Abstract
Here we use magnetic resonance imaging to study the impact of marathon running on brain structure in humans. We show that the signal for myelin water fraction-a surrogate of myelin content-is substantially reduced upon marathon running in specific brain regions involved in motor coordination and sensory and emotional integration, but recovers within two months. These findings suggest that brain myelin content is temporarily and reversibly diminished by severe exercise, a finding consistent with recent evidence from rodent studies that suggest that myelin lipids may act as glial energy reserves in extreme metabolic conditions.