, would affect some racers’ heart rhythms years later is impossible to know from an observational study like this one, however. But the rat marathon study, which closely examined the hearts of animals that had exercised strenuously, provides notable clues.
In the study, scientists at the University of Montreal had a group of adult, male rats run at a demanding pace for an hour almost every day for 16 weeks. By the end, the rodents were lean, fit athletes, with hearts physiologically different than those of a group of sedentary control animals.
Scans and other tests showed that the hearts of the runner rats had grown larger, especially in the atrial chambers, and now interacted with the animals’ nervous systems differently than the hearts of sedentary animals. Both of these alterations are believed to be generally normal and desirable after endurance training. But the runners’ hearts also showed signs of some cardiac muscle scarring. And when the scientists attempted to induce atrial fibrillation using drugs and other methods, the running rats’ hearts were much more likely to succumb and develop abnormal rhythms than those of the animals that hadn’t strenuously exercised.
It would seem, said Dr. Stanley Nattel, the Paul-David Chair in Cardiovascular Electrophysiology and senior author of the study, that some combination of fibrosis and nervous system alterations in the distance running animals’ hearts made them more susceptible to rhythm disruptions.
Whether exactly the same process occurs inside the hearts of human endurance athletes is unknown, he said. Scientists have not attempted and likely never will attempt to induce arrhythmias in marathoners. But the Vasaloppet study and others are suggestive.
Still, as Kasper Andersen, a professor at Uppsala University and co-author of the Vasaloppet study points out, “although we observed an increased risk of arrhythmias in the more trained athletes, physical activity and exercise have positive effects on the risk of other diseases.”
And, in fact, the outcome of the third new study of athletes’ hearts is quite encouraging in that regard. In it, rats ran for eight weeks at a slightly more moderate pace and for a shorter duration most days than in the Montreal experiment.
At the end of the eight weeks, the scientists tried to induce arrhythmias in the animals. But they focused on ventricular fibrillation, which is far more lethal than atrial fibrillation. And they found that the exercised animals’ hearts resisted ventricular fibrillation better than a group of sedentary control rats did.
According to Omer Berenfeld, a professor with the Center for Arrhythmia Research at the University of Michigan and co-author of the study, the finding suggests that vigorous exercise “may be pro-arrhythmic in the atria and may be anti-arrhythmic in the ventricles.”
Or it could be that the rats in the ventricular fibrillation study, which ran for half as many weeks and at a lower intensity than those in the Montreal study, hadn’t developed the slight scarring seen among the marathon rodents and so enjoyed only exercise benefits, Dr. Nattel said.
Scientists, in other words, still have a great deal to learn about what happens to the heart after months or years of intensive endurance training. But “I do not think that it has suddenly become dangerous to exercise,” Dr. Andersen said, adding that an earlier study he worked on found that Vasaloppet participants tended to live longer than other people in Sweden.
So, for now, his advice to anyone planning a Vasaloppet finish or similar endurance training is simple and commonsensical. “Carry on as usual” he said, “but remember to listen to your body and seek a doctor if you experience any symptoms from your heart.”