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CW: It’s settled–Long-term extreme endurance exercise is not heart healthy

My inbox lit up this week with links to mainstream media reports that extreme exercise causes heart damage.

Sure enough, a group of US researchers published an exhaustively complete review article on the Potential Adverse Cardiovascular Effects from Excessive Endurance Exercise. (Full text available.) Though published in the modestly circulated journal, Mayo Clinic Proceedings, this article makes a serious impact.

In this post I will attempt to recap–in my language—why these authors have garnered significant attention in the mainstream media. As I go through the ten topics addressed in the paper, I’ll report the findings and add in a typical naysayer’s view.

(I apologize in advance—this is a longer post than I like to write.)

Some basics:

This paper offered absolutely no new data. It was a review article—a really terrific one, but still a review. In fact, many of the studies cited have been previously discussed on this blog. Why the media buzz then? Perhaps it was the power of putting all the evidence together (including 68 scientific references) in one paper? Perhaps it was just that Yahoo news published the paper? Perhaps its appeal has something to do with non-exercisers reveling in the exposure of chinks in the armor of the super-fit. Hubris has this effect.

The review article was split into ten sections. I’ll call them chapters.

Chapter 1: Regular moderate exercise is beneficial:

The authors start smartly. They begin by citing the many studies that document the long-term benefits of modest amounts of regular exercise. Two studies in particular show a U-shaped pattern of benefit: that is, to get maximal health benefits, one must exercise enough, but not too much. This Taiwan Study of more than 400,000 patients published in the Lancet nicely demonstrates a leveling off of benefit after one hour per day of exercise. I won’t spend much time here because this is so obvious.

Chapter 2: Sudden Cardiac Death and Endurance Exercise:

In this section, the authors cite the recent NEJM article on the safety of marathon running. As previously noted on this blog, the risk of suffering cardiac arrest in a marathon is extremely low (1 in 100,000), and not changed with the increasing numbers of marathon participants. Triathletes have double this risk, mostly because of an increase in deaths during the open-water swim (for obvious reasons.)

Chapter 3: Animal Studies:

It’s no surprise that the authors led with the famous and elegant Marathon Rat study. This powerful study is worth a review: Canadian researchers studied two groups of rats. One group ran like crazed ultra-marathoners and the other group remained sedentary. The running rats developed enlargement of all four chambers of the heart, abnormal heart stiffness, an increase in heart scar tissue and an enhanced susceptibility to dangerous heart rhythm problems. Ouch.

Of course, the skeptics counter that this was a rat study, and therefore might not apply to humans. So noted.

Chapter 4: The Athlete’s Heart:

The authors nicely summarize what an “athletic heart” means. Here is their thought process: Long-term endurance exercise, especially chronic aerobic exercise, induces adaptations in the heart, including increases in right and left ventricle volumes and wall thickness, increases in the overall mass of the heart, and enlargement of both atria. The striking thing about these adaptive changes is that when they occur in normal people, they are thought to confer a bad prognosis. Which is it then? Bad or good? The authors go on to cite four studies supporting the idea that ‘athletic’ changes may not be entirely benign. One study even notes enlargement of the heart can persist long after de-conditioning.

Naysayers look at these reports and rightly point out that they are non-randomized look back studies of small groups of extreme athletes. Noted again.

Chapter 5: Biomarker Evidence with Extreme Exercise:

At least five studies confirm that up to 50% of marathon finishers show elevated levels of cardiac enzymes. You know, the same chemicals released during a heart attack.

The authors use the rest of this section to speculate on the significance of exercise-induced enzyme rises. Using phrases like, “this may reflect myocardial cell damage at sites of myocyte slippage,” it’s easy for a reader to take notice. Does exercise cause such violent slipping of heart cells so as to spill enzymes? For a blogger who has been on many a long run or ride, these are scary words.

To the authors’ credit, they admit to not knowing the significance of elevated enzymes post stressful exercise. Good for them.

Chapter 6: Extreme exercise and adverse structural remodeling of the heart:

This section details the scariest aspects of long-term exercise. Extreme exercisers might want to skip over the upcoming paragraphs.

Two important reminders: the human heart does not regenerate itself—scar remains scar—and, you only have one heart.

The authors set the tone well by telling us that intense exercise can increase the cardiac output from 5 liters per minute at rest to up to 25 liters per minute during exercise. Think about that for a minute—pumping 25 liters of blood per minute for a 12-hour Ironman! (Sorry for the hyperbole.)

Let’s look at a few of the studies that were reviewed. As I wrote about previously, this Australian study examined the immediate damage to the heart (particularly the right ventricle) after major endurance events. The sobering report had three major findings: 1) Athletes finishing major endurance tests had elevations of heart enzymes, and the amount of leak paralleled dysfunction of the right ventricle. 2) Longer duration events  predicted more right ventricular muscle weakness. 3) Scar was seen on MRI scans in 12.5% of athletes and was more likely to occur in longer events. The conclusion here was that extreme exercise can induce right ventricular abnormalities while largely sparing the LV. The only shred of good news was that in most (but not all) cases, the damage was transient.

Further implicating the right ventricle’s susceptibility, the review paper highlighted a study demonstrating a strong relationship between abnormal right ventricles seen on MRI scans with the presence of ventricular rhythm problems. More than half of athletes with ventricular rhythm issues had RV scarring.

Unfortunately, RV scarring is not the only exercise-induced structural change of the heart. Take this German study of 100 experienced marathoners that were compared with regular age-matched controls. Patchy scarring (on MRI scans) of the heart were seen in 12% of runners—three-fold more often than non-runners. The kicker here was that over 2 years of follow-up, marathoners had more coronary events than non-runners. Again, the review paper cites numerous other similar studies.

Skeptics will continue to emphasize the fact that these studies are small and non-randomized. Okay, I hear that. But the evidence is mounting.

Chapter 7: Extreme exercise and changes in the coronary arteries:

This section focuses on the well-established finding that marathon running increases the likelihood of having calcium deposits in the coronary arteries. It goes without saying that coronary arteries fare better when not hardened and stiffened by the presence of calcium. Most damning is this German study of marathon runners: not only did the runners have more calcium in their coronaries, but most remarkably, the supposedly extra-healthy runners experienced the same frequency of heart attacks as a group of patients with known heart disease! A similar study was presented at the ACC meeting in 2010, and was covered in this piece on

Again, the counter argument centers on the lack of comparative studies done over the long-term.

Chapter 8: The Pathophysiology of extreme exercise:

Pathophysiology is medical speak for abnormal adaptations that occur with long-term exercise. In this section, the authors mostly speculate that extreme exercise causes heart damage by inducing excess inflammation. A few studies confirm the presence of high levels of chemical markers of inflammation in extreme exercisers. There’s not much else to say here other than many studies implicate excess inflammation as the mediator of heart damage from excess exercise.

Chapter Nine: Extreme exercise and electrical abnormalities:

I’ll keep it succinct. The data speak for itself. Long-term exercise results in a five-fold increase in the risk of developing atrial fibrillation. The authors cite twelve papers supporting this obvious truth.

AF isn’t the only arrhythmia seen in the super-fit. As supported by five studies, ventricular arrhythmias occur commonly in endurance athletes. It’s worth recalling the marathon rat study, which showed a greater susceptibility to arrhythmia induction in the running rats.

I witness these findings nearly every day. Skeptics can deny the association of excess exercise with arrhythmias–but they would be wrong.

Chapter 10: Risk assessment:

We simply do not have cost-feasible means to identify those exercisers who either suffer heart damage or are at risk of damage.

One potentially promising strategy might involve doing coronary calcium CT scans in highly selected marathon runners. I hesitate writing that last sentence because it’s pure speculation, formed in my head by a couple of tragic cases of marathon runners who suffered heart attacks and at cath were found to have rock hard coronary arteries.

The Mandrola take home:

Step back and look at the big picture here. When you lay out all ten chapters on a canvas, the picture takes shape. Your conclusion should be the same as the authors:

In some individuals, extreme exercise causes severe heart abnormalities.

Many questions remain. The dose of exercise that causes damage is unknown, as is an individual’s susceptibility to varying training loads. We don’t know the ideal exercise regimen, or if there is one. I know what is too little exercise; I know what is too much; but there’s a large space in between.

I’ll finish with the obvious…

Just like everything else in life, even exercise has an upper limit.


21 replies on “CW: It’s settled–Long-term extreme endurance exercise is not heart healthy”

Excellent post by Dr. John – which together with the O’Keefe et al Mayo Clin Proceedings article makes for fascinating reading. For whatever methodologic errors there might be in assessing a difficult topic as this one – the nearly inescapable conclusion of the authors seems highly plausible – “that in some individuals, extreme exercise causes severe heart abnormalities”.

I hadn’t realized that there is a 5-fold increased incidence of AFib in highly fit endurance athletes (Dr. John ought to know … ).

BOTTOM LINE – of course is NOT that exercise is “bad” (on the contrary – moderate regular exercise is the best thing a person can do) – but rather John’s final sentence that “Just like everything else – even exercise has an upper limit”. THANKS for posting and reviewing the O’Keefe et al article!

Great summary, thank you. My only curiousity, and I could probably find the answer by reading all the studies, is at what point do the negative effects of exercise intersect at duration vs intensity of exercise? A 3 hour marathon is going to be performed at a much higher intensity than a 12 hour ironman.

All the studies seem to focus on efforts lasting 3+ hours (marathon, ironman, etc.). It would be great to see a study comparing structural changes of the heart in elite mile/5k runners vs. elite marathoners. Any predictions on the difference? All studies conclude that SOME people are prone to the adverse affects of chronic ET, just as some are prone to stress fractures. I assume that means, “some are not”. Wouldn’t it be possible for any one individual to create his/her own personal risk profile, rather than relying on the statistics. For example, get an echocardiogram, a CT calcium score, and an MRI to look for structural abnormalities.

Nice summary of a nice summary. A few random thoughts….& I’ll admit at the outset that I’m a skeptic of all sensational claims.

The heart can’t possibly know what form of activity it’s owner is engaging in. The studies are all done in runners, yet most general population exercisers are doing some other activity.

If there IS some threshold beyond which (for some athletes) harm comes to the heart, that threshold must certainly be related to some product of duration x intensity.

Enzyme release isn’t limited to long-distance running. It’s been documented in 5k runners after races that are just 20 to 30 minutes in duration.

From the data summarized in the report, benefit continues to accrue even at 2 hrs per day. Most marathon runners train far less than that, yet the sensationalized headlines malign marathon running in particular.

As I think about endurance sport, I wonder about swimmers…especially elite swimmers in their 20’s. These are athletes who’ve trained for upwards of 20 years and who have accumulated 15,000 to 20,000 hours of training. This is far more than almost any runner could accumulate over decades. Yet we never hear about heart problems in this group.

Lastlly, I’m not sure why it’s “obvious” that triathletes would be more prone than other athletes to race-related fatalities. We know from the French general population, for example, that cycling was the activity most related with sports-related sudden cardiac death.

USA Triathlon is just completing a review of race-related fatalities over the past 9 years. It turns out that the overall race-related fatality rate is about 1 per 76,000 participants. If the handful of traumatic deaths are excluded, the rate of deaths due to sudden cardiac arrest is about 1 per 86,000 participants….not very different from the rate at marathon run events.

I worry that the popular press does a tremendous disservice with stories like this week’s acknowledgment of the review article. Exercise is so obviously good. If even one individual decides not to exercise, it’s an opportunity lost.

Hi Larry. I am so glad you took the time to comment. I want to thank you personally, but also, I’m grateful because your words are so helpful to my 19 readers.

What I meant to emphasize about triathlon deaths was simply that arrhythmias that occur during open water swims are especially lethal. Heck, even benign tachycardia could be deadly in the water. I’m not sure people who haven’t done triathlons know the adrenaline surge that happens in the scrum of an open-water swim. I can remember thinking to myself while swimming way out in a foggy lake during a race–“I am really vulnerable at this moment.”

Your point about possible differential cardiac effects of various sports is important. The scariest adverse effects on the heart have been shown in runners. This makes intuitive sense. I have been a swimmer, runner and cyclist. For sure, running is the hardest on the body. One wonders whether cyclists and swimmers would fare better, though this JACC piece demonstrated persistent LV changes (by ECHO) in Tour de France cyclists.

This entire topic is hard for me to reconcile. I’m pulled in so many directions. On a personal level, my body seems to want a couple long rides 2-3 times per week. Averaging just seven hours per week of exercise doesn’t suit me. So yes, I agree hope, there’s no doubt that a one-hour per day maximum isn’t a hard and fast rule. Clearly, a one hour spin on a bike isn’t the same as a one hour run. And as you say, intensity must be factored in as well.

On a professional level, I witness the ravages of inactivity every day. It’s awful to see what happens to people who do not sweat. Despite what the NY Times over-hypes about exercise negatively effecting 10% of people, I believe moderate and long-term exercise constitutes the most effective and safe means of achieving wellness. Gosh, it’s hard to say anything negative about exercise.

But on a scientific level, this data cannot be ignored. It doesn’t tell us that endurance exercise is always harmful. It does however, strongly suggest certain individuals can exercise themselves into trouble.

It is interesting that the news story was out for 2 days before the actual paper was available. After reading it I got the same impression as you.

“We don’t know the ideal exercise regimen, or if there is one. I know what is too little exercise;”

I think that there was way too much hand waving to say that.

However, it was interesting that TheHeart did not directly cover it.

But they did report an study that was giving at a ACSM meeting

The only data that is available is in the meeting abstract.{FCDB1C1C-280A-4DF1-95F8-2DAA9AB6A8BE}

It will be interesting to see this a journal article.

But even with this limited data, as another person mentioned, it does not really help those that are not runners, but bike or swim.

And I am wondering where HIIT fits in this.

Yes, yes. Two interesting things here that I do not understand.

You are right–even my librarian could not get me the article for two days after the press report. It simple wasn’t there. That seems peculiar. I was alerted to this article when friends sent me the link on mainstream media outlets. As always, my reflex response is to get the original article and read it before reading the press reports. But in this case, the article wasn’t available.

You picked up another tricky thing. The study that you and I linked about the U-shaped curve of runners was an abstract presented last month. In the paper, it’s listed erroneously as a published paper, with page numbers that do not correspond to a study? An oversight perhaps?

Hi John,
as you know, I share both passions of you: EP and endurance sports. Thus, I liked your comments on this paper. I recently wrote an article for a german triathlon journal, also struggling with the data, that endurance sports could be harmful for some hearts. Anyhow, to my knowledge, there are no conclusive data, showing increased mortality in high level endurance athletes. But there is data showing increased longevity in former endurance athletes, i.e.
Sanchis-Gomar F, Olaso-Gonzalez G, Corella D, Gomez-Cabrera MC, Vina J. Increased average longevity among the “Tour de France” cyclists. International journal of sports medicine. 2011: 32: 644-647.
Sarna S, Sahi T, Koskenvuo M, Kaprio J. Increased life expectancy of world class male athletes. Medicine and science in sports and exercise. 1993: 25: 237-244.
Gajewski AK, Poznanska A. Mortality of top athletes, actors and clergy in Poland: 1924-2000 follow-up study of the long term effect of physical activity. European journal of epidemiology. 2008: 23: 335-340.
This data could also be the result of selection bias, but should be considered anyhow. Above all, most athletes enjoy sports not for health benefits but just for fun.
I am looking forward to your next posts!

Thanks Andreas. Great comment and excellent references.

Yes, like most things in Medicine, individuals exhibit great variation in their response to interventions.

Hope all is well.

As a marathoner and an avid cyclist I find your blog quite interesting. Extreme and intense exercise as a decripter is worthless because it’s too individualistic. A six minute mile is extreme for my body but for my friend it’s a warm up exercise. One thing not mentioned is the perception of athletes having healthy hearts which I believe may lull them, their family and even their care giver into ignoring what are obvious warning signs. Keep the post coming.

All of my friends over on cardiac athletes website will be squaking over this article! My take on it:

I have been told:

1) absolutely do not lift weights with an ICD, you might bust a lead….

My response: My toddlers (one in each arm) probably could cause the same damage or more than my careful presses and fly exercises… lifting weights is not prohibited in my world.

2) Watch your Max HR during exercise, make sure you don’t get too close to defib thresholds……or you might get zapped.

My response: Paying close attention to my HR does nothing but induce further anxiety….isn’t my threshold set to detect VFib? So if I have V-Fib please go ahead and zap me.

3) Your AFib seems to be increasing with activity level…..back off a bit, maybe it will go away.

My response: Maybe I will start eating Cheetos on the couch instead. AFib ain’t gonna kill me and my EF rocks….I’m not worried.

So- this magical article is nothing but hype in my mind. Everything has a possibility to kill us or damage us. Exercise in moderation with some common sense- for the vast majority of us, (without underlying disease process) is safe. We ain’t gonna DFO. And for those of us with a few hiccups, quirks, or right messed up rhythms, we are probably going to be OK too.

P.S. Don’t tell my EP I do HIIT too.

Hrm, lots of interesting and thought provoking points on both sides. That’s why I like your blog Doc.

I was watching Piers Morgan the other night, and noted with GREAT interest the guest for the evening, namely Lance Armstrong. Apparently (I wasn’t aware of this so please excuse my ignorance) he is venturing into the Ironman arena. I wonder if this is going to be good for him in the long run considering the other medical issues he’s had in his life. Frankly it concerns me.

Yes. Lance possesses an extraordinary aerobic capacity–perhaps the best ever. A scientist could learn a lot from studying his heart, lungs, cells and also his mind. Notwithstanding his hiccups in life, he remains in the category of World’s most remarkable athletes. Other members of that club include, swimmers like Mark Spitz and Michael Phelps, runners like Haile Gebrselassie, triathlete Mark Allen and of course, the ultimate heart and lungs–Sectetariat.

I am a long-time competitive runner who went into a-fib a few days after my first marathon. I was converted back to NSR with a CV (post-TEE) after 3 weeks. My echo and other tests were “normal.” Would the echo pick-up calcium deposits? I’m considering running another marathon and can live with the risk of going back into a-fib (and try to manage it: no more pre-race coffee or post-race cold ones), but have no idea how much comfort to take in the normal echo. My sense is that if I told my cardio that the echo wasn’t enough and I wanted further tests, he would probably think I’m crazy (maybe he’d be right).

Thanks for all the info. This blog has been a tremedous resource for me.

Dr. John – Another great post on a subject that touches close to home for me (someone who has been a runner for most of his adult life and has run several marathons and numerous half-marathons). My biggest concern at the moment is for my 13-year old daughter who has been a competitive swimmer for the last several years. Her club swim practices are at least 2 hrs, 6 days a week. Do have any sense whether this is “over the line” in terms of too much endurance activity? Are young people better able than adults to deal with such workloads? Would love any input you might have on this topic.


@Trey – What an important question you ask !!! So hard to tell a 13-year old you love that they are “doing too much” when all others on her team do the same. My sense (based on no data) is “No” (not ‘over-the-line’) – but the impact of your wonderful question is huge extending to numerous sporting activities over numerous levels of abilities over wide age range …


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