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Risk for multiple sclerosis (MS) increases at higher latitudes (i.e., with less sun exposure), and studies have suggested a possible benefit of sun exposure or vitamin D supplementation in lowering risk for MS. In addition, vitamin D prevents MS from developing in animal models. A depository of serum samples was used to retrospectively study the vitamin D–MS relation in a case-control study of 257 patients with definite or probable MS and 514 controls matched by age, sex, and race. Analyses were adjusted for latitude of residence and month of serum collection.

Serum 25-hydroxyvitamin D (25-OHD), a marker of bioavailable vitamin D, was measured at least twice in each patient before a diagnosis of MS was recorded. 25-OHD levels and risk for MS were associated only in white patients. A significant 62% relative reduction in risk was noted for subjects in the highest quintile of 25-OHD concentration (>99 nmol/L) compared with those in the lowest (<63 nmol/L). This risk reduction was even stronger when adolescent serum levels were used: 91% reduction for subjects who had levels 100 nmol/L before age 20, compared with those who had levels <100 nmol/L.

Comment: The authors emphasize that these observational data do not support clinical use of vitamin D supplementation to lower risk for MS yet. They do recommend a prospective clinical trial of vitamin D supplementation for lowering MS risk, although such a trial might not be feasible because of the number of subjects and length of follow-up that would be required. Evaluating 25-OHD in blacks will require a larger observational study.