Is Endurance Exercise Bad for Your Heart?

jogger - vascular system

It’s intuitive that being out of shape increases the risk of a heart attack and exercise decreases this risk.  However I also think many of us wonder about how much is too much. I have been practicing preventive cardiology for 20 years and, being based in Boulder, CO have had the privilege of seeing a large number of elite masters age athletes in my practice. 

In this blog post I would like to talk about why extreme endurance exercisers (mainly runners) can have more arterial plaque and whether or not this increases the risk of a heart attack.¹ I say mainly runners because we have the most data on them compared to cyclists, rowers or cross-country skiers. Many of us in the field of cardiology have been trying to figure out why some athletes get plaque and if this increases the risk of heart attack. Exercise is supposed to be good for you but does extreme exercise increase risk and at what level?

Too little physical activity is the problem for the majority of Americans, however in the last 10 years there have been increased reports of increased arterial plaque in experienced male endurance athletes.  Women are not typically studied because they are at lower cardiovascular risk (although I really think this is a mistake). Women develop atherosclerosis about 10 years later than men, often at an accelerated pace, typically after menopause. 

Male Endurance Athletes May Have More Arterial Plaque

Male endurance athletes may have more arterial plaque as determined by a coronary artery calcium test (CAC), also generally known as a “heart scan”.² A CAC is like a fancy x-ray of the heart, using computed tomography (CAT scan). Arteries develop calcium deposits at varying degrees as a result of atherosclerosis. The calcium is easy to see with an x-ray, just like bones are easy to see. Generally speaking: the more plaque, the more calcium and vice versa.  The CAC test is inexpensive and I like men over 40 and women over 50 to get one. Arterial plaque causes most heart attacks…especially when it ruptures. 

Vulnerable plaque is the worst…its unstable and can rupture clogging and artery downstream. Both higher CAC scores AND reduced cardiovascular fitness are strong predictors of future cardiovascular events such as stroke and heart attack.  Coronary calcium is a direct measure of arterial plaque. The scan cannot tell the difference between stable and unstable plaque but is still a good test for many reasons. I do a carotid intima media thickness (CIMT) ultrasound test and other tests to see if the plaque is the risky type.

The association between arterial plaque and extreme exercise began around 2008 when it was found that there was more coronary calcium in runners than non-runners. It was not clear as to why.  It was postulated that it could be personality type, past diet and/or lifestyle factors. Interestingly a follow-up study actually showed that plaque could increase in runners who did the Race Across America proving a direct link between extreme exercise and atherosclerosis. The Race Across America is a 140-day running race. Athletes run 25 miles a day.  Subjects were tested before the race and afterwards. Eight of the ten runners finished the race. One woman and 3 of men had plaque before the race and the plaque increased in all 4 of these runners. None of the runners without plaque prior to the race developed any plaque. This study supported the possibility that extreme amounts of exercise could accelerate plaque growth directly.³   

Why Would Exercise Increase Arterial Plaque?

One possible cause could be that exercise increases parathyroid hormone and parathyroid hormone increases blood levels of calcium. Another possibility could be that inflammation from muscle damage might also damage the endothelium. The endothelium is the first line of defense for an artery. When it is disrupted or damaged, plaque begins. I think runners are at greater risk because of the high impact nature of the sport, especially as compared to cycling or swimming. I also think that mechanical stress from millions of foot strikes could play a role. The heart is already beating quite a bit. The additional shaking from running is possibly causing microscopic endothelial damage.

Past Studies of Exercise on the Heart

In 1899 (that is not a typo) there was a study published called “Adverse Effect of Violent and Prolonged Muscular Exercise Upon the Heart.”  In this study, it was postulated that it was inflammation from muscle damage that was the problem. Through studying runners at the Boston Marathon using blood testing it was shown that post race inflammation markers were similar to someone who is having a heart attack.  As part of my masters athlete physicals I check these inflammation markers along with other blood tests such as cardiac enzymes and other factors associated with cardiac strain and endothelial health.

The Importance of Calcium Density in Endurance Athletes

Very fit extreme exercisers can get increased calcium density in the plaque which makes it more stable.  This is a complicated issue because although increasing coronary calcium score is associated with more risk, increased density of the plaque decreases this risk. Studies have found that athletes generally have increased calcium density and more stable plaque.4  It’s not easy to know if this is happening or not without further testing. Additionally, his brings up another contentious issue regarding the incidence of heart attacks during races.  Although small, there is an increased risk of heart attack during a race, but the fitter you are the lower the risk. By following a balanced long-term training plan the body will adapt to the demands of the sport.  I would be concerned about a first time marathoner over 40 with known vascular disease, high cholesterol and a high stress job who has not been training for years. All data support the fact that fitter populations have less “all cause mortality” which obviously includes fatal heart attacks.

Additionally, when we exercise our hearts become stronger and the blood supply increases due to more vascularization.  This is called collateral circulation and is an adaptive mechanism. The heart, being a muscle, responds a lot like skeletal muscle in that the harder you work it stronger it gets.  With increased oxygen requirements, the heart responds by growing more arteries, often bypassing diseased arteries. The benefit of this is that if there were a heart attack the chance of survival would be much higher because there is more blood supply and oxygen.

What Do the Latest Studies Say About Intense Exercise and Cardiovascular Health?

In the most recent study published in the Journal of the American Medical Association (JAMA), Dr. Benjamin Levine helps clarify the issue.  He studied 21,000 healthy men ages 40-80 between 1998 and 2013. The study subjects underwent coronary calcium testing. Most were runners but some were also swimmers, cyclists, rowers and triathletes.  He found that extreme exercise does not increase risk of heart attack and stroke. This is certainly good news for those of us who like intense exercise. Paul Thompson MD at Hartford Hospital is done a lot of work on this as well. When the data is put together he found that even with higher coronary calcium fitness mitigates the risk. 

Male Endurance Athletes Have Less Rupture Prone, Dangerous Plaque Compared to Sedentary Controls

Plaque Morphology Table

From: Prevalence of Subclinical Coronary Artery Disease in Masters Endurance Athletes With a Low Atherosclerotic Risk Profile. Circulation. 2017;136:126–137. In this study, athletes older than 40 with atherosclerotic risk factors for coronary artery disease report higher coronary artery calcium (CAC) scores compared with sedentary individuals. Calcified plaque is more usually more stable than mixed or non-calcified plaque! Masters athletes were >40 years of age, ran ≥10 miles or cycled ≥30 miles per week and have continued to do so for ≥10 years, and competed in ≥10 endurance events, including marathons (26.2 miles, 42.2 km), half marathons (13.1 miles, 21.1 km), 10 km races, or endurance cycling races ranging from 41.1 to 161.5 miles, 66 to 260 km) over a 10-year period.

Take Home Message

  • The latest study be Levine et al. showed that a high level of endurance exercise does not raise heart attack risk even though there may be a higher CAC score. 5
  • Consider aspirin. A study published in February 2019 strongly suggested that aspirin could help reduce the incidence of heart attacks during running races.  What they found is an increased incidence of heart attacks in patients after they discontinued aspirin. I have always been a believer in aspirin. Aspirin helps reduce the viscosity of the blood by making platelets less sticky. It also lowers inflammation markers.  I still recommend 81 mg per day.6 
  • Elite level male endurance athletes who have risk factors for atherosclerosis tend to have higher scores that sedentary controls but less overall risk. 
  • Get a coronary calcium test if you are male over 40 or female over 50. If it’s high we should talk. Know that Understanding how to work with athletes and interpret CAC tests is a specialty. I primary care provider should not be expected to be an expert on this.
  • Also get a carotid intima media thickness test (CIMT) to see if the plaque is stable or not.
  • Eat healthy food. Seriously, diet matters. Say no to junk food, excess sugar, processed food, fried food, high fat meat. Yes to nutrient dense not calories dense natural food like fruits and veg, plant fats, fiber, red wine, nuts, seeds, whole grains, fish, lean meat. Eggs and milk products are ok just not every day. Eat real food mostly plants.
  • If you have pain with exercise see a cardiologist immediately.
  • See someone well versed in preventive cardiology (and ideally an athletic population as well) once you have your CAC test. Conventional doctors typically are not trained in reading CAC tests and are more geared for treating patients after symptoms start. Because insurance does not cover CAC tests they are not routinely done in an insurance dominated setting. You do not need a doctor’s order for a CAC test.


High volumes of exercise are safe, even when coronary calcium levels are high. Coronary calcium scoring is not an exact science. I think more work needs to be done on runners versus low impact endurance athletes because the runners do tend to have more in comparison, in most studies. Much more than the CAC score must be taken into account when determining risk in an individual athlete. An individualized, precise workup should be done on anyone with elevated coronary calcium. Elite masters athletes are a rare breed and doctors encounter them infrequently. See a provider that is an expert in preventive cardiology to address these concerns.

To schedule an appointment with Dr. Steve Parcell call the front desk staff at 303.884.7557.

1 Too much exercise can damage the heart as well in different ways but that is a complex subject and best suited for separate article. Typically, an echocardiogram is needed to diagnose this condition.

2 Coronary Artery Calcification Among Endurance Athletes “Hearts of Stone”. Circulation. 2017;136:149–151

3 Fitness, Exercise, and Coronary Calcification. Originally published18 Jun 2018. Circulation. 2018;137:1896–1898

4 Relationship between lifelong exercise volume and coronary atherosclerosis in athletes. Circulation. 2017;136:138–148

5 Association of All-Cause and Cardiovascular Mortality With High Levels of Physical Activity and Concurrent Coronary Artery Calcification. JAMACardiol. Published online January 30, 20194(2):174–181. doi:10.1001/jamacardio.2018.4628

6 Aspirin to Prevent Sudden Cardiac Death in Athletes with High Coronary Artery Calcium Scores. Am J Med. 2019 Feb;132(2):138-141