Your Brain Might Start 'Eating Itself' During Strenuous Endurance Exercise

When the human brain is starved of energy, it may start to 'eat' its own fatty tissue for fuel, according to a pilot study on the brains of marathon runners.

The findings hint at a whole new form of neuroplasticity, which may help keep the human brain functioning during prolonged bouts of strenuous endurance.

Participants' brain scans suggest that during a marathon, when glucose in the brain runs dangerously low, some neurons can begin munching on myelin – a fatty sheath that forms around nerve fibers in the brain.

Myelin is known for helping neurons send messages more efficiently, but this protective covering is not just a simple static insulator, as scientists once thought. Emerging research suggests neurons can reuse these fatty sheaths and reshape their thickness to adapt to environmental changes.

Now, it seems some brain cells may even recycle myelin for fuel, but only if utterly necessary.

In MRI scans of the brains of 10 runners (8 men and 2 women) taken before and after a 42-kilometer (26.1-mile) race, neuroscientists in Spain noticed distinct changes in markers of myelin within the brain's white matter – the tissue where this fatty sheath is most concentrated.

Twenty-four to 48 hours after the marathon, runners showed signs of having lost a significant amount of myelin in brain regions associated with motor function and coordination as well as sensory and emotional integration.

Two weeks after the event, markers of myelin were beginning to bounce back, and by two months post-run, markers had re-stabilized in the six participants who continued with the scans.

The team, led by Pedro Ramos-Cabrer and Alberto Cabrera-Zubizarreta, says their findings "may open up a new view of myelin as an energy store ready to use when common brain nutrients are in short supply."

They think myelin is acting as a sort of metabolic 'safety net', allowing a temporarily 'starved' brain to draw fuel from restricted regions, while leaving the bulk of its white matter intact.

The researchers have called their hypothesis metabolic myelin plasticity.

In the past, some neuroscientists thought the brain largely avoided burning fat for energy, even when it was stressed for nutrients. But that may not be entirely true after all.

The sample size of the new pilot study is small and the association with myelin is only based on a proxy, but the findings agree with recent studies on mice that found myelin can be used as a fat reserve when glucose in the mammal brain grows scarce.

Myelin is essential to nervous system function, and extensive losses are associated with various neurological diseases, including multiple sclerosis.

By only taking myelin from some spots, the brain's metabolism could be inflicting temporary damage on its own tissue to protect the organ as a whole.

This certainly aligns with cognitive studies that have found runners display significantly slower reaction times and perform worse on memory tests directly after a marathon. Brain function then rapidly improves with recovery.

The most recently evolved parts of the human brain tend to host more myelin, which suggests this crucial fatty deposit is an evolutionary adaptation – one that may have been key to our species' success.

Maybe we have myelin to thank for allowing us to run down large game with relative physical ease, all while staying as cognitively alert as possible.

The study was published in Nature Metabolism.