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If you’ve raced a marathon, you’ve probably been there. By there, I mean back in the office a few days after the race: legs aching, stomach rumbling, and brain not working.
A new study suggests that brain fog may be unexpected, but it’s not out of the ordinary. The study, published by Spanish researchers in Nature, used magnetic resonance imaging (MRI) to study 10 road and trail marathoners to see what happens when the brain is subjected to severe exercise stress. The results are fascinating. As a neuroscientist, I will unpack what the findings mean and provide you with a valid excuse for being a bit slow after you race 26.2
When Running a Marathon Does to Your Brain
When you run a marathon, your body primarily relies on carbohydrates—specifically in the form of glycogen—as a source of energy to fuel you to the finish. That glycogen comes from muscles, the liver, the brain, and other organs. The problem is, runners often run out of that fuel source before they reach the finish line.
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As glycogen stores start to deplete during the race, the body turns to a second source of fuel: fat. Our bodies contain far more fat than carbohydrates, making it an excellent, albeit less efficient, fuel source for extended exercise like running a marathon.
One place our bodies store fat is in myelin, the electrical insulation that enwraps and surrounds axons in both the peripheral and central nervous systems, helping neurons communicate faster. In fact, myelin is made up of 70 to 80 percent lipids, a form of fat.
So, when your brain enters a glycogen-depleted state, researchers hypothesize that it may actually start using the myelin coating the axons as fuel. It’s a concept called “metabolic myelin plasticity,” an ingenious way the body adapts to energy stress on the brain.
This Is Your Brain on Empty
To explore this hypothesis, the researchers used an advanced MRI technique called multicomponent relaxometry to examine the amount of myelin in the brains of 10 marathon runners. Each study participant—eight men and two women, aged 45 to 73—raced a road or mountain marathon, including the famed Valencia Marathon and Zegama-Aizkorri.
The researchers examined myelin water fraction (MWF), which serves as a proxy for the amount of water between layers of myelin. This, in turn, indicates whether myelin lipids contribute to brain activity.
The runners received brain scans two days before the race, within 48 hours of finishing, two weeks later, and two months later. While the sample size was very small and gender-skewed, the pre-race scans showed that the overall distribution of MWF across the brain was very similar between runners, with only slight variations. Their MWF levels were also close to those seen in healthy people who don’t run, across different ages.
For the 24- to 48-hour post-race scan, the researchers segmented the brain into 50 white matter regions, since white matter is composed of myelinated axons. Out of the 50 areas, 12 showed lower MWF levels after the race—meaning there was less myelin. The biggest drops (about 28 percent and 26 percent) were in two areas that help with movement and coordination. These areas also help process feelings and sensory information.
One question that emerged was whether dehydration contributed to these changes in MWF. But after conducting follow-up experiments, researchers found no changes in the volume of various brain regions, including total gray matter, total white matter, cerebellum, deep brain, and brainstem. So dehydration likely wasn’t the cause.
By the two-week follow-up scan, MWF levels had improved, although they were still below pre-race levels. After two months, MWF levels had fully recovered.
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What the Results Mean for You
The brain experiences reductions in myelin content immediately after a marathon, particularly in regions related to movement and coordination—suggesting that it uses the fat from myelin as a source of fuel. But it bounces back fully within a couple of months.
You might be wondering if you can get out of your marathon brain fog faster and if there are ways to boost myelin. So far, the research isn’t very clear, and most (if not all) of it has been done with diseased populations (e.g., people with multiple sclerosis, which is a myelination disease) or animal models (e.g., mice). One study on mice found that a diet high in saturated fat combined with moderate exercise promotes remyelination. There’s also some evidence to suggest that learning new motor skills can promote remyelination. And, of course, getting solid sleep helps.
While marathon running may temporarily tax the brain’s energy reserves and alter its structure, the brain proves to be as resilient as the body it supports. These findings don’t suggest we should stop running marathons—far from it. Instead, they highlight the remarkable ways our brain adapts to metabolic demands and recovers in stride, reminding us that endurance isn’t just a physical feat but a neurological one, too.
