Marathon Running May Temporarily Damage Brain Insulation, Study Finds

Running a marathon is not just a test of physical endurance—it’s a mental marathon too. But new research suggests that the brain might be doing more than just helping runners push through the final grueling miles. It may actually tap into its own reserves, breaking down part of itself for emergency energy.

A recent study from Spain has revealed that during extreme endurance efforts like a 26.2-mile race, the brain may temporarily degrade myelin—a fatty, protective layer that insulates nerve fibers—to use as a backup fuel source. In simple terms, under intense physical stress, the brain may begin to “consume” itself.

Myelin is essential for fast and efficient communication between nerve cells. It is critical for motor control, sensory input, and emotional regulation. The study found that when conventional energy sources like glucose run low during a marathon, the body switches to fat for fuel—and this may include myelin.

To explore this phenomenon, researchers scanned the brains of 10 marathon runners—eight men and two women—before and after completing a full marathon. MRI results showed that all participants experienced a reduction in myelin levels, particularly in brain regions responsible for movement, sensation, and emotions.

The good news? This change appears to be temporary. Follow-up scans revealed that the runners’ myelin levels returned to normal within two months after the race.

“Myelin seems to act as an energy reservoir when other brain nutrients are depleted during prolonged physical activity,” said Dr. Pedro Ramos-Cabrer, Ikerbasque research professor at CIC biomaGUNE, a Spanish nonprofit research center. “However, more extensive studies are needed to understand how widespread this effect is and how it may impact brain health in the long term.”

While the study did not assess whether the temporary loss of myelin affected the runners’ cognition or physical performance, researchers caution that extreme endurance training could carry potential risks—especially for individuals genetically predisposed to neurological conditions.

This discovery opens new avenues in brain research. Understanding how myelin regenerates quickly in healthy individuals could eventually help in developing treatments for conditions like multiple sclerosis (MS) and amyotrophic lateral sclerosis (ALS), where myelin breakdown leads to lasting damage.

Dr. Carlos Matute, professor of anatomy and human embryology at the University of the Basque Country, highlighted the broader implications: “Studying how the brain recovers from this temporary myelin loss could offer insight into managing demyelinating diseases.”

Although moderate physical activity is well-known to support brain health, researchers suggest that extremely strenuous exercise may need to be approached with caution in certain populations. For now, runners shouldn’t panic. The findings underscore the brain’s remarkable adaptability—but they also highlight the importance of understanding what happens when we push our bodies and minds, to their limits.