A new study suggests that during intense endurance exercise, the brain may break down parts of itself for energy. Researchers found that when glucose levels drop too low, the brain starts using myelin—a fatty layer that helps nerve cells communicate—as an emergency fuel source.  The study was published in Nature Metabolism.

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What the study found 

The research, conducted by neuroscientists in Spain, examined the brains of 10 marathon runners (8 men and 2 women) before and after a 42-kilometer race.

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MRI scans revealed a significant depletion of myelin—a fatty sheath that insulates nerve fibres—particularly in brain regions responsible for motor coordination, sensory processing, and emotional integration.  

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However, the brain seems to recover. Two weeks after the race, myelin levels started to return, and by two months, they were back to normal in runners who continued with follow-up scans.  

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Why does the brain do this? 

The researchers believe this process, called metabolic myelin plasticity, is a survival strategy. Instead of shutting down when fuel is low, the brain temporarily sacrifices some of its protective coating to keep functioning. This might explain why marathon runners experience slower reaction times and memory issues right after a race.