The brain draws on myelin when other brain nutrients are depleted

A study by the UPV/EHU, CIC biomaGUNE and IIS Biobizkaia, carried out on marathon runners, proves unexpected myelin behaviour.

According to a study published in Nature Metabolism, marathon runners experience reversible changes in brain myelin. These findings indicate that myelin (a substance that surrounds neurons) exhibits a previously unknown behaviour that contributes to the brain’s energy metabolism when other sources of energy are in short supply. Understanding how myelin in runners recovers quickly may offer clues to developing treatments for demyelinating diseases such as multiple sclerosis.

Long-term exercise forces the human body to draw on its energy reserves. When running a marathon, for example, the body consumes mainly carbohydrates, such as glycogen, as a source of energy, but turns to fats when glycogen is depleted in the muscles. Myelin, which surrounds neurons in the brain and acts as an electrical insulator, is composed mainly of lipids, and previous research in rodents suggests that these lipids can act as an energy reserve under extreme metabolic conditions.

A study carried out by researchers from the UPV/EHU, CIC biomaGUNE and IIS Biobizkaia has shown that people who run a marathon experience a decrease in the amount of myelin in certain regions of the brain. According to the study published in Nature Metabolism, this effect is completely reversed two months after the marathon.

Carlos Matute, professor of Human Anatomy and Embryology at the University of the Basque Country and researcher at IIS Biobizkaia, and Pedro Ramos Cabrer, Ikerbasque professor at CIC biomaGUNE, together with Alberto Cabrera Zubizarreta, radiologist at HT Médica, used magnetic resonance imaging to obtain images of the brains of ten marathon runners (eight men and two women) before and 48 hours after the 42-kilometre race. The researchers also imaged the brains of two of the runners two weeks after the race, and those of six runners two months after the race as a follow-up.

By measuring the myelin water fraction in the brain – an indirect indicator of the amount of myelin – the authors found ‘a reduction in myelin content in 12 white matter areas of the brain, which are related to motor coordination and sensory and emotional integration,’ explains Carlos Matute. After two weeks, ‘myelin concentrations had increased substantially, but had not yet reached pre-race levels,’ adds Pedro Ramos. The authors found that myelin content had fully recovered two months after the marathon.

Myelin, fuel for the brain

The researchers conclude ‘that myelin appears to behave as an energy source when other brain nutrients are depleted during endurance exercise, and that further research is needed to establish how extreme exercise relates to the amount of myelin in the brain. Tests in a larger cohort are needed,’ says Ramos-Cabrer.

This study reveals that ‘brain energy metabolism is more complex than previously thought. The use of myelin as brain fuel opens up new insights into the brain’s energy requirements,’ explains Matute. Furthermore, according to the authors, further studies are needed to assess whether these changes have any effect on the neurophysiological and cognitive functions associated with the aforementioned regions, but they point out that most of the myelin in the brain is not affected.

The results of this work break new ground on the energetic role of healthy, aged and diseased myelin in the brain. ‘Understanding how myelin in runners recovers rapidly may offer clues for developing treatments for demyelinating diseases such as multiple sclerosis, in which the disappearance of myelin and, therefore, of its energy supply, facilitates structural damage and degeneration,’ says Matute. The researchers also want to stress that running marathons is not harmful to the brain; ‘on the contrary, the use and replenishment of myelin as an energy reserve is beneficial because it exercises the brain’s metabolic machinery’.