Brain endurance, mitochondria, and the desire to exercise


As of September 2017, new Sweat Science columns are being published at Check out my bestselling new book on the science of endurance, ENDURE: Mind, Body, and the Curiously Elastic Limits of Human Performance, published in February 2018 with a foreword by Malcolm Gladwell.

- Alex Hutchinson (@sweatscience)


Endurance training causes new mitochondria — the “power plants” that use oxygen and glucose to produce ATP — to grow in your muscles. This, in a nutshell, is why your endurance improves, because you’re able to keep your muscles aerobically fuelled for longer. These adaptations take place mainly in the muscles you use during training: legs for runners, arms and legs for swimmers, and so on.

But the muscles aren’t the only place where oxygen and glucose are needed: at rest, your brain sucks up 20 percent of your body’s oxygen supply and 25 percent of its glucose. A neat new study in the American Journal of Physiology (press release here; full text of study freely available here) suggests that aerobic exercise causes new mitochondria to grow in your brain as well as your muscles, which has a couple of interesting implications. The study was done in mice: an eight-week treadmill running program produced the usual changes (increased time to exhaustion, higher mitochondria in muscles), but also produced a series of changes suggesting that new mitochondria had grown in the brain.

One reason this is significant is that figuring out how to boost mitochondria in the brain would be helpful for “various central nervous system diseases and age-related dementia that are often characterized by mitochondrial dysfunction.” That includes, for example, Alzheimer’s disease.

The other is the possible role of brain mitochondria in “central fatigue,” which the researchers define as “the progressive reduction in voluntary drive to motor neurons during exercise” (this is a controversial topic, I should note). The idea is that your body’s absolute top priority is making sure that your brain ALWAYS has enough energy. During intense exercise, your muscles are using oxygen and energy so rapidly that your brain’s oxygen levels start to drop. To prevent disaster, your brain automatically starts to recruit fewer muscle fibres for a given level of effort, so that more resources can be diverted to the brain. You experience this as fatigue: you’re pushing just as hard as before, but you’re getting slower/weaker. But if you have more mitochondria in your brain, you can make use of available energy more efficiently so you won’t have to shut down your muscles quite as soon:

[I]t is reasonable to hypothesize that increased brain mitochondria may play an important role in reducing fatigue through their influence on cerebral energy status.

Another interesting wrinkle in the discussion:

We have also shown a positive association among brain mitochondrial biogenesis [i.e. growing new mitochondria in the brain], voluntary activity and endurance capacity…

What they mean by “voluntary activity” is how much mice, when left to their own devices, decide to run on a wheel in their cage. Researchers have found that the “impulse to exercise” tends to decline with age — so before your body starts to fail, your brain just isn’t as enthusiastic about doing lots of exercise as it used to be. There are some possible hints here that this phenomenon could be linked to declining levels of brain mitochondria. In other words, regular exercise doesn’t just preserve your ability to exercise — it also preserves your desire.

19 Replies to “Brain endurance, mitochondria, and the desire to exercise”

  1. Hi–I have been enjoying your blog immensely, and learning a great deal. Your recent post about exercise-induced mitochondria biogenesis in the brain, and increased desire to exercise is intriguing. It reminds me of related recent findings which may at first appear unrelated, but which in fact have interesting implications for exercise and training. Here is a link to a general interest article in the New York Times. The article should contains links to additional studies and books. The general idea is that a group of related qualities, such as the ability to make decisions, willpower, and resistance to pain (all important for exercise) are in fact a finite quantity, able to be “used up” or “depleted” like energy stores in a muscle. Furthermore, these quantities can be re-charged, by maintaining glucose levels in the blood. This would appear to relate directly to things like the motivation to train, as well as endurance in long events. And, now that i write this, I realize that it might be linked more directly to mitochondria in the brain, for example, perhaps the cognitive changes associated with hard decisions and the application of willpower are related to glucose depletion, and other effects, operating at the level of mitochondria in neurons in the brain….All the best.

  2. Very interesting finding.

    I also wonder if the extra mitochondria in the brain might lead to more efficient use of glucose and improved executive control, better access to working memory,less ‘ego depletion’, more self-control etc… or any of above?

  3. Funny, I made my post before I could see the comment Bruno made. We were thinking along similar lines although he explained and detailed his comment more clearly.

  4. Thanks for the comments, folks. @Bruno and @Seth, that’s a very interesting point about the potential link between these new findings and the previous discussion about “decision fatigue” (which, as an aside, I blogged about here). The relation between the two hadn’t occurred to me, but the idea definitely makes sense. This seems to be a really interesting field of study that researchers are just starting to unravel.

    (And Seth: yes, Bruno’s post was held up for several hours waiting for me to approve it, which is why you didn’t see it when you made your first post!)

  5. Hi,

    Does strength training increase mitochondria in muscles? Or is it only endurance training. And if it does, is there a rep range that works better than others?



  6. @rick: Mitochondria growth is primarily a response to endurance training. It’s possible to get some endurance response from strength training done in a continuous circuit format, but you really have to focus on keeping the workout nearly continuous. Strength training is great, and is a crucial part of any well-rounded fitness regimen — but for mitochondrial growth, endurance training is far more effective.

  7. I just came across another study linking cardio-respiratory fitness and related brain effects. Those interested in this thread may also find this study supportive.

    Using Path analysis on older adults the follwoing model was tested

    * fitness levels predict hippocampal volume
    * hippocampal volumes predicts working memory function performance
    * working memory performance predicts subjective forgetfulness

    I haven’t read the full study however this blogger indicates that

    The authors found their predicted model held up in the analysis:’.

  8. @Seth: Thanks for pointing that one out — interesting stuff! I’ll try to put up a separate post about it.

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