Multiple sclerosis (MS) is thought to be caused by a combination of factors — genetics, immune responses, and environmental conditions — and interactions among them. Environmental factors associated with an increased risk of MS include smoking, vitamin D levels, and exposure to certain viruses. One of the earliest discovered environmental factors is geography, or where a person lives.
Through research on the epidemiology of MS (the distribution of MS in different populations), scientists have found that MS incidence follows geographical patterns. On average, MS is more prevalent in populations that live farther from the equator.
It may be difficult to understand how MS, an autoimmune disease of the central nervous system, is associated with where a person lives. Scientists are still trying to understand why MS shows this geographical pattern, but they have several theories.
According to the National Multiple Sclerosis Society, areas farthest from the equator have the highest prevalence of MS. Conversely, MS is rare in populations that live near the equator.
A 2016 study of the geographical distribution of MS found areas with the highest prevalence of MS to be North America, western Europe, and Australasia (New Zealand and Australia). Researchers found the lowest rates of MS in central and eastern sub-Saharan Africa and Oceania (the islands of the Pacific Ocean, including Fiji, Guam, Samoa, and Tonga). However, study authors reported that many countries in the low-prevalence regions did not have data on MS, so the estimates of MS prevalence in these countries may not be accurate.
According to an MS prevalence classification system created in 1975, the highest rates of MS are found in:
Medium rates of MS are found in:
Countries with low rates of MS include those in:
North-south prevalence patterns also have been found within individual countries. In the United States, there are higher rates of MS in the Great Lakes region of Michigan (one of the northernmost parts of the country) as compared to other areas of the country. Within Great Britain, rates of MS are higher in Scotland (which is farther north) as compared to England. In Japan, MS is rare, but researchers still see that MS is more prevalent in the north of the country as compared to the south.
Scientists have found that moving from a geographical area with a low risk of MS to an area with a high risk of MS before puberty (roughly from ages 12 to 15) increases a person’s risk of developing MS. Similarly, moving from a high-risk area to a low-risk area before puberty will decrease a person’s risk of MS. This phenomenon shows that there may be other environmental factors contributing to a person’s risk of MS that emerge early in life.
Scientists believe that several factors associated with geography contribute to the geographical patterns of MS risk.
Temperature is associated with geography — temperatures get colder, on average, the farther an area is from the equator. Populations that live in temperate regions, where there is more variation in temperature, have a higher prevalence of MS compared to populations that live in tropical regions with more constant temperatures.
Temperature is suspected to be related to MS because it influences factors like the spread of viruses. Viruses such as the Epstein-Barr virus are another environmental risk factor for MS. In cold temperatures, people tend to spend more time in closer, more contained areas. This closer contact could increase the spread of viruses and affect the risk of MS. Conversely, warmer temperatures may allow for populations to be more spread out, which might reduce the spread of viruses and the development of MS.
Vitamin D is a vitamin created in the body through exposure to sunlight. It helps the body absorb calcium and contributes to the proper functioning of the immune system and brain. Populations of people who live closer to the equator get more sun exposure year-round and have naturally higher levels of vitamin D.
According to the National Multiple Sclerosis Society, adequate levels of vitamin D may lower the risk of developing MS. Vitamin D supplementation can play an important role in reducing the risk of MS. Furthermore, vitamin D supplementation may lessen symptoms in people already living with MS.
Scientists don’t fully understand how vitamin D affects MS. They suspect that it involves how vitamin D interacts with the immune system. Research supports that vitamin D may play a role in the association between the risk of MS and geography.
Diet is another factor that could influence the link between the risk of MS and geography. Diet is associated with where people live and the foods they have access to. Eating foods with different vitamins and nutrients can affect the risk of diseases like MS.
For example, populations that live closer to the ocean have lower risks of MS. These populations tend to consume more fish, which is high in omega-3 fatty acids and vitamin D — both nutrients shown to reduce inflammation and potentially play a role in the risk of MS.
Conversely, populations that live farther from the ocean tend to have diets that are high in saturated fats found in meat and dairy products, which generally increase inflammation in the body. These populations also tend to have a higher prevalence of MS. Findings from a research study in Norway showed that populations that lived in coastal regions and ate more fish had a lower incidence of MS as compared to populations that lived inland and consumed less fish.
While areas where people eat more fish have lower MS rates, scientists cannot say that a diet rich in fish would lower the risk of MS. However, because diet is associated with regions and the food available to them, diet could be a factor in the association between geography and the risk of MS.
While the prevalence of MS shows geographic patterns, research also shows that geography alone cannot predict someone’s risk of MS. More likely, risk is determined by both genetic and environmental factors. For example, the countries of Malta and Sicily are very close in geography and climate, yet they have genetically different populations with very different incidence rates of MS.
Another example can be found by comparing rates of MS in cities with similar latitudes (distance from the equator) — Rochester, Minnesota, in the United States; London, Ontario, in Canada; and Vancouver, British Columbia, in Canada. While these cities are similar distances from the equator, Rochester, Minnesota, has nearly double the rate of MS as compared to the two Canadian cities.
Scientists believe a combination of environmental, immunologic, and genetic factors ultimately determine a person’s risk of MS. If you’re living with MS, talk to your neurologist or health care provider about the best therapies and treatments for your symptoms.
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