Author response: 25-Hydroxyvitamin D deficiency and risk of MS among women in the Finnish Maternity Cohort

Although a randomized clinical trial of vitamin D supplementation on primary multiple sclerosis (MS) prevention would provide more compelling evidence of causality than observational studies, challenges including cost, duration, and compliance make it unlikely that such a study will be done. As is often the case in medicine, recommendations must be based on the best available observational evidence. Our study1 contributed to a large body of...