Your brain starts eating itself during a marathon, study finds

Marathon running may seem like the ultimate sport for staying in shape. That said, it can take a heavy toll on the body, potentially damaging the kidneys, upsetting the stomach, increasing the risk of heart failure, and triggering musculoskeletal injuries. Let’s not forget that the original marathon runner, the ancient Greek soldier Pheidippides, is said to have dropped dead immediately after reaching his destination.

Now, a new study suggests that marathons also affect the brain — causing the organ to eat itself to make up for fuel lost during the run.

In the research, published March 24 in the journal Nature Metabolism, scientists observed declines in a key biomarker of myelin, a fatty tissue that insulates the connections between brain cells, in marathon runners just after a race. The study authors say this suggests the brain consumes myelin as an energy source under these extreme conditions.

Luckily for long-distance runners, these effects appear to be reversible.

Related: 6 ways an athlete’s brain differs from an average person’s

Once the body has exhausted the supply of its standard energy source — glucose, or sugar — it turns to backup generators, metabolizing fats as an alternative fuel source. The brain is, metabolically speaking, more of a Humvee than a hatchback; the energy-hungry organ devours 20% of the body’s energy, meaning it can quickly suffer damage if fuel sources run short.

Pedro Ramos Cabrer — a neuroscientist at CIC biomaGUNE, a nonprofit research institute in San Sebastian, Spain, and a co-author of the study — told Live Science that he and his colleagues wanted to identify the brain’s reserve energy sources. They suspected that fatty myelin could be the answer. Previous research in rodents suggested that fatty acids produced from myelin breakdown could help boost cell survival in the brain. Whether these preclinical findings would be replicated in humans was still a mystery, though.

“We needed to really deplete all the sources of energy of a body to prove this,” Cabrer told Live Science.

The team scanned 10 runners’ brains 48 hours before they ran a marathon and then again two days, two weeks and two months afterward. They used MRI, which can reveal the presence of water molecules trapped between myelin layers.

The brain scans showed that, two days post-run, the MRI signals in 12 brain areas were depleted compared with before the race — in some cases, by up to 28%. Changes to the brain’s overall myelin levels were not statistically significant, however, suggesting that any changes were very isolated to specific regions.

“The areas that we saw have more significant changes were those related to motor circuits and the center of emotional control of the brain,” Cabrer said. He suggested that this reflects the mental and physical effort the brain must maintain to get through a marathon.

For any runners concerned about this cerebral cannibalism, there’s good news: After two months, all of the runners’ myelin measures had recovered to their baseline levels.

Klaus-Armin Nave, director at the Max Planck Institute for Multidisciplinary Sciences in Germany, was not involved in the study but has also studied how myelin stores energy, using lab mice. Nave said the paper’s findings aligned with how neuroscientists think myelin is maintained in the brain.

Related: How far can a person run without stopping?

“Myelin is constantly made and degraded,” he said. “It’s like a bathtub in which you constantly fill in water and constantly drain it.” He added that Cabrer’s work presents “very strong evidence” that, after a marathon, the brain’s metabolic needs increase. Thus, the fuel stored in myelin is depleted more quickly than usual, leading to brain shrinkage.

The findings don’t mean runners should avoid marathons on their brains’ behalf. Both Nave and Cabrer noted that the study’s small size means more work will be needed to draw concrete conclusions about marathons’ effects on the brain. The very limited effects on overall myelin also raise the need for further research, they added.

Nevertheless, these effects may carry increased risks for certain athletes. For example, in people with amyotrophic lateral sclerosis (ALS), metabolic molecules typically made by myelin are in short supply, and the researchers think that excessive exercise could potentially exacerbate the problem.

Further research will be needed to confirm this theory, Cabrer said.