During a marathon the brain even eats away at the nerves

Legend has it that the first person to run a marathon, the Greek Pheidippides, died of exhaustion just minutes after completing the feat. It must be taken into account that the poor soldier did not expect that on that historic day in the 5th century BC, instead of ending up pierced by a Persian spear on the beach at Marathon, he would have to run tirelessly the 42.195 kilometers that separated the battlefield from Athens to announce victory over the invading troops.

To face physical challenges of this magnitude, it is better to train well, because a body subjected to such stress will have to resort to extreme measures to continue functioning. Even "eating" parts of its own nerves, as a group from the University of the Basque Country led by the neuroscientist Carlos Matute, which could have more beneficial effects than it seems.

Less myelin

Myelin is a substance made of fat and water that surrounds axons, the projections of neurons through which nerve impulses circulate. Without myelin, communication between neurons would be slow and defective. In one article published in Nature metabolismMatute's team analyzed the amount of myelin in the brains of ten amateur but experienced runners, aged between 45 and 73, two days before and after running a marathon. They did so using magnetic resonance imaging (MRI) techniques, which indirectly measure the presence of myelin by identifying the water molecules contained in the brain. They found that the amount of myelin decreased especially in neurons in areas of the brain related to motor control and the processing of sensory and emotional stimuli, functions that runners must use most during the race.

When the body needs energy, it first mobilizes sugar reserves, which are the molecules it can release more of most quickly. When it runs out, it turns to fat, a very effective long-term storage system. It can even be obtained by destroying muscle, if necessary. It's no wonder that during an effort as intense as a marathon, the body seeks every possible way to generate energy, and the brain, the organ that needs it most of all, uses exceptional tactics to guarantee its supply. After all, myelin is right there and accounts for 40% of the central nervous system's weight: it seems like a quick and affordable source of fuel.

Beneficial for the nerves

Although it may sound negative, the truth is that there is no data to suggest that marathon running is bad for the brain. No loss of cognitive ability is seen, even temporarily, at least after sufficient rest to recover from the ordeal. It is possible that the amount of myelin consumed is not enough to have negative effects on the nerves, and that would be logical, because it would make no sense for the body to jeopardize the functioning of the very organ it is trying to feed. Matute proposes that "eating" myelin could actually have beneficial effects on nerve health, because it would force them to regenerate. Indeed, the amount of myelin returns to pre-marathon levels after a couple of months at most, and this renewal of the nerves' covering would perhaps allow them to function more efficiently and repair themselves better if they ever get injured. This would be good news for those, like Matute, who are avid long-distance runners.

It would be interesting to study whether this phenomenon is related to autophagy, a cellular process that, as its name suggests, consists of digesting one's own parts to clean out outdated components and thus recycle materials that can be used to build new ones. It is known that when autophagy doesn't work well in cells, tissues age more quickly. Thus, a periodic replacement of myelin could protect against neuronal degeneration, for example, the myelin loss seen in multiple sclerosis. It would also be worth considering what type of activity, beyond a marathon, can activate this response.

We must keep in mind that evolution hasn't made us runners, like the gazelle and the cheetah. Our bodies are more adapted to climbing trees than to the great physical effort involved in a long-distance race. However, with proper training, we are capable of great feats, such as completing 42 kilometers in one go without falling dead. This has not traditionally been seen as a healthy lifestyle strategy, because engaging in this type of intense exercise regularly takes a toll on our skeleton. There are even studies that link competitive sports with premature aging. For the majority of the population, consistent moderate physical activity should be better than strenuous tasks that push the body to its limits. But the myelin study suggests there may be hidden benefits to these excesses that we weren't yet aware of. More research is needed to understand the scope and significance of these findings.