Multiple Sclerosis (MS) and Environment | Merck Manual Consumer Version

Environment has a role in multiple sclerosis. Where people spend the first 15 years of life affects their chance of developing multiple sclerosis. It occurs as follows:

In about 1 of about 2,000 people who grow up in a temperate climate
In only 1 of about 10,000 people who grow up in a tropical climate
Much less commonly in people who grow up near the equator

These differences may be related to vitamin D levels. When the skin is exposed to sunlight, the body forms vitamin D. Thus, people who grow up in temperate climates may have a lower vitamin D level. People with a low level of vitamin D are more likely to develop multiple sclerosis. Also, in people who have the disorder and a low vitamin D level, symptoms appear to occur more frequently and are worse. But how vitamin D may protect against the disorder is unknown.

Where people live later in life—regardless of the climate—does not change their chances of developing multiple sclerosis.

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