Multiple Sclerosis (MS) is a slowly progressive, often disabling disease of the nervous system. It consists of regions of nerve injury characterized by demyelination or loss of the protective fatty coating of the nerves. When this fatty myelin coating is damaged, it is akin to removal of the plastic coating of the wires in your home. If the wire connecting a light to a wall outlet loses its insulation, the light will go out because the electricity stops being conducted. When nerves lose their myelin, or insulation, the same thing happens. Conduction from one nerve to the next is interrupted. If this develops in a group of nerves responsible for sensation on the hand, for example, numbness will be the result. The disease produces multiple and varied neurological symptoms. There are activations and remissions during the course of the disorder. The exact cause of MS is unknown. Inheriting genetic risk factors for MS is not sufficient to cause this disease. Exposure to environmental risk factors is also required. MS may thus be preventable if these unidentified factors can be avoided.

An interesting observation regarding the incidence of MS is that it increases with decreasing exposure to solar radiation. Incidence of MS is lowest in the tropics.This suggests that sunlight may be protective for MS. Since the vitamin D system is exquisitely responsive to sunlight and MS is highest where environmental supplies of vitamin D are lowest, it has been suggested that vitamin D may protect genetically susceptible individuals. Studies on MS and vitamin D have found that periods of low vitamin D precede the occurrence of high MS activity, while periods of high vitamin D precede low MS activity.

To study the relationship between vitamin D and the risk of developing MS, Dr. Walter Willett and his associates at Harvard and the University of California at Irvine used data from two large prospective human cohorts to determine whether or not vitamin D intake is associated with risk for developing MS.

Over 180,000 women were followed for 10 years. Diet and total vitamin D intake (including that from vitamin supplements) were assessed at baseline and every 4 years thereafter. During the follow up period, 173 cases of MS were confirmed. The study results identified a 40% reduction in the risk of developing MS in those subjects who used daily supplemental vitamin D in amounts greater than or equal to 400 IU, versus those who took no supplemental vitamin D. The authors interpreted the results as supporting a protective effect of vitamin D intake on the risk of developing MS.

In the 1950s, it was noted that the average annual sunshine exposure and the winter daily solar radiation at a person's birthplace correlated strongly and inversely with the life time incidence of MS. This was subsequently confirmed in other studies. It was later observed that the season of birth in those who later developed MS differed significantly from the general population. The consensus of these studies appears to suggest a higher risk if the first or second trimester of pregnancy were during the fall or winter, the seasons of low or absent synthesis of vitamin D by the skin in mothers living at moderate to high latitudes.

There are numerous examples where simple dietary supplementation has been quite effective in preventing specific diseases. This is best exemplified by the connection between folate supplementation during pregnancy and prevention of neural tube defects (spina bifida and anencephaly). It seems this preventive effect works not by correcting a simple, nutritional deficiency, but by influencing specific pathways in early brain development. A related influence may be true for vitamin D due to its beneficial impact on oligodendrocytes (the cells responsible for making the fatty coating of nerves (myelin)). For this reason, if these arguments have validity, the potential health and economic benefits of vitamin D supplementation in areas of high MS prevalence are large. Routine vitamin D provision in pregnancy and childhood is a simple and cost-effective strategy to try and reduce the burden of a potentially devastating disease that destroys many lives.