Multiple sclerosis (MS) is more common among people who live in countries with temperate climates, such as the northern United States, Canada, Europe, New Zealand, and southeastern Australia, than among those living in warmer zones.
This difference may be due to greater sunlight exposure (ultraviolet or UV light), which allows for the body to produce greater amounts of vitamin D naturally. Growing evidence suggests vitamin D may play a role in MS, because vitamin D is thought to support immune function. Indeed, research has found that MS becomes more common the farther away from the equator a person lives, likely because people there are exposed to less sunshine. Scientists also report that people with ample exposure to sunlight during childhood and adolescence have a lower risk of developing MS as adults.
Studies have also shown that people who were born in an area thought to carry a higher risk of MS (possibly because of its climate), who then moved to a lower-risk area before the age of 15 (generally, the age of puberty), have the same risk as others in their new area. This may suggest that there are environmental agents that predispose people to develop MS in later life.
On occasion, very high numbers of MS cases are also found at specific time periods and in specific locations. These are called “clusters,” and may provide clues to environmental or genetic risk factors for MS.
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