April 5, 2025, 9:42 am | Read time: 5 minutes
In principle, endurance sport is considered a healthy way to protect yourself from lifestyle diseases. However, as experts repeatedly advise, you should not overdo it. A recent study shows what happens during extreme running sessions using the example of marathon runners: Their brains apparently digest themselves when their bodies starve!
Regular exercise has countless positive benefits for our health. In particular, endurance sports such as swimming, running, or cycling strengthen the cardiovascular system, protect against high blood pressure, improve blood lipid levels, strengthen our resistance to stress, and even boost our mood — to name just a few of the positive aspects.1 However, as with everything, you shouldn’t overdo it. There is scientific evidence that the brain begins to digest its own fatty tissue and use it as an energy source during very long endurance sessions such as a marathon. And this can have negative consequences.
Overview
The Brain Does Not Actually Have a Fat Store — This Was Previously Assumed
It is always amazing to see the new insights researchers gain about our bodies. In endurance sports, it was previously clear that the body begins to tap into fat reserves when our carbohydrate stores (glycogen stores) are depleted. An effect that many people use to lose weight or define muscles. However, the brain itself does not have any fat stores that could be used — this was previously assumed. Although this is still true with regard to fat storage, fatty tissue in the brain is apparently broken down when the body is starved. This has now been shown by Spanish researchers in a recent study of marathon runners.2
How the Study Was Conducted
Marathon runners were recruited for the study to find out how extreme endurance exercise affects people. Of the ten volunteer participants, eight were male, and two were female. The test subjects were well-trained and aged between 45 and 73. MRI scans were carried out to determine how endurance exercise affects the brains of the marathon runners.
- The first MRI scans were performed 24 to 48 hours before and after a marathon.
- The second MRI scans were carried out two weeks after the marathon.
- The third and final MRI scans were performed two months after the marathon.
Less Fatty Tissue in the Brains of Marathon Runners After a Race
The evaluation of the MRI scans showed that there had been a significant change in the markers for myelin in the white matter of the brain after the marathon. White matter refers to parts of the central nervous system that consist mainly of pathways or nerve fibers. This also includes myelin. This is an insulating layer that is located around the processes that connect nerve cells, also known as axons.3 Myelin is primarily responsible for the rapid transmission of electrical signals between nerve cells. And it consists largely of fat.
In the brains of marathon runners, significantly lower amounts of myelin were present 24 to 48 hours after the marathon, especially in brain regions that are responsible for motor skills and coordination as well as sensory and emotional regulation.
However, just two weeks after the marathon, the amount of myelin in the brain increased again. This was a sign that the signal-transmitting layer between the nerve cells had recovered. Two months after the run, the myelin had fully regenerated in six of the ten participants.
Myelin Apparently Serves as a Kind of Energy Store
In the study analysis, the researchers came to the conclusion that myelin apparently serves as an energy store, which is only tapped into when the usual brain nutrients are scarce. They describe it as a kind of metabolic “safety net” that enables a temporarily “starved” brain to draw energy from a specific brain region without causing major damage. And because the myelin apparently regresses, according to current knowledge, there are probably no long-term consequences to fear. The researchers call their hypothesis “metabolic myelin plasticity.”
Although this study is a small sample with only ten participants, there is another study on mice that supports this hypothesis.4 Here, too, the researchers found that myelin can be used as a fat reserve when glucose becomes scarce in the mammalian brain.
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Negative Consequences of Myelin Degradation
According to the study results, myelin regresses after a few weeks after it has been used as an energy store. Nevertheless, the small amount of myelin in the brain has negative consequences, at least in the short term. Previous studies have shown that marathon runners had cognitive impairments directly after the run.5 They had significantly slower reaction times and performed worse in memory tests. However, cognitive performance improves relatively quickly after recovery.
Myelin is important for the function of the nervous system. A significant and, above all, permanent reduction is associated with various neurological diseases. These include multiple sclerosis and dementia. This raises the question of whether endurance athletes who frequently run marathons or take part in other extremely strenuous sports could suffer long-term damage to the brain. However, larger long-term studies are needed to find an answer to this question.