January 21, 2026, 11:19 am | Read time: 4 minutes
Endurance sports are generally considered a healthy way to protect against lifestyle diseases. However, experts consistently advise against overdoing it. A recent study on marathon runners illustrates what happens during extreme running sessions: Their brains apparently begin to digest themselves when their bodies are starved!
Regular exercise offers countless benefits for our health. Endurance sports such as swimming, running, or cycling strengthen the cardiovascular system, protect against high blood pressure, improve blood lipid levels, enhance stress resistance, and even boost mood—to name just a few positive aspects.1 However, as with everything, moderation is key. There is scientific evidence suggesting that during very long endurance activities, such as a marathon, the brain begins to digest its own fat tissue and use it as an energy source, which can have negative consequences.
Overview
The Brain Actually Has No Fat Storage
It’s always fascinating what new insights researchers gain about our bodies. In endurance sports, it was previously understood that the body starts tapping into fat reserves when our carbohydrate stores (glycogen stores) are depleted. Many people use this effect to lose weight or define muscles. However, the brain itself does not have fat stores that could be used—this was the assumption until now. While the lack of fat storage remains true, it appears that fat tissue in the brain is broken down when the body is starved. Spanish researchers have demonstrated this in a recent study with marathon runners.2
How the Study Was Conducted
To determine how extreme endurance stress affects people, marathon runners were recruited for the study. Of the ten volunteers, eight were male and two were female. The participants were between 45 and 73 years old and well-trained. To assess how endurance stress affects the brains of marathon runners, MRI scans were conducted.
- The first MRI scans were conducted 24 to 48 hours before and after a marathon.
- The second MRI scans were performed two weeks after the marathon.
- The third and final MRI scans were conducted two months after the marathon.
Less Fat Tissue in the Brains of Marathon Runners After a Race
The evaluation of the MRI scans showed a significant change in the markers for myelin in the brain’s white matter after the marathon. White matter refers to parts of the central nervous system that consist mainly of nerve fibers or pathways, including myelin. Myelin is an insulating layer that surrounds the extensions connecting nerve cells, known as axons.3 Myelin primarily ensures the rapid transmission of electrical signals between nerve cells and is largely composed 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 responsible for motor skills, coordination, and sensory and emotional regulation.
However, two weeks after the marathon, the amount of myelin in the brain increased again, indicating that the signal-transmitting layer between nerve cells was recovering. Two months after the race, myelin had fully regenerated in six of the ten participants.
This effect occurs when you consistently drink too little water.
Former soccer player André Schürrle’s time in the half marathon
Myelin Apparently Serves as a Type of Energy Storage
In the study’s analysis, researchers concluded that myelin serves as an energy reserve, tapped only when typical brain nutrients are scarce. They describe it as a kind of metabolic “safety net” that allows a temporarily “starved” brain to draw energy from a specific brain region without causing significant damage. Since myelin appears to regenerate, no long-term effects are expected according to current knowledge. The researchers call their hypothesis “metabolic myelin plasticity.”
Although this study involved a small sample of only ten participants, another study on mice supports this hypothesis.4 Researchers found that myelin can be used as a fat reserve when glucose in the brains of mammals becomes scarce.
Negative Consequences of Myelin Breakdown
According to the study results, myelin regenerates after a few weeks of being used as an energy reserve. However, the reduced amount of myelin in the brain has at least short-term negative effects. Previous studies showed that marathon runners experienced cognitive impairments immediately after the race.5 They had significantly slower reaction times and performed worse on memory tests. However, cognitive performance improves relatively quickly after recovery.
Myelin is crucial for the nervous system’s function. Significant and especially permanent breakdown is associated with various neurological diseases, including multiple sclerosis and dementia. This raises the question of whether endurance athletes who frequently run marathons or engage in other extremely demanding sports might suffer long-term brain damage. Larger long-term studies are needed to answer this question.
The German original of this article was published in April, 2025.