[Research Rundown] The Neuroscience of Brain Fog After Long Runs

Aug 25
4 min read

Have you noticed that after a long run, tough workout, and especially after a marathon, you experience “brain fog”? It’s not just in your head (pun intended). A 2024 study suggests that there’s a real, biological reason why your brain feels off after endurance events. It’s related to myelin, a fatty substance that insulates your brain’s wiring.

The Study: How Does Distance Running Effect Our Brains?

The study used advancing brain imaging (MRI) to examine how running a marathon affects the brain. When you run a marathon, your body taps into stored energy, using carbohydrates (glycogen) first and switching to fat once glycogen stores run low. This metabolic shift has been well documented in muscles and the liver - this is why people “bonk” during races” - but scientists began to wonder if the brain could also be drawing on fat-based energy during extreme endurance events as well.

This is where myelin comes in. Myelin wraps around nerve fibers as protection and helps send signals efficiently and quickly. It’s composed of 70-80% fat, making it a potential energy source in times of stress. The researchers hypothesized that, during a marathon, the brain may dip into its own “fat stores,” or myelin, to help power neural activity when glucose is in short supply.

To test this, scientists scanned the brains of ten participants (eight men and two women, aged 45 to 73), using a special MRI technique that measures myelin water fraction (MWF). MWF is a validated marker that reflects how much myelin is present in different regions of the brain.

The Findings: Long-Distance Running May Temporarily Reduce the Brain’s Protective Myelin Layer

The runners, (who completed either the Valencia Marathon or the Zegama-Aizkorri trail marathon) were scanned at four time points:

48 hours before the race
24–48 hours after
2 weeks after
2 months after

Volumes (average intensity projection) of MWF maps from five consecutive coronal slices centred in motor descending pathways from an individual before the marathon (pre-run), two days after the marathon (post-run, 2 d) and two weeks after the marathon (post-run, 2 w), as well as after two months (2 m) of recovery (Source: Ramos-Cabrer et al)

In the 24-48 hour post-race scans, researchers segmented the brain into fifty white matter regions (white matter is rich in myelin) and found that in 12 of these regions, MWF has decreased. This indicated a temporary reduction in myelin content. The biggest drops, 28 and 26% respectively, were in areas that help with movement and coordination, processed feelings and sensory information.

Notably, the researchers ruled out dehydration and other confounding factors for why myelin may have dropped. Levels gradually rebounded over time, showing improvements at the two week-benchmark scan and returning to baseline in all runners by the two month follow-up scan.

What the Findings Mean

Researchers have identified a phenomenon called "metabolic myelin plasticity," a process where, under extreme physical stress and low glucose availability, the brain may temporarily break down myelin (the protective coating around nerve fibers) to use its fatty acids for energy.

Animal studies support this, showing that glial cells (supportive brain cells) can tap into myelin fat to maintain energy during low-glucose states. In simple terms, when your brain runs out of fuel, it might dip into its own protective layer to keep things running.

For most healthy runners, this is temporary and reversible. Myelin is restored with adequate rest and nutrition. The brain’s ability to adapt like this is impressive, but it also has its limits.

This process may pose greater risks for individuals with neurological conditions like MS or ALS, where myelin is already compromised, or for people with restrictive eating disorders, whose bodies have fewer energy reserves to draw from.

Takeaways for Runners

Brain fog after a long race? Temporary myelin breakdown could be part of the reason, alongside factors like dehydration, inflammation, and electrolyte imbalances.
Recovery matters. Prioritize rest and post-race refueling to help restore myelin and support full brain and body recovery.
Fuel early and often. Avoid hitting dangerously low glucose levels by fueling consistently during long runs and races.
Respect your limits. Just because you can push through doesn’t mean you should. Chronic under-fueling or overtraining can increase the strain on your nervous system.
This is one more reason to ditch the “no pain, no gain” mindset. Your brain is literally sacrificing its own insulation to keep you going—give it the respect and recovery it deserves.

What Comes Next

Like many early-stage studies, this one has limitations. The sample size is small, just ten runners, and has a major gender imbalance, making it harder to generalize the findings to the broader running population. THat said, the researchers noted that pre-race scans showed a consistent distribution of MWF across participants, with only slight variations. These baseline values were also similar to those seen in healthy non-runners across various ages, strengthening the validity of these results.

Still, more research is needed to answer some big questions:

Are certain runners, based on age, sex, fitness level or health conditions, more vulnerable to these temporary myelin changes?
How do factors like nutrition, hydration, or energy availability influence the degree of myelin reduction and recovery?
Can we support or accelerate myelin recovery through specific recovery strategies, fueling approaches, or training modifications?

This study opens the door to fascinating new areas of exercise neuroscience, and suggests that our brains are adapting and recovering from endurance efforts in ways we’re only beginning to uncover.

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Sources

Grant DD. Yup, Running a Marathon Makes You Dumber. RUN | Powered by Outside. Published April 28, 2025. Accessed June 18, 2025. https://run.outsideonline.com/nutrition-and-health/general-health/yup-running-a-marathon-makes-you-dumber//

Ramos-Cabrer P, Cabrera-Zubizarreta A, Padro D, Matute-González M, Rodríguez-Antigüedad A, Matute C. Reversible reduction in brain myelin content upon marathon running. Nature Metabolism. Published online March 24, 2025. doi:https://doi.org/10.1038/s42255-025-01244-7.

Tarun Sai Lomte. News-Medical. News-Medical. Published March 26, 2025. Accessed June 18, 2025. https://www.news-medical.net/news/20250325/Marathon-running-temporarily-reduces-brain-myelin-levels.aspx

asel). 2024;12(8):226. Published 2024 Aug 20. doi:10.3390/sports12080226.

This article was researched and written by Sophie van Leeuwen, social media and podcast assistant at Running Explained.