The association between vitamin D and multiple sclerosis (MS) has been studied for several decades; numerous studies have reported that low serum levels of vitamin D increase the risk of MS and that vitamin D suppresses disease in experimental autoimmune encephalomyelitis, an animal model of MS. However, it is not clear whether vitamin D truly protects against MS or whether it can reduce the number of relapses or lower disability in MS. These issues are difficult to address, in part because there are several different vitamin D metabolites and it is not clear which metabolite(s) is most associated with lowered risk for MS. In humans, the two relevant …