Brain endurance, mitochondria, and the desire to exercise
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.