Most people assume runners are the healthiest people in the room. And for the most part, they are right. But when running stops being a hobby and starts becoming an extreme endurance sport, think marathons, ultramarathons, and multi-day events, the picture gets more complicated.
The heart can handle a lot. Over time though, years of pushing it to the limit leaves marks. Some of those marks are fine. Some are not.
Here is what the actual research says.
What Does Extreme Endurance Running Do to Your Heart?
Every time you run a marathon, your heart works at a level it was not designed to sustain for hours on end. Blood has to move faster, chambers have to pump harder, and the whole system runs under prolonged stress.
Three things happen consistently after extreme endurance running:
- Cardiac stress proteins called troponin T, troponin I, and NT-proBNP flood the bloodstream, the same markers doctors use to diagnose heart attacks.
- The right ventricle, which drives blood to the lungs, temporarily weakens.
- The upper chambers of the heart (atria) can stretch and enlarge under the pressure.
A 2025 systematic review published in BMJ Open Sport and Exercise Medicine analysed 69 studies covering 3,274 runners aged 27 to 63. Every single one of those cardiac stress markers exceeded standard clinical thresholds within the first hour of finishing a marathon.
Now, before that sends you straight to a cardiologist, most of this is temporary. Researchers call it “cardiac fatigue.” In healthy athletes with no underlying conditions, the heart recovers within about 72 hours. The heart is not breaking. It is responding to an extreme demand.
But here is where it gets more complicated. Repeated stress, year after year, starts to leave permanent changes behind.
Does Long-Term Endurance Running Damage the Heart Permanently?
Not for most people. But for some, particularly men who have been training at high volumes for a decade or more, the evidence points to real structural changes.
What the research shows:
- Endurance athletes show more coronary atherosclerosis (plaque build-up in arteries) than sedentary people. A 2025 systematic review and meta-analysis found this consistently. However, those plaques tended to be stable rather than obstructive, meaning they were less likely to cause a heart attack than the raw numbers might suggest.
- Cardiac fibrosis, scarring of heart tissue, has been detected in around 17% of triathletes in some cohort studies, with total training distance as a meaningful predictor.
- Left atrial enlargement is a common structural adaptation in marathon runners, and it directly raises the risk of atrial fibrillation.
None of this means that running is bad. What it means is that extremely high training volumes over many years push the heart into territory where beneficial adaptation and harmful remodelling start to look similar.
What is the Link Between Extreme Endurance Running and Atrial Fibrillation?
This is probably the most important cardiovascular risk for extreme endurance runners, and it is the one most commonly underestimated.
Atrial fibrillation (AF) is an irregular heart rhythm: It happens when the electrical signals in the atria fire chaotically rather than in an organised pattern. Left untreated, it raises stroke risk four to five times.
Endurance athletes develop AF at significantly higher rates than the general population. A meta-analysis of 13 studies covering 6,816 athletes and 63,662 controls found that endurance athletes carry roughly a two-fold higher risk of AF. A more recent analysis puts that figure closer to four times the risk compared to non-athletes.
For marathoners specifically, AF incidence ranges from 0.43 per 100 person-years to 4.4% depending on the population studied.
Why does this happen?
Years of sustained high-volume training do several things to the atria:
- The chambers stretch and enlarge from repeated pressure load.
- Inflammation after races leaves behind small amounts of scar tissue over time.
- That scarring disrupts normal electrical conduction.
- Even a single mountain marathon can temporarily slow atrial electrical signals.
The risk is not spread evenly. Here is how it breaks down by group:
|
Group |
AF Risk |
|
Recreational runners, moderate volume |
Low – no significant elevated risk |
|
Men aged 40–65, high-volume training history |
Elevated – 2-4x general population |
|
Elite female endurance athletes |
Elevated – HR 2.56x, rising to 3.67x after adjustment |
|
Lifelong high-volume male athletes |
Highest documented risk group |
The middle-aged male recreational athlete who has been running 60–80 miles a week for fifteen years is the person cardiologists worry about most. Not the occasional half-marathon runner.
What is the Actual Cardiac Arrest Risk During a Marathon?
The rate of cardiac arrest during long-distance races is approximately 1 in 184,000 participants. Sudden death occurs at a rate of roughly 1 in 259,000 participants.
Those are reassuring numbers. But there is something important underneath them: the majority of cardiac arrests during marathons happen because of pre-existing heart conditions the runner did not know they had. Hypertrophic cardiomyopathy is the most common culprit in younger athletes. Undetected coronary artery disease drives most cases in men over 40.
The race does not create the problem from nothing. It reveals what was already there.
Men are significantly more affected than women across almost every metric, cardiac arrest, sudden death, and AF. The likely reasons are higher rates of undetected hypertrophic cardiomyopathy and earlier-onset atherosclerosis in men.
Should You Be Worried About Troponin Rising After a Race?
Troponin is a protein that leaks into the bloodstream when heart muscle is under stress. When someone comes into an emergency department with chest pain and elevated troponin, doctors treat it as a potential heart attack.
After a marathon, many healthy runners show troponin above the normal clinical cutoff, with completely normal heart scans and no symptoms whatsoever.
So no, a troponin rise after a marathon does not automatically mean something went wrong.
The difference that matters:
- Exercise-related rise: Troponin goes up, comes back down within 24-48 hours. No chest pain. No breathlessness. Normal ECG. Normal imaging.
- Actual cardiac injury: Troponin stays elevated. The person has chest pain, breathlessness, dizziness, or collapses.
The real danger is what happened in a reported UK case, a 42-year-old runner with genuine chest pain was reassured because “athletes always have high troponin.” He later died from a heart attack caused by underlying coronary disease that was never investigated properly.
Chest pain during or after a race is not a training inconvenience. It is a red flag that needs medical attention.
Who Faces the Highest Cardiovascular Risk From Extreme Endurance Running?
Research points clearly at specific groups. If you fall into one of these, your risk profile is meaningfully different from the average runner.
Higher-risk groups:
- Men aged 40-65 with more than 10 years of high-volume training.
- Anyone with undiagnosed hypertrophic cardiomyopathy or coronary artery disease.
- Athletes who train more than 10 hours a week at sustained high intensity with little recovery.
- Runners who dismiss symptoms, palpitations, chest tightness, unusual breathlessness, or dizziness during exercise.
Lower-risk groups:
- Recreational runners at moderate volumes
- Athletes with regular cardiac screening
- Runners who use a polarised training approach (mostly easy, occasionally hard)
- Younger athletes without structural heart conditions
How Can Extreme Endurance Runners Protect Their Heart?
- Get a cardiac screen before committing to high-volume training: An ECG and echocardiogram can identify most structural and electrical problems before they become dangerous. This is not excessive, it is basic due diligence for anyone running more than 50 miles a week.
- Train in a polarised way: Most elite endurance coaches and sports cardiologists recommend that 80% of training sits at genuinely low intensity (easy conversation pace) with only 20% at higher effort. Sustained high-intensity training day after day is where cardiac stress accumulates most aggressively.
- Take symptoms seriously, every time: Palpitations, chest pain, unusual fatigue, or dizziness during a run are not things to push through. Each one of those warrants a cardiology conversation.
- Prioritise recovery as part of training: The heart remodels during rest. Skipping recovery does not make you fitter, it just means the heart never finishes adapting.
- Follow current clinical guidance on AF: The 2024 Heart Rhythm Society consensus statement recommends that athletes with known AF work with a cardiologist on a shared return-to-activity plan rather than making those calls alone.
So, Is Extreme Endurance Running Safe?
For the average recreational runner, yes. The cardiovascular benefits of regular distance running far outweigh the risks for most people.
For the extreme end, ultramarathons, decades of 70-mile weeks, back-to-back race schedules, the answer is more qualified. The risks of atrial fibrillation, cardiac remodelling, and in rare cases sudden cardiac events are real, particularly for older male athletes with long training histories.
The research does not say stop running. It says to know your risk, screen appropriately, train with intelligence, and never ignore what your body is telling you during exercise.
The heart is an extraordinary organ. It will adapt to almost anything you ask of it. The question is whether you are giving it what it also needs to recover and stay healthy over the long term.
Key Takeaways
- All three major cardiac stress markers spike after a marathon, mostly temporary, but worth understanding.
- Endurance athletes face a 2-4x higher risk of atrial fibrillation than the general population.
- Cardiac arrest during a race happens in roughly 1 in 184,000 runners, usually from undetected pre-existing conditions.
- Middle-aged male athletes with long high-volume training histories carry the most documented risk.
- Cardiac screening, polarised training, and taking symptoms seriously are the most effective protective steps.
