Phun Physiology: Is excessive endurance exercise bad for the heart?

Maybe. This according to a study published this past week (June 4) in Mayo Clinic Proceedings by a team of cardiologists that bases its claims on a review of a number of animal and human studies.

Because the blogosphere was quick to pounce on the story (here, here, here, and here), I thought a short commentary might be appropriate. One that addresses the original article and whether it pertains to endurance cycling known as randonneuring.   

First of all, the researchers are not referring to daily, moderately intense workouts. Or to 2-3-mile runs four times a week. Nor are they referring to short intense interval workouts 2-3 times per week. In fact, the researchers acknowledge that these kinds of exercise regimens are extremely beneficial and may even add seven years to one’s life.

So what kind of activities does this particular team of cardiologists believe qualify as “excessive endurance exercise?” According to the article,   

[C]hronic training for and competing in extreme endurance events such as marathons, ultramarathons, ironman distance triathlons, and very long distance bicycle races . . .

Cycling? In an interview, the lead researcher, James O’Keefe, mentions the Tour de France and 200-mile bicycle races as examples of excessive endurance exercise.

Inferring from the article, it seems that for an endurance exercise regimen to qualify as “excessive,” it must be 1) almost daily, 2) hours at a time, 3) intense (racing), and 4) chronic (yearly).

The researchers have hypothesized that over time in some athletes excessive endurance exercise begins to remodel the heart in unhealthy ways. I’ve lifted the types of heart changes and their timelines from the article: 

. . . transient acute volume overload of the atria and right ventricle, with transient reductions in right ventricular ejection fraction and elevations of cardiac biomarkers, all of which return to normal within 1 week.

Over months to years of repetitive injury, this process, in some individuals, may lead to patchy myocardial fibrosis, particularly in the atria, interventricular septum, and right ventricle, creating a substrate for atrial and ventricular arrhythmias.

Additionally, long-term excessive sustained exercise may be associated with coronary artery calcification, diastolic dysfunction, and large-artery wall stiffening.

The phrase “patchy myocardial fibrosis, particularly in the atria” refers to tissue scarring in the heart’s two upper chambers—the atria. The heart’s electrical circuitry known as the pacemaker (SA-node) is located in the right atrium. Consequently, scarring there increases the likelihood of arrhythmias.

One type of arrhythmia known as atrial fibrillation can be particularly problematic, if not diagnosed and treated, since it can lead to sudden death. I shall say more about atrial fibrillation below.

  

It is important to note that the researchers view their thesis regarding the adverse cardiovascular effects of long-term excessive endurance exercise as tentative and that not all individuals may be susceptible to the aforementioned pathologies. In fact, the researchers note that “lifelong vigorous exercisers generally have low mortality rates and excellent functional capacity.”

It is extremely important to keep in mind that the occurrence of SCD [sudden cardiac death] during marathons, triathlons, and collegiate athletic events is rare and should not deter individuals from participating in vigorous ET [exercise training]; the benefits of regular PA [physical activity] to the individual and to society as a whole far outweigh potential risks. At the same time, long-term training for and competing in extreme endurance events may predispose to CV [cardiovascular] issues that are not seen in more moderate forms of PA.

Now the question: What does this study have to do with randonneuring? Nothing, unless randonneurs are also involved in daily, long-distance racing.  

But what about other studies? Of direct concern for randonneurs is the potential for atrial fibrillation, the chance of which increases in some individuals, who may have trained for but never competed in endurance-type races. This is the contention of Dr. Luis Mont, a Spanish physician, who 

reports that atrial fibrillation is more frequent in middle-aged individuals who formerly took part in competitive sports and continue to be active, or simply in those involved in regular endurance training without having actually participated in competitive sports.

Dr. Mont adds that

long-term endurance sport participation may well increase the incidence of cardiac arrhythmias, particularly atrial fibrillation, atrial flutter, sinus node dysfunction, and right ventricular premature beats.

Again, this pertains to a small proportion of the general population of endurance athletes. 

Here is a personal story—worth reading—of one cyclist who developed atrial fibrillation while on a bike ride and the outcome.

While many athletes use heart monitors as training aids, the usefulness of such devices extends well beyond training to include the detection of abnormal physiological states such as sudden spikes in heart rate—a possible sign of atrial fibrillation.

Should inexplicable heart-rate spikes occur in someone with whom you are riding, he or she should be strongly encouraged to get it checked out. Atrial fibrillation is something that is easily diagnosed and in many cases is treatable.

  

Be aware and be safe.

Let’s ride!

Update: Here is a link to Iron Rider, which has an important and related blogpost on atrial fibrillation in endurance athletes.