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Multiple sclerosis, a disease of the brain and spinal cord that produces myriad symptoms, does not strike the population evenly. Your risk for the disease is determined by genetics, gender and even where you live. All three factors hint at a role for the environment in the disease.

If you are an identical twin who acquires the disease, your twin, who has nearly the same genetic material as you, has a 20 to 30 percent chance of getting it, too. Among fraternal twins, who share half their genes, the risk is only 5 percent. To date, researchers have identified about 50 genes that appear to increase the risk to some degree. All these genes play a role in immune function, but no one gene or genes have been identified as the culprit.

Together, however, these genes explain only about 10 percent of a person's risk of acquiring MS, with the three genes with the strongest ties to the disease accounting for around just 4 percent of the risk. Thus, additional genes, coupled with environmental exposure, are very likely to be involved as well.

Three to four times as many women as men are diagnosed with MS, a statistic that parallels the increased risk for autoimmune disease in women. No one knows the cause of the gender discrepancy in MS, but one clue, which hints at an emergent environmental factor, is that the gap has widened in recent decades. In the 1950s a woman's risk of developing the disorder was only about 1.4 times that of a man's.

MS is more common among Caucasians, especially those of northern European ancestry, and much less so among Asians and people of African descent. Although such differences could reflect genetics, geography also appears to be important. MS seems to be more common at higher latitudes where the climate is temperate—that is, in Europe, the U.S., Canada, New Zealand and Australia—and rare in the tropics (parts of Asia and Africa). The highest prevalence is on the Orkney Islands, north of Scotland, where residents have the disease at the rate of 270 per 100,000 (compared with 90 per 100,000 in the U.S.). Interestingly, migration studies suggest that if you move from a low-risk country to a high-risk one before adolescence, you acquire the risk of your new home country. If you move after adolescence, however, you retain the risk profile of your country of origin. This pattern suggests that something in the environment affects risk, possibly an infectious agent or insufficient levels of sunlight.

Sunlight, in particular, affects vitamin D levels. Some studies have found that these amounts are lower in MS patients and that those with lower levels are more likely to have attacks, or relapses, than patients with higher levels. Vitamin D has also been shown to influence immune function, making a link between the vitamin and MS plausible. To date, however, there is no definitive proof that taking vitamin D supplements will prevent MS or help treat it. Nevertheless, because the vitamin is generally safe, many neurologists will recommend vitamin D as a supplement.

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DMITRY MERKUSHIN iStockphoto (people icons); ISTOCKPHOTO (sun and globe icons)


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