Holmer, brain fog after a marathon shows up in the data
Two studies from one research group found myelin dips after 26.2 and comes back in about a month.
Your head feels like mush for a few days after a marathon. Brady Holmer says there is research that explains it.
In a post on X, Holmer walks through two studies from the same research group. The first found that brain myelin drops after a marathon. Myelin is the fatty insulation around nerve fibers that helps signals travel efficiently. Levels came back to normal after about a month. One idea Holmer raises is that late in the race, when glucose runs low, the brain may burn some of those fats as backup fuel.
It protects the essential stuff and lets the demanding stuff slide.
Your recovery should include your brain, not just your legs.
What the follow-up tested
The second study looked at brain function rather than brain structure. It tested 43 marathon runners and 29 non-runners, with everyone tested before the race, within 48 hours after it, and again a month later.
The headline result is that most things were fine. Basic neural signaling held up, and Holmer says there was no sign that nerve signals were slowed even with less myelin. Basic visual and motor tasks were normal. So were attention and executive tasks.
Two tougher thinking skills did dip, and only briefly. Processing speed lagged. So did the ability to tune out distractions and stay on the right answer, measured with the Stroop test, where you name the color of a word rather than read the word itself. By one month, everything had returned to normal.
What it actually means
This is not a brain damage story, and Holmer is explicit about that. His read is that the marathon brain is metabolically stressed and adapting for a while rather than hurt. The stuff you need to keep working keeps working. The expensive stuff gets throttled for a few days.
Holmer has run four marathons himself, and he says that after every one his brain feels like mush for a few days. The practical takeaway he draws is simple enough. Recovery should include your brain, not just your legs.
Worth keeping the scale in mind. This is two studies from one group, the follow-up compared 43 runners to 29 non-runners, and the dips the researchers found resolved inside a month. Holmer is not claiming the effect is permanent or that anybody should change how they train. He is saying that if you ran Chicago and the next few days at work felt foggy, there is a measurable reason for it, and it goes away.
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