Ultraviolet sunlight is too low to produce adequate amounts of vitamin D3, and vitamin D insufficiency lasting 4 to 6 months of the year at latitudes of ≥42° is common in individuals with low vitamin D intake. Vitamin D has strong immunoregulatory effects, and vitamin D supplementation prevents experimental autoimmune encephalomyelitis (EAE), an autoimmune disease in animals that is used as a model of MS.