What Happens to Your Brain After Running a Marathon?

Introduction

It turns out the idea that endurance runners have "bigger brains" isn’t just motivational fluff—it’s backed by science. Studies show that endurance athletes tend to have larger brain volumes, including more grey and white matter, compared to sedentary individuals. These structural differences are particularly noticeable in areas critical for memory, decision-making, and motor control, like the hippocampus and prefrontal cortex.

Over time, running appears to help preserve brain volume and function, offering protection against age-related decline. But while the long-term benefits are clear, new research is shedding light on what happens to the brain immediately after intense endurance efforts—like a marathon.

The Study: What Happens to the Brain After a Marathon?

In a recent study, researchers investigated how marathon running impacts brain structure in the short term—specifically, whether it temporarily reduces myelin, the fatty substance that insulates nerve fibers and enables fast, efficient communication between neurons.

Using a specialized MRI technique called myelin water fraction (MWF) mapping, researchers scanned the brains of 10 experienced marathon runners before, shortly after, and weeks after their race.

The findings were striking: up to a 28% drop in myelin was observed in key motor and sensory regions of the brain within 48 hours post-marathon. These included areas responsible for coordination, emotional processing, and voluntary movement. However, this effect was temporary. By two weeks, partial recovery was evident, and by two months, myelin levels had fully returned to baseline.

Why Does This Temporary Myelin Loss Happen?

The study proposes an intriguing explanation: during prolonged exercise, when the body’s glucose stores are depleted, the brain may turn to myelin lipids as an emergency energy source.

The cells responsible for myelin, called oligodendrocytes, can metabolize fatty acids through β-oxidation. This process, termed “metabolic myelin plasticity,” allows the brain to adapt during energy stress by tapping into its own structural reserves.

Interestingly, the myelin loss was region-specific—affecting only the pathways most involved in marathon performance, not the entire brain. This suggests a targeted, adaptive response rather than widespread damage.

Should Marathon Runners Be Concerned?

Probably not. While the initial findings sound alarming, the study's small sample size and lack of cognitive performance testing make it hard to draw firm conclusions. More importantly, the changes were fully reversible, and long-term research consistently shows that endurance exercise protects the brain, not harms it.

Some experts even suggest this temporary myelin shift may be part of how the brain stays plastic and resilient. It’s a sign of adaptation, not dysfunction.

Final Thoughts

Running, especially over long distances, puts unique demands on the brain. While marathon running may cause a temporary dip in myelin levels, this seems to be the brain’s way of maintaining function during energy stress—not a sign of harm.

In the long run (pun intended), consistent aerobic exercise appears to build a stronger, more resilient brain. So, if you’re lacing up for your next marathon, your brain might just be growing stronger with every mile.

Study Reference: https://www.nature.com/articles/s42255-025-01244-7