Known as one of the most the physically demanding challenges, a marathon is not for the faint of heart – less than 0.1% of the world’s population is estimated to have completed a marathon. But while everyone agrees that your body is working hard, few realize that your brain is as well. Recent research has shown that your brain begins to break itself down to fuel you through those 26.2 miles.
The broken-down part is myelin, a fatty insulating sheath that forms around the nerves in the brain. Myelin increases the speed of the electrical signals and prevents them from being lost, ensuring an efficient nervous system – damaged myelin slows or even stops nerve impulses and can lead to neurological issues. But it may have another function. After your glycogen stores have been depleted, the brain begins searching for an alternative energy source and surprisingly finds it in the lipids and proteins in the myelin sheath.
A 2025 study published in Nature Metabolism used MRI to assess the Myelin Water Fraction, a valid and noninvasive marker to estimate myelin levels, in the brain before and after a marathon race (Ramos-Cabrer et al., 2025). The research showed a temporary reduction in myelin content after a marathon, specifically in the pontine and corticospinal tract, which are associated with voluntary control/coordinating body movements and pain processing respectively. A 2023 bioRxiv study aligns with these results, finding widespread reduction in myelin after a marathon, especially in motor control (Mario Matute-González et al., 2023). Initially, this may seem horrifying. Why is the brain degrading a vital part of itself while running?
But this reduction reflects your brain’s metabolic plasticity, or its ability to adapt to extreme physical stress by reallocating energy resources. Both studies included follow up scans indicating that myelin had increased two weeks after the marathon, and the brain returned to original myelin levels after around two months. This recovery suggests that the brain doesn’t carelessly sacrifice itself – it intentionally uses itself to satisfy your current energy needs and then activates a mechanism to repair itself after physical change. Instead of seeing this as harmful, it demonstrates the resilience of the brain: the brain can utilize itself to fuel the demanding needs of running a marathon and able to restore itself afterwards.
So the next time you’re running through a brutal long run and wondering if you’ve lost your mind, well, you kind of are. But don’t stress. Your brain is just lending itself to help you achieve your goal. Keep running – just like you, your neurons are tougher than they look.
References:
Mario Matute-González, Alberto Cabrera-Zubizarreta, Daniel Padró et al. Widespread drastic reduction in brain myelin content upon marathon running, 27 October 2023, PREPRINT (Version 1) available at Research Square [https doi.org/10.21203/rs.3.rs-3484516/v1://]
Ramos-Cabrer, Pedro, et al. “Reversible Reduction in Brain Myelin Content upon Marathon Running.” Nature Metabolism, 24 Mar. 2025, https://doi.org/10.1038/s42255-025-01244-7.
Asadollahi, Ebrahim, et al. “Oligodendroglial fatty acid metabolism as a central nervous system energy reserve.” Nature neuroscience 27.10 (2024): 1934-1944.