Glad you guys brought this up...this has come up again recently and the new thought is that it could be related to sunlight...
1) There appears to be an association between latitude and the risk of MS, with the risk increasing from south to north.
2) In an analysis from the Nurses' Health Study for example, the adjusted rate ratios were 3.5 for the northern United States and 2.7 for the middle tiers relative to the southern tier.
3) What is REALLY interesting is that people migrating from a high to low-risk area after the age of puberty carry their FORMER high risk with them, while those that migrate during childhood seem to have the risk associated with the new area to which they migrated.
4) One proposed explanation for the association with latitude is that EXPOSURE TO SUNLIGHT MAY BE PROTECTIVE either because of an effect of ultraviolet radiation or of vitamin D.
5) A case-control study in Tasmania found that higher sun exposure during childhood and early adolescence was associated with a reduced risk of MS
6) In a large prospective study analyzing data from the Nurses Health Study, the risk of developing MS was significantly reduced for women taking 400 IU/day of vitamin D