Imagine pushing your body to the absolute limit—running a marathon, cycling for hours, or swimming long distances.
Your muscles scream for energy, your heart pounds, and your body taps into every available fuel source.
But what if your brain is doing something just as extreme—breaking down its own structure for energy?
A groundbreaking study suggests that during strenuous endurance exercise, the brain might resort to an unexpected fuel source: its own myelin, the fatty sheath that insulates nerve fibers.
This challenges long-standing assumptions about how the brain powers itself and opens the door to a new understanding of neuroplasticity and energy consumption.
Researchers scanned the brains of marathon runners before and after a race and found significant changes in myelin markers—indicating the brain might be temporarily sacrificing its protective insulation to keep functioning.
This discovery is redefining our understanding of brain metabolism and could have profound implications for both athletes and individuals with neurological conditions.
More Than Just Brain Insulation
For decades, myelin was thought to be a static, protective coating around nerve fibers, helping electrical signals travel faster.
But emerging research has revealed that myelin is far more dynamic than we once believed.
Scientists now recognize that myelin can reshape, thin, or thicken depending on environmental demands and brain activity.
In essence, our brains actively remodel myelin throughout our lives.
But here’s where it gets even more surprising: under extreme energy deficits, such as during prolonged endurance exercise, the brain may start consuming its own myelin to compensate for glucose shortages.
A Closer Look at the Evidence
To explore this possibility, neuroscientists in Spain conducted a study on 10 marathon runners (8 men, 2 women).
They performed MRI scans before and after a 42-kilometer (26.1-mile) race and found shocking changes in brain structure:
- 24 to 48 hours after the marathon, the runners showed reduced myelin markers in white matter regions linked to motor function, coordination, and sensory processing.
- Two weeks later, some myelin markers began to recover.
- Two months post-marathon, the myelin levels had stabilized in the six participants who continued with follow-up scans.
These findings suggest that myelin isn’t just a passive structure—it acts as a backup fuel reserve, available when brain energy levels are critically low.
The researchers named this phenomenon “metabolic myelin plasticity”.
Challenging the Old Assumption: Does the Brain Burn Fat?
Traditional neuroscience holds that the brain relies almost exclusively on glucose for fuel, even under extreme conditions.
The idea that the brain could metabolize fat—especially from its own myelin—was once unthinkable.
However, the evidence is mounting. Studies on mice have shown that when glucose levels plummet, neurons can break down myelin for energy.
Now, this latest human study suggests the same mechanism may exist in us.
This could explain why long-distance runners often experience cognitive slowdowns post-race.
It’s not just physical fatigue—their brains might be running on emergency reserves.
What This Means for Athletes and Everyday Health
If myelin serves as an energy safety net, does this mean endurance exercise is harmful to the brain?
Not necessarily. In fact, the brain appears to have built-in recovery mechanisms:
- Myelin levels rebound within weeks after intense exercise.
- Endurance training may actually strengthen myelin over time, much like muscle adaptation.
- The brain’s ability to temporarily sacrifice myelin may be a feature of evolution—allowing early humans to chase prey for hours while staying mentally alert.
However, the study raises concerns for those with neurological conditions like multiple sclerosis (MS), where myelin damage is already an issue.
Could excessive endurance exercise worsen symptoms or slow recovery? Future research will need to explore this.
The Evolutionary Advantage: Did Myelin Help Humans Outrun Their Prey?
The human brain is uniquely rich in myelin, especially in areas linked to higher thinking and complex movement.
Scientists speculate that this might have evolved as an adaptation to our ancestors’ endurance-based hunting strategy—known as persistence hunting.
In prehistoric times, humans lacked the speed of other predators, but we excelled at chasing prey over long distances until they collapsed from exhaustion.
This required both physical stamina and sharp cognitive abilities—precisely what metabolic myelin plasticity may have helped support.
By allowing the brain to borrow energy from its own insulation, early humans may have been able to push their limits without losing mental focus, giving them a survival advantage.
A New Perspective on Brain Resilience
This study isn’t just about athletes—it’s about how resilient the human brain truly is. The ability to repurpose its own structure for energy showcases an unprecedented level of neuroplasticity.
For endurance athletes, the findings offer a new perspective on post-exercise brain recovery.
For neuroscientists, they challenge old assumptions about brain metabolism.
And for those with neurological disorders, they may open new doors for research on brain repair and energy use.
One thing is clear: the brain is far more adaptable than we ever imagined.
The study was published in Nature Metabolism.
