Something Strange Happens to Brains During a Marathon

It’s not just our legs that get put through the wringer during a marathon, new research suggests. Scientists have found evidence that our brains can undergo dramatic changes while we’re running an extreme long distance race.

Researchers in Spain conducted the study, published Monday in the journal Nature Metabolism. They scanned the brains of people before and after running a marathon, finding that runners experienced a significant, but temporary reduction of myelin in certain regions. The results indicate that our brains can use the fat stored in myelin as an emergency source of energy when pushed to the limit, the researchers say.

Marathons are grueling tests of endurance, requiring participants to run 26.2 miles. While running a marathon or any kind of prolonged exercise, we initially rely on stored carbohydrates (glucose) for fuel. But eventually, this fuel runs low, prompting our bodies to burn off stored fat instead. Research in rodents has suggested that brains can use myelin as one such source of fat when needed. Myelin provides a protective coating to the brain’s neurons while boosting the speed at which our brain cells can communicate with each other; it’s also mostly made out of lipids, or fat (between 70% and 80%).

To test this hypothesis, the researchers performed MRIs on 10 city and mountain marathon runners up to two days before the long run. All ten were given MRIs a day or two after the run as well, and some got scans two weeks and two months later.

Before the marathon, the runners had similar levels of myelin (measured using an indirect biomarker). Afterward, they had “substantial” decreases of myelin in specific brain regions known to be involved in motor coordination as well as sensory and emotional integration. In runners scanned two weeks later, myelin levels increased but not all the way back to normal; two months later, myelin levels finally reached their pre-marathon baseline.

“These findings suggest that brain myelin content is temporarily and reversibly diminished by severe exercise, a finding consistent with recent evidence from rodent studies” suggesting that myelin lipids can be used as an energy reserve during times of need, the researchers wrote.

The authors do note that their study had a small sample size, so more research will be needed to confirm these findings and to better understand the phenomenon if it is genuine. It’s unknown whether myelin is used differently depending on the emergency situation, for instance. In other words, would different kinds of prolonged activity or stress make our bodies borrow myelin from somewhere else in the brain? The researchers also say more research should be done to figure out if these reductions in myelin lead to any temporary physical or cognitive changes.

Marathon and other kinds of elite runners are generally very healthy and probably live longer than average, so it’s unlikely this momentary drop in myelin has any huge adverse impact on the average runner’s health. But the lessons learned from studying these runners might help scientists gain insight into how our brains metabolize and use energy during other situations, such as when we develop certain neurodegenerative disorders tied to the permanent loss of myelin.

“[T]he potential relevance of the findings described here to brain energy metabolism warrant additional studies,” the authors wrote.

Personally, having run a marathon a decade ago, I’m just glad that my brain didn’t have to completely starve before I was able to gorge myself on the Chinese takeout I ordered immediately after my race.