The brain has a secret survival trick to endure a marathon

National Geographic

21-04-2025

A study of long-distance runners shows the brain's remarkable ability to adapt—by burning its own fat. A marathon taxes more than just muscles—it pushes the brain to its metabolic limits. New research shows that during extreme endurance events, the brain may burn its own fat for fuel, revealing just how adaptable—and surprising—the mind can be. Photograph by Ben Stansall, AFP/Getty Images
Carlos Matute wasn't looking for a scientific breakthrough when he laced up his shoes for a long run. But as the miles ticked by, the neuroscientist from the University of the Basque Country found himself chasing a persistent question: 'How do we make it to the finish line, being conscious, after having our energy stores depleted?'
That curiosity led to a surprising possible answer. In a recent study in Nature Metabolism, Matute found that myelin—the fatty insulation wrapped about nerve fibers—provides energy when fuel runs low, such as during a grueling endurance event like a marathon. Some headlines have sensationalized the finding as the brain 'eating itself,' but Matute says the truth is far more nuanced—and promising. Instead, it adds evidence of how the brain adapts and changes, even well into adulthood. Your brain burns fat during a marathon
For his study, Matute and colleagues scanned the brains of 10 recreational runners using MRI before and after a marathon, and again two weeks to two months later. In the two days post-marathon, myelin was reduced in 12 of the 100 areas evaluated; the affected areas were involved in motor coordination, sensory integration, and emotion, regions frequently used during running. Two months after the marathon, myelin levels recovered.
(This is why running is the ultimate cardiovascular sport.)
The team determined the results were not due to dehydration. Instead, it seems that the brain is using myelin, which is mostly lipids or 'brain fat' as Matute calls it, as fuel. 'When there is no glucose coming from the bloodstream, then [the brain] has to use whatever it has at hand,' says Matute. For a neuron, this means myelin.
Participants ranged in age from 45 to 73, suggesting that even older brains remain remarkably adaptable. 'Nobody foresaw that this would happen, that the structure of our brain may change so rapidly, in the time which it takes you to run the marathon,' says Matute.
Matute doesn't believe the temporary drop in myelin leads to noticeable cognitive changes. He suspects the brain routinely taps into myelin for fuel, just at levels too low to detect—except during extreme events like a marathon.
But there's no need to panic, he says. The changes were short-lived—and may even be healthy. 'You exercise everything, including your myelin metabolism,' he says. What running does to your brain
Running has long been linked to brain benefits. It boosts mood-regulating chemicals like dopamine, serotonin, and norepinephrine, which can ease anxiety and depression. That famous 'runner's high,' once chalked up to endorphins, may come from endocannabinoids—the brain's natural version of cannabis.
(Here's how to get high on your hormones—naturally.)
Aerobic exercise also seems to spur the birth of new neurons in the hippocampus, the brain region critical for memory and learning. Over time, these changes may protect against cognitive decline.
Marathon running has been studied less than running in general, but some studies show these trends hold up over 26 miles. One study reported that some regions of the hippocampus were larger in marathon runners than in healthy controls. Astrid Roeh, a physician at the University of Augsburg, found that marathon runners exhibited fewer depressive symptoms than sedentary people over six months and reported the best moods the day after the marathon.
(Humans were built for long-distance running.)
In another study, Roeh found that marathon running led to changes in the brain's electrical activity, as measured by electroencephalogram (EEG). Like Matute's findings in myelin, these changes were transient. 'The changes in EEG patterns indicate that there are alterations in how neural circuits are functioning and communicating,' she says.
This reorganization in response to experience is called neuroplasticity. She hypothesizes that regular physical activity may prime the brain for plasticity. 'If someone is already physically active, then intense challenges like a marathon might have a stronger — or at least more coordinated — neuroplastic effect,' she says. 'The system is ready to respond more effectively.'
However, not all brain changes are positive. Marathoners and endurance athletes are particularly prone to overtraining syndrome, says Luana Main, a professor at Deakin University. 'A lot of endurance athletes are still of the mindset that more is always better,' she says. But doing more without sufficient rest, especially when sick or stressed, taxes the body too much.
Overtraining syndrome leads to injury and poor performance and can take months or years to recover from. It also affects the brain. 'Changes in mood state are often one of the first changes that occur before you see a decrement in performance,' says Main. Overtrained athletes tend to feel grumpy, overwhelmed, and tired. 'They're just not loving life,' she says.
(No time to exercise? Just five minutes still has a big impact.)
In a review, Main and her colleagues found that cognitive function, particularly reaction time, was impaired when athletes were overreaching or overtraining. Main says that because cognition is affected, people are more likely to make errors and become injured.
Paying attention to these cognitive effects can be helpful. In a different review of the research, Main and colleagues found that subjective measures, such as mood and perceived stress, were more sensitive to increased training loads than objective measures, such as heart rate and oxygen consumption. By tracking reaction times with online games and moods with daily journaling, athletes can know when they may be overdoing it.
Avoiding overtraining requires incorporating adequate rest and recovery into training. Main emphasizes that risk is not a reason to avoid exercise; however, the numerous benefits of exercise outweigh the bad. But, for the small part of the population doing huge training loads, 'More is not always better,' she says. 'Sometimes less is more.'
Matute agrees. 'For people who worry, 'Should I stop running?' I would say, 'No, never, as long as you train properly.''