Is intense exercise bad for your heart?

People are well aware of the heart protective effects of exercise, and recommendations that people increase exercise quantity and intensity are common. But as I’ve written about previously, evidence from observational studies suggests that the sweet spot is reached at just 35 minutes of intense exercise per day (or two hours of moderately intense exercise), after which exercise confers no additional health benefit. After an hour of intense exercise, you are actively harming your health.

There are plenty of examples of people who were thought to be supremely healthy up to the point they dropped dead of heart disease. One such example is that frequently comes to my mind is Micah True, the ultra-runner and central character in the book “Born to Run”, who died of a heart attack at the age of 58. Another is Bob Harper, the celebrity fitness trainer and host of Biggest Loser, who had a cardiac arrest at the age of 52 (but survived thanks to rapid intervention by bystanders).

It’s not hard to imagine why more is not always better when it comes to intense exercise. During a bout of intense exercise, the systolic blood pressure is often over 200 mmHg, a level that would in other situations immediately result in a person being placed on multiple blood pressure lowering drugs. The strain on the heart leads to increased blood levels of troponins, molecules that normally exist inside heart muscle cells, but which are released when they suffer damage, and which are used clinically as a means to detect heart attacks. And intense exercise increases oxygen needs massively, resulting in a dramatic increase in free radicals. The longer the exercise goes on for, the harder it is for the body to maintain sufficient mechanisms to neutralize the free radicals, and the greater the probability that they will succeed in causing damage.

It’s been known for the last decade or so that professional athletes have an increased risk of developing atrial fibrillation, a condition in which the atria of the heart stop contracting synchronously and instead “wobble” in a disorganized manner. Blood clots frequently form in the fibrillating atria, and can travel from there to the brain, causing a stroke – which is why people with atrial fibrillation are usually put on blood thinners. None of this is controversial. What is it about prolonged intense exercise that causes atrial fibrillation?

Heart muscle damage, which leads to scar tissue, which leads to disorganized pathways for the cell-to-cell signals that cause the heart to contract.

Unfortunately it’s not just the heart muscle that gets damaged by too much exercise. The arteries take a hit too. Multiple studies have found that people who engage in a lot of high intensity exercise have an increased risk of having significant coronary artery calcification, even when compared with people who don’t exercise at all. The lowest risk of having significant coronary artery calcification across the studies was found among those who engage in a moderate amount of exercise. The reason this matters is because a higher amount of calcification usually means an increased risk of experiencing a heart attack.

What can we conclude? Some exercise is good, a lot of exercise is bad. As mentioned above, the optimal health benefits appear at around 35 minutes of intense exercise per day (or 2 hours of moderately intense exercise, such as walking, if you don’t like getting sweaty).

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30 thoughts on “Is intense exercise bad for your heart?”

    1. If you are over 30 then jogging is a great way to accelerate sarcopenia with a bonus of possible foot and knee injuries.

      1. Depends where you jog and how you jog…

        Jogging on a rubberized track or grass is much harder on joints than jogging on concrete.

        Jogging so that your quads take most of the shock is easier on the joints.

        Jogging improperly on improper surfaces can cause knee and back problems.

        Body lifting (e.g. deadlifts) with improper technique may also cause knee and back problems.

        The word for the day is “improper.”

      2. Um, I got things reversed in one statement…

        “Jogging on a rubberized track or grass is much harder on joints than jogging on concrete.”

        should have been

        “Jogging on a rubberized track or grass is much easier on joints than jogging on concrete.”

      3. @theasdgamer how do you answer Michael’s truthism about the methabolic damage of long duration, middle intensity aerobic exercise?

  1. While I agree with limiting high intensity exercise within a certain parameter, I disagree with the reasons why and the studies in their ultimate conclusion that a lot of exercise may damage your heart etc. because these are based upon a metabolism / physiology primarily using CHO as the energy source. Fat Adaptation, getting the body back to using the aerobic energy source it is meant to burn, fats, changes everything about this paradigm.

    1. Absolutely! Without looking at the dietry intake of persons in these studies and anecdotal references any conclusions are meaningless.

  2. I’m a dance / fitness teacher and I structure my 60 minute class so that the heart rate is steadily elevated – and then allowed to slow and then elevated again and this goes on continuously throughout the class – until around ten mins before the end whereby the heart rate and intensity of exercise is gradually reduced back down to ‘normal’.
    I guess this is a gentle form of HIIT – what’s your opinion on this method of exercising?

    1. It’s hard to go in to specifics, since the studies usually don’t look at specific routines. I think a reasonable routine for a one hour session would be one where there is a 10-15 minute warm up session at the start, where the intensity is more comfortable, and a 10-15 minute cooldown session at the end, so that the truly intense period is limited to around 35 minutes.

  3. What is the influence of inadequate magnesium levels on exercise?
    Asking because I used to get afib during exercise but taking Mg Threonate seems to have significantly reduced incidence and severity.
    NB most of us are deficient according to e.g. Caroline Dean MD.

  4. I’m wondering how you define “intense”. Could mean different things to everyone. People sweat at different levels of exercise. Can this be clarified by correlating with sustained heart rate levels of ?? Some other way?

    1. Well, it’s more about how much pressure you’re putting on your cardiometabolic system than the specific type of exercise you’re doing. The way most people do strength training doesn’t put much strain on the cardiometabolic system since you’re usually lifting weights for less than a minute and then resting a couple of minutes before you go again.

  5. Bob Harper heart attack was due to his undiagnosed Lp(a) not his presumed excessive exercise !
    As I’m sure you’re aware, elevated Lp(a) affects more than 20% of the population, and currently there is no drug or medical therapy (other than plasmapheresis ) available, although there are some exciting new drugs in phase 3 trials. Everyone should know their Lp(a) status, which remains stable after age 5.
    The vast majority of exercisers under exercise, not over. The type of cardiovascular training that should be emphasised is “zone 2 training” or what Phil Maffetone refers to as MAF (maximal aerobic function).
    These zones are ‘fat burning zones’ that stimulate increased mitochondrial density and quality, resulting in enhanced defences against oxidative stress and inflammation. Longevity depends on healthy mitochondria. To achieve this one ideally should aim for 4 sessions per week of 45minutes in zone 2. HIIT sessions should make up less then 10% per week. Peter Attia has written extensively on this and has many illuminating interviews on his podcast about this core subject for those with a deep interest in health span and longevity.

    1. Thanks for the interesting critique. I’m impressed at how quickly you put it together, and many of your points are valid.

      As to atrial fibrillation in professional athletes – I chose a study more or less at random. There are in fact many studies that confirm this association.

      It’s true that most of the evidence in this sphere is observational, and that’s unlikely to change – it’s hard to randomize people to lots of exercise, some exercise, or no exercise and then follow them for a decade to see what differences there are in cardiovascular outcomes. But in general the observational nature of the data should decrease differences, not increase them, since endurance athletes are on the whole more conscious about what they eat, less likely to smoke, less likely to consume excessive alcohol, and so on.

      Is there evidence that endurance athletes who supplement magnesium have less cardiovascular issues?

      1. Dr Rushworth: “As to atrial fibrillation in professional athletes – I chose a study more or less at random. There are in fact many studies that confirm this association.”

        I accept that. For myself, no test or symptoms have ever shown it, but I suppose I have years to go. However, about 10 years ago in the best-trained condition of my life, I developed a missed-beat heart rate pattern, followed by an extra-powerful beat—very annoying and hard to sleep! So that at least confirms for me the effects of intense training on the heart. I discussed with cardiologist and it is apparently quite common in highly trained athletes. I don’t train that hard anymore (late 50’s now). It went away as the off-season came.

        Back then and now, my resting HR is typically 42 bpm. I have seen brief periods recorded while sleeping as low as 33 when I was younger. HR was 39 bpm when tested for health insurance—nurse said lowest she’d ever seen. But all my life I have been a faux bradycardia person. Is that because of training, or genetic? I think it’s a propensity increased by training for years. But all ECGs I’ve ever had always perfect.

        I’d bet that a lot of cases of AFIB relate to electrolyte problems with Mg deficiency at the core. Of course, some of these athletes might actually have heart damage. But what if 90% of it cases are Mg deficiency, and no one bothered to check? And/or it could be a factor predisposing. The general public is Mg deficient, not just retired athletes.

        I reference my commentary here on BMJ: “Subclinical magnesium deficiency: a principal driver of cardiovascular disease and a public health crisis”
        direct link to paper:

  6. I like the ideas of Prof. Hiroaki Tanaka with his slow jogging method. Worth reading about if you’re interested.

  7. Dr Rushworth: “Is there evidence that endurance athletes who supplement magnesium have less cardiovascular issues?”

    Once I fixed my electrolyte intake for double centuries, all my gastro and appetite and fatigue issues disappeared and never returned. I attribute that to Mg in particular. But that doesn’t speak to cardio issues.

    I wish there were even one study on Mg in elite athletes, but I’ve never seen that investigated (could’ve missed it). Basic chemistry suggests a possible benefit, as Ca likes to precipitate without Mg; the cellular balance being critical. I was hoping to get some evidence for myself via CT heart calcium score progression (or not) following MgCl supplementation, but an 18 month recovery after COVID wrecked those plans, and the experiment.

    Magnesium is a prolific area of research in general, but most of it is using nearly useless MgO, along with serum magnesium tests (highly unreliable) instead of ionized magnesium in tissues. MgCl is the only reliable form of supplementation, with nearly 100% absorbed and no diarrhea, so you could do studies that have credible data on actual amount of intake. But no one does that, which is crazy. I’ve taken as much as 4000mg MgCl in a day without diarrhea; if body stores are low no problem, but if topped-up (months later), then that amount is best for toilet training. Try that any time with Mg citrate of only 1000mg (nominal) and you’ll curse supply chain woes.

    Research on elite athletes is scattershot in all areas, let alone the Mg thing. My cardiologist friends find it all pretty sketchy to conclude much about elite athletes.

    What is clear to me personally is that training of up to 13000 KCal per week (8000 KCal average) draws down all sorts of electrolytes (K, I, Mg, etc), and it’s not feasible to eat enough food to replace Mg in particular, and often iodine. After some months, body stores of Mg necessarily plummet, hence the “June problem” after 6 months of training and competing.

    My hope is that restoring Mg levels to “topped off” levels in body tissue will halt further accrual of calcium deposits in my heart. But at this point, I have only a hypothesis, and not even anecdotal personal data as per my intro above.


  8. I only quickly skimmed thru this article. But one thing comes to mind… Isn’t this common sense?

    Any machine, mechanical or biological or whatever, will fail quicker if put to stress frequentely. The same if it at a stand still for long periods. Common sense, do not try to go too much above and beyond the sweet spot or you will face increased wear and tear plus the increased risk of injuries.

    So, what about the professional athletes that we see on TV and at sports event?
    I applaud them for their achievements of course. But, can we consider them healthy? Isn’t it becoming sort of a freak-show / experiment of how far we can push our bodies.

    Keeping in good health is a long-term game, much longer than a marathon… Common sense, isn’t it?

    1. I agree! I look at a football team and think that few of them are working towards longevity in their life journey. The profession of sports has created ultra-competitive, get rich doing it, push to the limit people. They aren’t healthy and coaches don’t look healthy either.
      Obesity puts a strain on your heart, stress puts a strain on your heart, and extreme exercise puts a strain on your heart. But most doctors just prescribe pills for your blood pressure or recommend a low fat diet and exercise without actually helping patients find the help they need to change their lifestyle.
      Once you’ve addressed diet and possible supplements, exercise should become consistent but not necessarily intense! And it should be a mix of aerobic where your heart rate goes up and strength! Most people are not lifting weights and it’s SO important as your get older.

  9. You got me worried now!
    How would you define intense, is it not different for everyone?
    I go to the gym almost every day and I get really sweaty! But don’t feel breathless, only when I do a high intensity work out say twice a week.
    I’m 47.

    1. It is absolutely different for everyone. I I think a good general rule of thumb is that if you’re having noticeable difficulty talking, then the exercise is ”intense”. So what can be considered intense depends on your underlying fitness level.

  10. One important question that the article doesn’t address is the recovery period after 35 minutes of intense exercise. I would expect that to vary by age and would not expect there to be much direct data on it. But maybe there is enough secondary data to venture a high confidence guess about the recovery period for various age cohots.

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