Story at-a-glance Multiple sclerosis (MS) is an autoimmune condition attacking myelin in the central nervous system. Symptoms vary based on nerve damage location and often begin as clinically isolated syndrome (CIS) Low vitamin D is consistently linked to higher MS risk, with people living closer to the equator having lower MS rates due to greater sun exposure A 2025 clinical trial showed that high-dose vitamin D delayed MS progression in CIS patients, doubling time before new disease activity appeared compared to placebo Vitamin D stimulates myelin-rebuilding cells, boosts neurotrophins, reprograms microglia from inflammatory to healing states, and protects the blood-brain barrier Optimal vitamin D levels (60 to 80 ng/mL) can be maintained through sensible sun exposure or D3 supplementation, with regular testing recommended to adjust intake accordingly