from http://knol.google.com/k/multiple-sclerosis# of July 2008
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Vitamin D
For a very long time it has been noted that MS occurs at a greater frequency at locations away from the equator. MS prevalence is strongly tied to geography, especially latitude. Although some of this so-called “latitudinal gradient” is related to genetic factors (people at the greatest genetic risk likely come from places distant from the equator), recent evidence has illustrated that exposure to sunlight and the related production of Vitamin D probably plays some role in determining risk for MS. After World War II, it was noted that the rate of MS was highest among those veterans who grew up in states from the northern part of the US (Minnesota, Washington). Furthermore, researchers noted twenty years ago that people who performed inside desk jobs had higher rates of MS than those who worked outdoors (and presumably had a greater level of sun exposure).
Vitamin D is not really a “vitamin” at all (an essential nutrient obtained strictly from diet). It is, in fact, a prohormone (a molecule without hormonal effects on its own but which is converted to molecules with hormonal activity) synthesized in the skin from a cholesterol by-product. It is then further converted in the liver and then in the kidney. The first important step in its production takes place in the skin and then only in response to stimulation with ultraviolet light (in the high frequency A and B spectrum of 270-290 nm wavelength). The production of Vitamin D is therefore heavily dependent on exposure to sunlight. There are few dietary sources of Vitamin D and these probably cannot supply an adequate amount of Vitamin D. Traditionally doctors and physiologists had focused on the importance Vitamin D in the production and maintenance of bone because severely Vitamin D-deficient children were noted to develop the bone disease rickets, and adults the bone disease osteomalacia. However, in recent years attention has turned to all the other important functions of Vitamin D – including a role for Vitamin D in the function of the immune system. Many types of immune cells respond to Vitamin D (and have sites on their surface intended to respond to stimulation with Vitamin D). Activation of this vitamin D receptor plays a role regulating the activity of certain classes of immune cells.
Recently some important studies have highlighted the role of vitamin D in MS. One very elegant study showed that people with low vitamin D levels are at nearly a 3x higher risk for developing MS when compared with those with the highest levels. This difference is particularly pronounced for those patients younger than 20 and implies that vitamin D exposure in childhood and adolescence may be most important. Another study suggested that among identical twins in which only one twin had MS (discordant twins), the twin who developed MS was much more likely to have a history of sun avoidance behavior. There is even some evidence for a seasonal variation in relapse rate, with a higher rate seen in winter, especially at more extreme latitudes, meaning that exacerbations may also be tied to vitamin D levels.