This Disturbing Side Effect of Marathon Running Caught Scientists Off Guard
Pushing your body past its limits takes on a whole new meaning.
Marathons are the ultimate fitness flex—revered by some, questioned by others, and viewed as borderline insanity by most. Powerlifters scoff, sprinters nod in respect, and the general public just wonders why. But no matter the motive, one thing’s certain: running 26.2 miles takes serious grit, unwavering determination, and a toll on the body like no other.
Despite the benefits of training for one, the marathons itself is undoubtedly hard take on the body, and now, research says it might even make it cannibalize itself just to keep going. In other words, your brain quite literally starts eating itself.
The study, published in Nature Metabolism, found that marathon runners showed signs of decreased myelin, the fatty tissue around brain cells, two days post-race. Myelin is one of the main things that allows your brain to function properly, and researchers believe it may be burned for fuel when you run out during a race (here's your reminder to never forget your energy chews).
Related: This Common Recovery Trend Might Be Sabotaging Your Gains, New Study Shows
In the study, 10 runners were given an MRI scan on their brains 48 hours before and two days after their marathon, and again two weeks and two months post-race. They saw up to a 28% drop in MRI signals in areas tied to movement and emotional regulation—but the good news? Myelin levels bounced back to normal after two months.
Scientists think that this form of "self-cannibalism" is the brain's way of surviving extreme conditions, such as marathon running, when your glucose and fat stores are depleted. Think of myelin like a backup generator that the brain uses as fuel for emergencies.
"Myelin is constantly made and degraded," Pedro Ramos Cabrer, a neuroscientist at CIC biomaGUNE and a co-author of the study, told Live Science. "It's like a bathtub in which you constantly fill in water and constantly drain it."
While the sample size was pretty small and the changes localized, the findings are a reminder of how intense endurance exercise can push both body and brain to the limit. But researchers noted that, given the size of the study, more research is needed to come to concrete conclusions. Plus, the effects on overall myelin were minimal.
No need to cancel your next race—but it’s a good reminder to prioritize rest, recovery, and proper fueling.
