Some research articles seem to warn people not to exercise too long or too hard, but the overwhelming scientific and epidemiological evidence is that vigorous exercise strengthens a healthy heart and helps to prevent heart disease. Elite endurance athletes who exercise long hours each day outlive their less-active peers (Med Sci Sports Exerc, Feb 1993;25(2):237-44), as do Tour de France cyclists (Eur Heart J, Oct 2013;34(40):3145-50). A review of fourteen articles showed that endurance athletes live longer than the general population and suffer far less from heart disease (J of Sci and Med in Sport, July 2010;13(4):410–416). However, three studies raise the possibility that extreme exercise such as running a marathon could possibly increase risk for heart problems, particularly in people who are not adequately trained for their events.
Elevated blood enzymes: In one study, 69 runners tested after finishing a marathon had markedly elevated levels of troponin, an enzyme in the bloodstream that rises after a person has a heart attack. None of the runners actually had a heart attack (Circulation, December 3, 2018).
Increased plaques in arteries: Master athletes who compete in endurance sports have more plaques in their arteries than men of the same age who do not exercise (Circulation, 2017;136:138-148). It is true that most heart attacks begin with plaques in arteries leading to the heart breaking off to block the flow of blood to the heart muscle, but any form of exercise stabilizes plaques so they are far less likely to break off and lead to a heart attack (Med Sci Sports Exerc, Nov 2015;47(11):2251-6). Exercise also helps to prevent clotting, which is why regular exercisers have a much lower risk for heart attacks than those who do not exercise.
Increased risk for irregular heartbeats: Older athletes in endurance sports are at increased risk for developing atrial fibrillation, in which the upper part of the heart starts to beat irregularly (J Atr Fibrillation, Dec 2015;8(4):1309; EP Europace, May 1, 2008;10(5):618–623). This can cause clots to form in the upper part of the heart and the clots can cause heart attacks and strokes.

Elevated Troponin to Diagnose Heart Muscle Damage
When any muscle is exercised vigorously, it releases enzymes from within its cells into the bloodstream. For example, all skeletal muscles can release SGOT and SGPT into the bloodstream. In the same way, vigorous exercise can cause heart muscle cells to release troponin enzymes into the bloodstream. The authors of a recent study emphasize that it takes far more intensity of exercise to raise blood levels of troponin significantly, so that mild exercise does not cause high blood levels of troponin and that the novice runner, rather than the elite athlete, is the one who is at the highest risk for heart damage during extreme effort such as running a marathon (Circulation, December 3, 2018). The same applies to skeletal muscle damage. The highest blood levels of skeletal muscle enzymes occur in people who attempt major intensity and endurance events for which they are not adequately trained. High blood levels of heart troponin after moderate exercise can be a sign of heart damage that needs immediate medical attention (European J of Prev Cardiology, Feb 10, 02017). The greater the stress of exercise on the heart, the greater the chance of suffering heart damage. Heart attacks occur in 1 in 50,000 runners and the vast majority occur in marathons, not in shorter or less demanding races, and they usually occur in men over 35 years of age.

Increased Plaques in Arteries
Competitive older endurance athletes may have more plaques in their arteries than non-exercisers do, but they have the type of plaques that are far less likely to break off and cause heart attacks (Circulation, April 27, 2017;136:138-148; May 2, 2017;136:126-137). A heart attack is usually caused by a plaque breaking off from an artery leading to the heart, followed by bleeding and clotting. Then the clot extends to obstruct all blood flow to that part of the heart muscle and it dies, which is a heart attack. Exercise helps to prevent heart attacks by:
• stabilizing the plaques so that they are far less likely to break off,
• reducing the risk of clotting,
• widening the arteries, and
• increasing the ability of coronary arteries to widen even more during exercise to send more blood to the heart to prevent heart muscle damage. A 90 percent blockage of an artery does not cause a heart attack because blood can still flow through and the heart muscle can still get oxygen

A CT scan called a calcium score can show the difference between stable plaques that are safe and those that are unstable and more likely to break off to lead to heart attacks (American Journal of Roentgenology, March 2015;204(3):W249-W260). Signs of plaque stability include extensive surface calcification, less fat-rich areas and increased fibrous areas.

Exercise does not increase plaque formation. Men over 60 who had run marathons for 26 to 34 years had plaques in their arteries that were related to their own risk factors for heart attacks and not to the number of miles or marathons they had run (Med & Sci in Sports & Ex, July 17, 2017). Plaques in arteries are not caused by endurance training, but are caused by other factors such as a pro-inflammatory diet, high blood pressure, high cholesterol and previous use of tobacco. See Exercise and Plaques

Irregular Heartbeats in Older Athletes
About twenty years ago, doctors noted that men over 80 who competed in cross country ski races longer than 100 kilometers (64 miles) were at increased risk for irregular heartbeats called atrial fibrillation (BMJ, Jun 13, 1998; 316(7147): 1784–1785). Extensive studies have since confirmed this increased incidence of atrial fibrillation in healthy older world-class endurance athletes (Scan J Med & Sci in Sports, Nov 21, 2013;24(4)). However, most of these men are healthy athletes and have no obvious problems with their hearts. Non-exercisers with atrial fibrillation are at significantly increased risk for forming clots in the less active upper part of their hearts (the atrium), and then the clots can travel to the brain to block the flow of blood to cause a stroke. One study found no evidence of irregular heartbeats from damage to the right ventricular heart chamber in elite lifetime-endurance athletes (Circulation, May 17, 2016;133(20):1927-35). However, at this time, doctors may treat atrial fibrillation in most patients, including healthy athletes, with:
• drugs to slow heart rate,
• drugs to prevent clots,
• electric pacemakers to change the maximum heart rate,
• in extreme cases, electrical waves to destroy heart nerves in an effort to change the irregular heartbeat.

We do not know if the healthy older athletes need these treatments, but rightfully, doctors are afraid not to treat the athletes because of the possibility of strokes. Most doctors prescribe anti-clotting medicines for almost all of their patients who suffer atrial fibrillation, but we do not know if the older athletes are as susceptible to heart problems as are non-exercisers. See Irregular Heartbeats in Senior Athletes and Exercisers

My Recommendations
Exercisers live longer and have far less heart damage than non-exercisers. I believe that everyone should exercise to help prevent diseases and prolong lives. However, if you want to start or increase the intensity or duration of your exercise program, if you have questions about your health, have any specific medical conditions or have heart attack risk factors, check with your doctor.

CAUTION: Intense exercise can cause a heart attack in a person who has blocked arteries or heart damage. Check with your doctor before you start a new exercise program or make a sudden increase in the intensity of your existing program.

Checked 10/7/22