Background

Moderate amounts of exercise produce a multitude of health benefits.  Both the American Heart Association (AHA) and World Health Association (WHO) now recommend 150 min per week of moderate intensity exercise for adults.

Some recent observations, though, have raised the question:  when it comes to exercise, can there be too much of a good thing?  And, more specifically, can too much exercise somehow be harmful to the heart.  With the increased popularity of adult recreational and competitive sports–particularly in cycling, running, and triathlon–there is a growing number of adults who are pursuing exercise far in excess of the AHA and WHO recommendations.  The questions surrounding the issue of “too much exercise” are very pertinent.

A few recent articles in the popular press summarize some of the findings and frame the debate:

• Can Too Much Extreme Exercise Damage Your Heart?–at Cleveland Clinic online
• Can Too Much Exercise Harm the Heart?–by Gretchen Reynolds at NY Times Well Blog
• Is Too Much Exercise BAD for the Heart?–by Anna Hodgekiss at The Daily Mail
• Extreme Exercise and the Heart–by Lisa Rosenbaum at The New Yorker

The worrisome observations have generally been made in small numbers of symptomatic athletes, in groups of athletes whose athletic history–or “dose” of exercise is poorly characterized, or in studies that were designed for some purpose other than determining the effects of “too much exercise” on the heart.

I’ve said previously that these observations deserve our attention, but that we really need more targeted investigation into this issue.  In a study just released online (ahead of print) in the medical journal, Circulation, Philipp Bohm and colleagues from the Institute of Sports and Preventive Medicine at Saarland University in Germany bring us an important new look at “extreme” exercisers.

The Study

This new study focuses on a group of 33 healthy, male, Caucasian competitive elite master endurance athletes.  This group of athletes was selected so that it included only athletes with a 10+ year continuous training history of 10+ hours per week; the average training was 16.7 hours per week and the average training history was 29 +/- 8 years.  These athletes had an average age of 47 years (range, 30-60 years).  This group of athletes included:

• Sixteen were former elite professional athletes
• One Ironman world champion and several 2nd and 3rd place finishers
• The world record holder at the long distance triathlon
• A 2nd ranked cyclist of the Vuelta a Espana (Tour of Spain)
• Six Olympic athletes in the sports of triathlon and rowing
• A former winner of the Munich Marathon.

A control group consisted of 33 healthy Caucasian men who were pair-matched for age, height, and weight.  This control group was selected to include only individuals who exercised <3 hours per week.

All of the subjects underwent a comprehensive evaluation that included:

• History and physical examination (to exclude any athlete with a history of overt heart disease, high blood pressure, smoking history, or other risk factor for heart disease)
• Resting EKG
• Cardiopulmonary exercise testing
• Echocardiography, including tissue-Doppler imaging and speckle tracking
• Contrast-enhanced cardiovascular magnetic resonance imaging (CMR).

Interestingly, none of the athletes presented with, or reported a history of, atrial fibrillation.

There were several unsurprising, and expected differences between the athletes and the controls.  First, the resting heart rate (HR) for the athletes (48 +/- 7 beats per minute) was slower than for the controls (65 +/- 11 beats per minute).  Second, the size of the athletes’ hearts was significantly greater.  The left ventricular (LV) mass for the athletes’ hearts (188 +/- 26 g) was significantly greater than the controls (124 +/- 23 g).  Similarly, the right ventricular (RV) mass for the athletes’ hearts (70 +/- 13 g) was significantly greater than the controls (49 +/- 11 g).  Among the 33 athletes, 22 met a traditional definition of “athlete’s heart,” with a heart volume of 13+ mL/kg of body weight.  As expected, the VO2 max of the athletes (60 +/-5 ml/kg/min) was significantly greater than controls (37 +/- 6 ml/kg/min).

The important results of the study were those that showed no difference between the athletes and the controls.  With echocardiography, there was no difference between athletes and controls in LV longitudinal strain or RV longitudinal strain–measures of the strength of contraction.  Using CMR, there was no difference between athletes and controls in LV ejection fraction (EF) or RV EF–again, measures of the strength of contraction.  One athlete (3%) had a LV EF slightly less than normal, at 45%.  No athletes or control subjects had abnormalities of the RV that could be suggestive of the potentially life-threatening problem of arrhythmogenic right ventricular cardiomyopathy (ARVC).  One athlete (3%) had late gadolinium enhancement (LGE) on CMR that suggested previous, asymptomatic inflammation of the pericardium, the sac in which the heart sits.  LGE analysis showed no evidence of unusual fibrosis or scarring in either athletes or controls.

My Thoughts

Kudos to the investigators here.  The study is apparently self-funded.  It’s expensive to perform this kind of testing; in the United States, the costs of this study would easily run into the many hundreds of thousands of dollars.  Kudos, too, to the editors at Circulation.  There is a tremendous bias against publishing so-called “negative” studies, where no important differences are found between study and control groups.  Many “negative” studies are left on the editing room floor–and we never hear about them.

This is an important study because it is the first to gather and study a group of long-term endurance athletes with a substantial, and defined, training load over an extended period of time.  The results deserve our attention.  At nearly 17 hours per week of exercise or training, these athletes obviously far exceeded the contemporary recommendations for 150 minutes of moderate exercise per week.  Just doing some quick math, the average cumulative exercise “dose” is more than 25,000 hours.  As I’ve said many times before, it’s worth asking the question if such an exercise pattern can be harmful to the heart over the long term.  This is a terrific group of athletes to study in order to help answer that question.

We must keep in mind that, with just 33 athletes, this is a small study.  With only 33 athletes, it’s obviously possible to miss something that would be found in the 34th athlete.  We must also keep in mind that the study only involves male athletes.  Female athletes are not immune from heart problems and deserve study, too.

It is a striking finding that no athlete was found to have atrial fibrillation–either now, or in the past.  Moreover, no athletes experienced arrhythmias during the cardiopulmonary exercise test.  A number of previous studies have reported a 2- to 5-fold increase in atrial arrhythmias among long-term endurance athletes.  Like the current study, all of those previous studies have involved small number of athletes.  None, though, have focused on athletes like these, with such extensive exercise and training histories.  In my opinion, endurance athletes broadly can take some comfort from the findings of this new study with regard to the potential risk of atrial arrhythmias.

It’s noteworthy that the LV and RV function of the athletes was no different than the controls.  One athlete had mildly depressed LV function, for reasons that are not clear.  In short, though, the study found no evidence of cardiac damage–at least, in terms of the pumping function–that accrued over the long term.  We know that there is some depression of LV and RV function immediately after an intense bout of exercise (eg, marathon, long-distance triathlon, long-distance cycling event), but we also know that these changes resolve within days to weeks afterward.  The current study argues against the hypothesis that repeated episodes of intense exercise (ie, many marathons or triathlons over a lifetime) might result in a decrease in LV or RV function.  Again, this is encouraging news for endurance athletes.

Finally, the CMR and LGE results are important.  Aside from the 1 athlete with possible previous pericarditis, there were no worrisome findings of fibrosis or scarring that might be attributable to injury from repeated episodes of intense, strenuous exercise.  These LGE findings are at odds with some observations of unexplained fibrosis in other cohorts of long-time runners, even if the consequences of such findings remain uncertain.  This area of investigation deserves further attention.  For now, I’d say that long-time participation in endurance sports does not necessarily result in unexplained fibrosis in the heart.

Related Posts:

1. Thoughts on the recent VeloNews article
2. PRO/CON:  Prolonged intense exercise leads to heart damage
3. Do elite athletes live longer?