L'physical exercise regular Allows you to live longer and in better health. For the body, this is an opportunity to burn excess sugars and fats. The marathon is a particularly intense endurance test, during which the muscles must draw from their reserves to maintain the effort to the end. Researchers have discovered that the brain also had its own energy reserves, which it can mobilize during the race, when its nutrients are insufficient.
Lipids are molecules made up of fatty acids and glycerol. More abundant in the body than carbohydrates, they constitute a source of lasting energy, ideal for an effort of prolonged endurance. Excess fats are stored in the abdomen (visceral fat) and under the skin. They can also accumulate on the walls of the vessels, potentially causing health problems. What we know less is that lipids are also in myelin – the insulating membrane that surrounds the axons of neurons.
The brain of marathonians observed in detail
The main function of myelin is to increase the propagation speed of electrical impulses (action potentials) along the nerve fiber. She also plays the role of electric insulation. All organic membranes have a lipid bicouche. However, myelin is much richer in lipids than other types of cell membranes. They constitute 70 to 80% of its composition.
To function properly, the brain needs a constant glucose intake. Aside from the glycogen grains present in astrocytes, it has no other known energy reserve. In a study published last year, researchers showed that, in mice, lipid metabolism of oligodendrocytes – the glial cells that form the myelin sheath – can serve as a reserve of energy in the event of a lack of glucose.
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Based on this research, another team has hypothesized that myelin lipids can contribute to brain activity, just as body fat feeds muscles.
The idea germinated in the head of one of the researchers, Carlos Matute, himself a marathoner. How did the human body manage to accomplish such demanding races? Since myelin is very abundant in the brain and that it is rich in lipids, Matute wondered if the brain was able to use this substance if necessary.
To test this hypothesis, his team and studied him, by imaging by magnetic resonance, the impact of the practice of the marathon (42.1 km) on the structure of the brain. Ten runners (eight men, two women), aged 45 to 73, have engaged in the exercise. The team scanned their brain 48 hours before, then 48 hours, two weeks and two months after their participations in different marathons in 2022 and 2023.
A significant loss of myelin after the effort
Imaging has revealed a significant reduction in the fraction of myelin water – an indicator of the myelin content – 24 to 48 hours after a marathon. The team observed that this loss was particularly marked in 12 brain regions. These were regions of the brain involved in motor coordination, sensory perception and emotions. These are all areas that we expect to see active during a marathon. “” We feel a lot during the race and we have to talk to each other a lot to continue “, Explains Matute to Nature.
Changes in the fraction of myelin water in the regions of the white substance before, then after (48 hours, 2 weeks, 2 months) having run a marathon. Credits: Ramos-Cabre et al., Nature Metabolism (2025)
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Two weeks after the race, myelin levels began to recover. Then, two months later, they returned to their original levels, in all brain areas.
These results suggest that the brain's myelin content is temporarily and reversibly reduced by intense physical exercise. The team defines this process as “the metabolic plasticity of myelin”. The study also confirms the results of mice research, which suggested that myelin lipids could act as energy reserves under extreme metabolic conditions.
A new form of neuroplasticity
The plasticity of myelin is fundamental for the adaptation of the brain to neural activity. We know that modifications of structure and density can occur, especially during learning or other experiences (positive as negative).
This innovative study highlighted a new form of plasticity, in response to the endurance exercise. This plasticity aims to support the brain function to the detriment of the use of myelin lipids.
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This research nevertheless includes some limits, starting with the small number of participants, who were also mostly men. In addition, they were all experienced runners. Matute thinks that the temporary reduction in myelin would be more important in people in less good physical shape. Conversely, ultra-marathon runners, accustomed to even more intense efforts, could present less important myelin losses.
The team also points out that imaging did not directly measure myelin, but the water molecules trapped there. As a result, myelin content measures can lack precision.
Better understand certain neurological disorders
The results of this study are nonetheless interesting. The team is now wondering if the loss of myelin transiently affects the neurophysiological and cognitive functions associated with affected brain regions. This will undoubtedly be the subject of future research.
A study published in 2022 had already highlighted significant cognitive declines in ultra-marathonians after a race. Their reaction time decreased by around 30%, and their ability to distinguish different objects by around 23%. In addition, the memory of the participants decreased by around 8%.
The researchers also emphasize that in view of the results, the endurance exercise could constitute an additional danger for people with a genetic risk of amyotrophic lateral sclerosis (SLA). Physical activity has already been identified as a major risk factor for this disease. This study shows that sustained endurance greatly changes highly myelinized and vulnerable motor areas.
On the other hand, people with certain neurological disorders, including ALS, have a significant degradation of myelin. This study, which highlights a complete recovery of this substance, could lead to knowledge likely to improve the treatment of these disorders. “” We may be able to get clues for potential treatments if we understand what's going on in runners “Concludes Matute.
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