Marathon runners tap brain-cell insulation for racetime fuel The finding could lead to treatments for neurological diseases

MRI scans suggest runners’ brains might use the fatty substance myelin as fuel

Marathon runners saw myelin levels drop temporarily post-race in areas of the brain involved in motor control and sensory and emotional processing.Credit: Saeed Khan/AFP/Getty

A fatty substance that insulates the electrical signals transmitted by nerve cells might also be a source of energy for the brain — especially when reserves are running low. This intriguing possibility has been raised by scans of long-distance runners’ brains, taken before and after they ran marathons.

The scans suggest that levels of the insulating substance — called myelin — decreased in brain areas involved in motor control and sensory and emotional processing after these gruelling endeavours, only to bounce back to normal within two months of the races.

Although the idea of myelin as an energy source is not completely new, nobody had thought of examining whether this was the case for runners, says Carlos Matute, a marathon runner and neuroscientist at the University of the Basque Country in Leioa, Spain, who led the research, which is published today in Nature Metabolism¹. “These findings open the way to consider that myelin lipids contribute to brain energy metabolism, at least in certain conditions.”

The temporary loss of myelin after a race is not something that runners should be concerned about, Matute adds. His team is currently conducting studies to examine whether the reduction in myelin has a temporary effect on cognitive function, and have so far come up empty, suggesting the effect is either very small or non-existent. “There are no gross changes in brain function,” he says. In fact, Matute suspects that the use and replenishment of myelin is beneficial because it “exercises the brain’s metabolic machinery”.

Mustapha Bouhrara, who studies brain imaging and ageing at the US National Institutes of Health in Baltimore, Maryland, agrees. The reduction in myelin lasts for only a short time, so is not concerning, he says, and the process teaches the brain how to quickly repair myelin, and “could be very, very beneficial”.

Marathon inspiration

The idea for the study came to Matute, who has run 18 marathons, during training. He pondered how people are able to complete such demanding races. Given myelin’s abundance in the brain — it comprises up to 40% of the central nervous system by weight — and its fatty composition, Matute wondered whether the brain might use the substance “strategically”, allowing it to power on when other energy sources are low.

His team used magnetic resonance imaging (MRI) to scan the brains of ten runners — eight men and two women — within the 48 hours before and after they competed in various marathons in Spain in 2022 and 2023. The authors found that myelin levels were significantly lower in 12 brain regions after the race, compared with before. “It was not much, but it was a clear reduction in certain areas of the brain,” says Matute.

The areas affected are involved in motor coordination, sensory perception and emotions, and are regions you would expect to be active during a marathon, says Matute. “We feel a lot of things during the run and we have to talk to ourselves quite a bit to continue,” he says.

The researchers imaged the brains of some of the runners again in the weeks and months after the races. They observed that some ‘remyelination’ had occurred after two weeks and that myelin levels had fully recovered after two months.