Myelin content temporarily decreased in marathon runners, opening new questions into long-term brain health.

Carlos Matute, a neuroscientist at the University of the Basque Country in Leioa, Spain, wasn’t just logging miles when he trained for marathons. He was also turning over scientific questions in his mind, including whether it was possible to complete such long distances when the body’s readily available energy stores were nearly depleted. ¹

Endurance exercise such as long-distance running benefits the brain, with studies showing that it can improve cognition and reduce the risk of neurodegenerative disease.² It also places significant metabolic demands on the body, using up essential energy resources. For Matute, this raised an intriguing question: Could the brain be making structural trade-offs to adapt to this depletion?

Matute’s curiosity led him and his colleagues to focus on myelin, a lipid-rich substance that wraps around neurons, insulating and speeding up the brain’s electrical signals. Given its high fat content and crucial role in neural communication, myelin stood out as a plausible energy source. While relatively stable in adulthood, myelin loss is a hallmark of neurological diseases such as multiple sclerosis (MS)³. In MS, myelin breakdown slows neural signaling and causes symptoms such as muscle weakness, poor coordination, and cognitive decline. Researchers now suspect that myelin may be dynamic even in healthy brains, a possibility Matute’s study tested by asking whether the brain uses it as fuel during marathon running.

To explore this question, Matute collaborated with Pedro Ramos-Cabrer, a neuroimaging expert at the Center for Cooperative Research in Biomaterials in San Sebastián, Spain. Together, their teams used advanced magnetic resonance imaging to scan the brains of 10 marathon runners before and after their races. Within 48 hours of completing the marathon, runners exhibited a significant drop in myelin water fraction,⁴ a well-established imaging biomarker and proxy for myelin content. This decrease was observed in key white matter regions, including the corticospinal tract and cerebellar peduncles, which are critical for movement and coordination. However, follow-up scans revealed a reassuring finding: by two months post-race, myelin levels recovered fully.

“Our findings suggest that the brain is highly adaptable,” said Matute. “While endurance exercise temporarily alters brain structure, these changes are not permanent and likely reflect a dynamic, energy-efficient process.”

Mustapha Bouhrara, chief of the Magnetic Resonance Physics of Aging and Dementia Unit at the National Institute on Aging who was not involved in this work, finds the study impactful. “While the idea of the brain using lipids, including those in myelin, as a fuel source is not new, the suggestion that myelination could recover quickly after intense physical activity raises intriguing questions,” Bouhrara said. “If confirmed, it could offer insights into rapid remyelination, with potential implications for brain aging and neurodegenerative diseases.”

Matute and his team propose that myelin could be temporarily used as fuel under conditions of energy depletion, akin to how the body shifts from using carbohydrates to fats during prolonged exercise. This idea aligns with findings from rodent studies, which suggest that myelin lipids can serve as an energy source when glucose levels are low.⁵

“Dehydration, inflammation-related changes in extracellular water, and other technical considerations may influence the results. Nonetheless, this work provides a foundation for further research and underscores the need for additional technical adjustments,” Bouhrara said.

Looking forward, these findings raise important questions: How might repeated cycles of myelin loss and recovery affect brain health? Could controlled fluctuations in myelin be therapeutic for demyelinating diseases such as multiple sclerosis? For now, this research adds yet another layer to understanding the remarkable plasticity of the human brain.