The recent deaths of Olympic Trials marathoner Ryan Shay and ultrarunner Micah True (Caballo Blanco from Chris MacDougal’s Born to Run) (11), both of whom were elite level or veteran endurance athletes, revealed that even high-level athletes are not immune from sudden cardiac death. The autopsies on both revealed some degree of myocardial fibrosis or scarring, which fostered the idea that too much endurance activity actually may be bad for your heart. Can excessive endurance activity really damage the heart?
It has long been known that regular moderate exercise has been shown to have many benefits. Exercise reduces the risk of heart disease, hypertension, heart failure, depression, and diabetes (6). Those who perform regular exercise live longer and are more functional for the duration of their lives (4). The American College of Sports Medicine as part of its Exercise is Medicine® initiative recommends 150 minutes per week of moderate physical activity (1). A recent study in Taiwan demonstrated a small but measureable benefit from exercising as little as 90 minutes per week, with minimal additional benefit from exercising longer than 1 hour per day (24). Similarly, in a study of runners, those who ran up to 20 miles per week received a mortality benefit, whereas those who ran more mileage per week did not gain a significant additional benefit (9). It seems that a dose-response relationship between exercise and benefit gained exists and that some is good but more may not always be better.
The running boom of the 1980s created a culture of marathoners as endurance junkies, and those who completed a marathon were considered extreme. However, in the last 20 years, the number of people participating in marathon races has increased 20-fold (23). With marathons becoming more “mainstream,” endurance aficionados have moved toward iron-distance triathlons and ultramarathons to distinguish themselves as hardcore endurance athletes. These ultradistance events require many hours of training per week, and the competitions last hours to even days. Multisport or ultraendurance athletes indeed may be exercise addicts who engage obsessively in a pattern of compulsive and excessive daily exercise and may often continue to train despite pain or other signs from their body, which may further increase the risk that excessive exercise may lead to health consequences (25).
Long-term endurance training leads to adaptations termed the “athlete’s heart.” These adaptations include an increased left and right ventricular volumes, as well as increased left ventricular wall thickness and increased left atrial size (17). These adaptations typically are at the upper end of the “normal” range and rarely become grossly abnormal. Also, because in endurance athletes the amount of blood the left side of the heart ejects is maintained, these adaptations have long been viewed as benign (10). However, recent research would suggest that excessive endurance training actually may lead to damage to the heart.
Many studies support that blood cardiac troponin levels, a chemical released by heart cells when damaged, increase as much as 50% after marathon running (14,15,20). It is unclear whether the increase in troponin actually is indicative of heart muscle damage or whether it occurs transiently after extreme endurance events or secondary to some of the remodeling changes that occur in the heart with endurance training (20). Also, several studies of veteran marathon runners demonstrated an increase in coronary artery plaque (measured with coronary artery calcium score) compared with nonrunners (12,19). When looking at two commonly used indices that evaluate coronary health, both the troponin and coronary artery calcium score were found to be abnormal in those with a history of prolonged endurance training.
Although many discussions about sudden cardiac death in sports focus on left ventricular abnormalities such as those found in hypertrophic cardiomyopathy, it seems that right-sided remodeling may lead to chronic structural changes in the endurance athlete. Several studies have demonstrated an acute dilation of the right atrium and right ventricle as well as decreased right heart output that occurs immediately after prolonged endurance events but will return to normal within 1 week (8,22). What proved concerning was that one in eight of these athletes demonstrated scarring of the heart muscle on magnetic resonance imaging of the heart. Those with scarring had the longest cumulative experience in endurance events (8). This suggests that prolonged repeated endurance training acutely causes reversible changes to the right ventricle but, in the long-term, may lead to myocardial fibrosis or scarring.
Well-trained endurance athletes develop electrical changes on their electrocardiogram, such as slowing of the heart and signs of enlargement of the left heart, which when interpreted by common computer algorithms will register as abnormal but may indeed be normal in endurance athletes (5). Elite high-level endurance athletes have a much higher incidence of irregular heartbeats (atrial fibrillation) than their age-matched nonathletic counterparts (13). These may be caused by the excessive parasympathetic nervous system tone (resulting in a lower resting heart rate), increased inflammation, scarring of the heart muscle, or the dilation of the atria that occurs with excessive endurance training. Other potentially life-threatening heart rhythms can originate from the “abnormal” right ventricle or the muscular septum between the two ventricles (2,3). It also is thought that the area of heart muscle scarring found in some veteran endurance athletes may be the origin for these potentially dangerous heart rhythms (16).
Why do these potentially dangerous changes to the heart occur? It is thought that individuals who do long-term endurance training and maintain an elevated heart rate, blood pressure, and cardiac output may generate inflammation and free radicals. When maintained for a long-term repeated basis, these free radicals may lead to some of the abovementioned maladaptations (8). Furthermore, this repetitive cycle may stimulate the immune system to secrete signals that cause adverse changes in the cardiac muscle leading to fibrosis (7). It has been demonstrated by previous research that the acute structural changes caused by endurance activity resolve within a week (8) and perhaps allowing for a recovery period between intense endurance activities may mitigate the risk of developing maladaptations.
How then should we advise people who desire to pursue endurance sport? It is clear from many studies that regular exercise reduces overall risk of death; however, the risk of sudden death may rise during vigorous exercise in susceptible individuals (21). Those who participate in long-term endurance exercises still may have risk factors despite regular aerobic exercise. Exercise will not protect you if you have existing cardiovascular risk factors such as high blood pressure, high cholesterol, diabetes, smoking history, or strong family history of cardiovascular disease. Good advice for people older than age 40, or those with existing risk factors, is to consult with a physician before beginning (and maintaining) an extreme endurance training program. Although standard exercise stress testing has not been found to be helpful in screening extreme endurance athletes, coronary calcium scoring, although more expensive, may have a role as a method for risk stratification. If the calcium score is greater than 100, the athlete may be at higher risk for a cardiac event during exercise (18). This doesn’t mean that all athletes with a high calcium score should not exercise, but an elevated coronary calcium, especially in light of other existing risk factors, may necessitate advising to run fewer than 20 miles per week and to keep intensity moderate rather than running longer or more extreme.
In conclusion, although we have more information regarding the potential long-term effects of excessive endurance activity on the heart, there still is much to learn. Further research is needed to determine the exact nature of the risk. It seems that some individuals may be more susceptible to long-term damage, whereas others have no long-term risk. The evidence to date would suggest that extreme or obsessive exercisers, as well as those with existing cardiac risk factors, are at more risk for cardiac issues than those who practice moderation. Because the benefits of regular exercise do not seem to increase after 1 hour per day, and the risk level may indeed rise after this threshold, susceptible individuals should be advised to adhere to moderate exercise because there is plenty of evidence to support the benefits of long-term moderate physical activity.
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