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Multiple sclerosis involves the dysfunction of the immune system, leading to an autoimmune attack on the myelin sheath that covers nerve cells in the central nervous system. The exact cause of the immune response in MS is unknown, but certain environmental and genetic risk factors are associated with the disease.
Support for environmental risk factors in the development of MS comes from studying twins. Because identical twins share nearly all the same genetic material, a purely genetic model of disease would expect both twins to develop MS at a rate of 100 percent. However, research has found that the risk of one twin developing MS when the other twin is affected is just 30 percent, suggesting that environmental factors may also trigger the disease.
Here are the major environmental factors associated with MS:
The prevalence of MS around the world seems to occur in a gradient from north to south. Cases of MS near the equator are relatively rare, with a pattern of increasing prevalence the farther you get from the equator in either hemisphere.
Studies have shown that some people who move to a high-risk geographical area before the age of 15 have a higher risk of developing MS. For people over the age of 15, moving didn’t appear to have an effect. This finding suggests that exposure to certain environmental factors before puberty may influence an individual’s susceptibility to MS.
Numerous studies have established a correlation between low vitamin D levels in a person’s blood and the development of MS. Vitamin D may have several positive effects on the immune system, including promoting healthy immune function and another function called immune tolerance. Immune tolerance is important for preventing autoimmune diseases.
Vitamin D is available in foods and by supplementation, but perhaps the best source of vitamin D is through sun exposure. Vitamin D3 is made in the skin through a reaction with UVB light. Scientists believe that lack of sun exposure and lower levels of vitamin D may be related to the increased MS prevalence farther from the equator.
Read more about vitamin D and MS.
Several viruses and other infectious factors have been associated with the development of MS, including infections with Epstein-Barr virus (EBV), human herpesvirus 6 (HHV-6), and varicella-zoster virus (VZV).
EBV causes infectious mononucleosis, or “mono.” Infection with EBV that leads to mononucleosis is one of the strongest environmental risk factors associated with MS. Scientists have studied people with MS and found antibodies that react with both myelin and a viral protein from EBV in their blood. Antibodies are the proteins the immune system makes to attack invaders. The antibodies that recognize myelin and EBV may contribute to the autoimmune destruction and demyelination in MS.
Read more about the role of viruses in MS.
Smoking is also a strong risk factor associated with MS. People who smoke are approximately 1.5 times more likely to develop MS compared to nonsmokers. Smoking can also contribute to disease severity and progression in someone who is already living with MS. The toxins in smoke, as well as smoking’s production of inflammatory cytokines, may contribute to the onset or exacerbation of MS. Individuals who quit smoking can improve their health outcomes.
Read more about smoking and MS.
People who were obese during childhood or early adulthood, especially girls, are at higher risk of multiple sclerosis. Obesity can cause chronic low-grade inflammation, which may contribute to the onset of MS. Additionally, obesity may affect the diversity of bacteria in an individual’s gut, which may have an impact on their susceptibility to MS.
The microbiome is made up of the microbes, or bacteria, that live inside the human body. The human body coexists with about 100 trillion bacteria. Your body must live in harmony with “good” and “bad” species of bacteria in the microbiome. However, diet, obesity, antibiotic use, and chronic illness can alter the microbiome, throwing things out of balance, which affects the immune system. When the bacteria in the microbiome are out of balance, it’s called gut dysbiosis.
Studies have shown that some people with MS exhibit gut dysbiosis. It’s unclear whether the imbalanced microbiome is a risk factor for MS or a result of MS. We know that animal research has found that mice develop an MS-like disease after receiving gut bacteria from someone with MS, suggesting that the microbiome may be involved in causing MS. However, more research is needed on this subject.
Several studies have examined major head injury, as well as exposure to organic solvents and air pollution, but evidence linking the risk of these factors to MS is weak.
Multiple sclerosis symptoms may differ greatly from person to person, and many risk factors can contribute to its development. Understanding the environmental risk factors associated with MS can help you be more knowledgeable about the disease. As scientists and epidemiologists learn more about the environmental factors that contribute to MS, they may develop better interventions for disease prevention and treatment.
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