The authors of the review paper found that long-term exercise endurance can cause abnormal structural changes in the heart and large arteries. This is especially true for people who train in marathons, ultra-marathons, ironman distance triathlons, and very long bicycle races. Initially, this could cause short-term overloading of the heart, decreased heart pumping efficiency, and elevations on blood tests that are indicative of heart damage. The good news is that these transient effects return to normal in one week.
The bad news is that if this process of excessive repetitive exercise continues over months to years, it can lead to heart disease such as abnormal thickening of the heart valves and abnormalities in the heart muscle leading to stiffening of the heart tissue. This can also lead to abnormal heart rhythms. Other problems that can be caused by excessive exercise include coronary artery disease and stiffening of large arteries. Some of the risk factors are considered hypothetical and like most research areas there are some inconsistent findings. However, it is probably wise to consider the words of the ancient Greek philosophers to do things in moderation.
Reference: O'Keefe JH, Patil HR, Lavie CJ, Magalski A, Vogel RA, McCullough PA. (2012). Potential adverse cardiovascular effects from excessive endurance exercise. Mayo Clin Proc., 87, 587-95.
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