Extreme Mountain Ultra-Marathon Leads to Acute but Transient Increase in Cerebral Water Diffusivity and Plasma Biomarkers Levels Changes A group of researchers from different countries has studied what happens to the brain in ultra skiers.
After a 330 km long run, there was an increase in the intercellular volume in the brain, holding this state for a long time. All details can be found under https://pubmed.ncbi.nlm.nih.gov/28105018/