It would be difficult to find a sane and rational person who would disagree with the statement that ‘regular aerobic exercise is good for you.’ From the earliest studies by Drs. Jeremy Morris and Ralph Paffenbarger in the 1950s up to the present time, there have been thousands of publications documenting the beneficial effects of regular aerobic exercise on health and well-being. A partial list of these benefits can be found in Table 1.
Current public health guidelines for aerobic activity recommend a minimum of 150 minutes per week at a moderate intensity, or a minimum of 75 minutes per week at a vigorous intensity. Heart rate during exercise is used to differentiate moderate versus vigorous intensity. There is an established dose:response relationship between the amount of aerobic exercise and potential health benefits. Therefore, these same guidelines tell us that up to 300 minutes per week at a moderate intensity, or 150 minutes per week at a vigorous intensity is likely to yield greater health benefits than just meeting the minimal guidelines. While very few people would disagree with these guidelines, some important questions arise: 1) if doing 150 minutes per week is much better than doing nothing at all; and doing 300 minutes is better than doing 150 minutes, then does this relationship continue beyond 300 minutes per week? 2) What about ultra-endurance athletes who are spending ~900 minutes (15 hours) or more per week doing aerobic training; with some of it being quite vigorous?
To help answer these important questions, O’Keefe and colleagues recently published a review article on the potential adverse cardiac effects of excessive endurance training and competition. First, they note that sudden cardiac death (SCD) during marathon running is very rare, with 1 event per 100,000 participants. SCD during triathlons is twice that of marathons (2 events per 100,000 participants), with most events taking place during the swim portion. In younger people who experience SCD during endurance events, the most common cause is a genetic defect in heart structure or function (example: cardiomyopathy). In older people who experience SCD during endurance events, the most common cause is coronary heart disease (blockages in the arteries of the heart), resulting in heart attack.
The next portion of the O’Keefe paper focused on possible mechanisms by which excessive endurance training might be potentially harmful to the heart. They noted that various markers of cardiac damage appear in the blood of ~50% of participants during and following a marathon race. However, these changes are temporary; with values typically reverting back to normal within a few days. The clinical significance of the temporary increase in these markers of cardiac damage is unknown, and some argue that this might even be a normal response to long-term endurance training. Indeed, following a strength training session, markers of skeletal muscle damage appear in the blood temporarily; with values typically reverting back to normal within 48-72 hours. As long as the skeletal muscle damage is not extensive and does not cause severe muscle soreness, this is recognized as a normal, benign and beneficial response to strength training.
O’Keefe also cites studies where adverse structural changes in the heart have occurred with excessive endurance training, noting that ‘some individuals may be prone to the development of chronic structural changes…..that might predispose to serious arrhythmias such as atrial fibrillation and/or ventricular arrhythmias…….long-term training for and competing in extreme endurance exercise may lead to myocardial fibrosis and remodeling in a small subgroup.’
It is actually very well-documented that atrial fibrillation (a type of irregular heartbeat) is 5 times more common in endurance athletes than in the general population. I can personally attest to this. During my competitive running days (1972-1986) I put in nearly 35,000 miles and have stationary cycled well over 100,000 miles on a Schwinn Air-Dyne since that time. I’ve had brief bouts of atrial fibrillation a few times a year for the past 20 years. While it is annoying, my condition is benign. I cannot prove cause and effect between my many years of endurance training/competing and my atrial fibrillation, but I am reasonably sure that the former contributed in some way to the latter.
Putting Things in Perspective
In closing, a few points need to be strongly emphasized.