Tuesday, September 16, 2014

Can Too Much Exercise Harm the Heart?

by Larry Creswell, M.D.

Can too much exercise harm the heart? It’s an interesting question -- and probably an important question for many of the readers here at Endurance Corner. This question has received attention recently because of news reports in the popular press , so I thought I’d review the available data, offer some of my own thoughts and conclusions, and let you decide for yourself if you can have “too much of a good thing.”

An article written by Jonathan Knowles in the online edition of the New York Times (NYT) on March 9, entitled “When Exercise is Too Much of a Good Thing” caught my eye. Judging from the questions I received in the following week -- and more than a little discussion among fellow athletes and colleagues since then -- the article caught the eye of a good many athletes. Knowles shared the findings of a couple recent scientific studies that raise the possibility that significant damage to the heart might occur with long-term participation in endurance-type athletics.

Some Important Observations
I’ll review the details of the studies from the NYT article, but first let me mention some observations that are important to set the stage for this discussion.

  1. First, the general issue of long-term consequences to the heart from endurance athletics has not been well studied. There are few reports and the available studies have included very few athlete subjects.
  2. We do know from the available studies that endurance athletes are more likely in the long term to develop atrial arrhythmias (that originate in the upper chambers of the heart). The exact cause is not known, but speculation centers around enlargement of these chambers and possible structural changes in the heart walls that occur over time.
  3. We also know that endurance athletes are probably more likely to have complex ventricular arrhythmias (originating from the pumping chambers of the heart), but the cause and significance of these arrhythmias is still not understood.

A Hypothesis
These observations lead us to the relevant hypothesis for this discussion. Some investigators have hypothesized that repeated episodes of strenuous exercise cause injury to the heart muscle (the walls of the heart) that leads to scarring in the heart muscle (which we call fibrosis) -- and that areas of scarring in the heart walls predispose athletes to electrical problems that manifest as arrhythmias. This is a reasonable hypothesis because we know that fibrosis caused by other forms of injury to the heart (such as myocardial infarction, or “heart attack”) do indeed predispose patients to the development of arrhythmias by this same mechanism.

Until recently, though, it hasn’t been possible to examine patients (or animal subjects) for very small areas of heart muscle fibrosis except at autopsy. For that reason alone, the hypothesis remained conjecture. New and continued development of cardiac magnetic resonance imaging (MRI) has changed all that. With the use of the intravenous contrast agent, gadolinium, a technique known as late gadolinium enhancement (LGE) can be used to detect very small areas of fibrosis within the walls of the beating heart. This is a technique that can now be used experimentally as well as clinically.

The Scientific Studies
The first important scientific study on this topic was reported in 2009 by Frank Breuckmann and colleagues at the West German Heart Center in Essen, Germany (Breuckmann F, Mohlenkamp S, Nassenstein K, et al., 2009). These investigators studied 102 healthy male runners, aged 50 to 72 years old, who had completed at least five marathons in the previous three years. The runners were compared to a similar control group of 102 healthy non-runners. The investigators found more runners (12%) had abnormal areas of fibrosis than non-runners (4%). This finding received much attention, but the difference between the two groups was not statistically significant (p>0.07). Interestingly, during the following two years after the MRI images were made, there were significantly more severe coronary events (such as heart attack) among the subjects with the abnormal areas of fibrosis. This report has rightfully been criticized, though, because the long-term athletic history of the subjects was not defined and because there was a larger proportion of smokers among the runners.

A more recent study was mentioned in the NYT article. In this report (Wilson M, O’Hanlon R, Prasad S, et al.), Wilson and colleagues from the U.K. describe a cardiac MRI study of 12 lifelong male veteran endurance athletes (50 to 62 years old) and 20 age-matched control subjects. The veteran athletes reported a continuous history of endurance athletics of between 35 and 52 years at the time of enrollment into the study. Indeed, the participants were recruited from a running club whose membership required completion of 100 marathons. These investigators found abnormal heart fibrosis in 50% of the athletes and in none of the control subjects. They concluded that additional well-controlled studies are needed to determine if certain individuals are placing themselves at undue risk with lifelong participation in endurance sports.

The last study we’ll consider today was conducted by Benito and colleagues in Barcelona and reported earlier this year in the prominent journal Circulation (Benito B, Gay-Jordi G, Serrano-Mollar A, et al., 2011). These investigators established a rat model of long-term exercise, where the animals ran vigorously for periods of up to 16 weeks (perhaps 10 years on a human scale). They found that over the 16-week period, the rats developed fibrosis of the heart walls that was similar to the findings reported from the cardiac MRI studies above. It also turned out that the animals with the abnormal fibrosis were also more likely to have inducible arrhythmias. Perhaps most interestingly, they found that during an eight-week period of rest after the exercise period, that the fibrosis disappeared.

Some Thoughts on the Evidence
I’ve come across several thoughtful pieces about these findings online in recent weeks, including those by Kurt Harris, M.D., a blogger about paleonutrition and the culture of science and medicine; John Mandrola, M.D., a blogger about cycling, cardiology, and the practice of medicine; and Chris Carmichael, an endurance sport coach and prolific writer. No doubt there are others as well. I’d encourage you to read their takes on the subject. You’ll find that there’s a variety of interpretations out there.

Here’s my take. I started the year here at Endurance Corner with a column devoted to the health benefits of exercise. These benefits are simply undeniable. Nobody should stop exercising. Is it possible that too much exercise can harm the heart? I think the answer is yes. The scientist in me recognizes that there can be “too much of a good thing” in almost all biological and physical systems. Exercise and the heart should be no different. There may well be some threshold of accumulated exercise, beyond which there is heart damage. The recent studies that I’ve mentioned above make some interesting and preliminary observations that warrant further study. With millions of athletes in this country alone participating in endurance sports, we deserve to learn more about the long-term consequences to the heart of those activities. For the moment, though, there just isn’t enough information available to dissuade me (or make recommendations to other endurance athletes) to stop exercising. We’ll stay tuned for more information as it becomes available.

References
Benito B, Gay-Jordi G, Serrano-Mollar A, et al., (2011). Cardiac arrhythmogenic remodeling in a rat model of long-term intensive exercise training. Circulation 2011; 123:5-7.

Breuckmann F, Mohlenkamp S, Nassenstein K, et al., (2009). Myocardial late gadolinium enhancement: Prevalence, pattern, and prognostic relevance in marathon runners. Radiology 2009; 251:50-57.

Wilson M, O’Hanlon R, Prasad S, et al., (n.d.). Diverse patterns of myocardial fibrosis in lifelong, veteran endurance athletes. J Apppl Physiol (in press, published online in advance).


Larry Creswell, M.D., is a cardiac surgeon and Associate Professor of Surgery at the University of Mississippi Medical Center in Jackson, Mississippi. In addition to his regular column on Endurance Corner, he maintains The Athlete's Heart blog to offer information about athletes and heart disease in an informal way and to encourage exchange and discussion that will help athletes build a heart-healthier lifestyle. You can contact him at lcreswell@surgery.umsmed.edu.
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