Have you ever wondered what happens to your brain when you push your body to its limits? A new study suggests that during extreme endurance exercise, like running a marathon, the brain might actually break down parts of itself to keep working.

The study, published in Nature Metabolism, found that when the body runs low on sugar (glucose), the brain starts using myelin—a fatty coating that helps nerve cells send signals—as a backup energy source.

Scientists in Spain carried out this research by studying the brains of 10 marathon runners—8 men and 2 women. The researchers scanned their brains using MRI machines before and after a 42-kilometer race. They noticed that after the marathon, some of the runners’ brains had less myelin, especially in areas that control movement, emotions, and sensory information.

The good news is that the brain seems to recover after the race. Two weeks later, the runners’ myelin levels had started to bounce back. After about two months, their brains had mostly returned to normal, based on follow-up scans done by the researchers.

According to the scientists, this process—called metabolic myelin plasticity—is like a survival trick. When the brain doesn’t get enough fuel from food, it sacrifices some of its protective coating to stay active. This might explain why marathon runners often feel tired, slow, and forgetful right after a long race.

This study gives us a better understanding of how the brain adapts during extreme physical activity. It shows just how far the body and brain will go to keep us moving, even when energy is running low.

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