Studies of identical twins (who have the same hereditary background) show that environmental factors play a predominant role in the onset of the disease. Take the fictitious example of Julie and Sophie, real twins aged 30. Julie has had multiple sclerosis since the age of 25. The risk that Sophie suffers from multiple sclerosis like her twin sister is rated at 30%, while it should be 100% if the multiple sclerosis was only genetic. It is therefore mainly environmental factors that trigger the disease. This is probably a conjunction of many factors, not just one event.

The following risk factors are presented as ahypotheses. 

• Have a vitamin D deficiency. The distribution of multiple sclerosis cases around the world (more cases in less sunny countries) has led researchers to speculate a link between vitamin D and the risk of multiple sclerosis. Indeed, vitamin D is produced by the skin under the effect of sun exposure. Low sunlight, leading to vitamin D deficiency, could therefore be linked to the onset of the disease.

   

  Several studies have evaluated the link between the level of vitamin D in the blood and the risk of multiple sclerosis¹. In 2004, a study of 2 cohorts with a total of 187 nurses found that women who take a daily vitamin D supplement (563 IU or more) reduced their risk of developing multiple sclerosis by 400%.¹¹. In 2006, a study of American soldiers showed that those with the highest vitamin D levels had a lower risk of developing multiple sclerosis.². In an article published in 2013, it is estimated that the risk of developing multiple sclerosis is reduced by 30% in women with the highest levels of vitamin D, compared to those with the lowest levels. Multiple sclerosis would not be the first autoimmune disease to be associated with a low level of vitamin D (47).

   

  In addition, vitamin D levels are low in the majority of people with¹, especially at the onset of the disease. Finally, studies in mice show that vitamin D may reduce the number of relapses and slow the progression of the disease.¹⁹. Unfortunately, current data cannot determine whether vitamin D supplementation can influence disease progression in humans.

•  Have contracted the Epstein-Barr virus. This virus, involved in infectious mononucleosis, has been implicated by several studies in the onset of the disease. However, no formal proof of his involvement could be provided. In June 2010, a study of 900 people showed that the risk of multiple sclerosis increases after infection with the Epstein-Barr virus (EBV)³. In 2006, the same researchers had shown that people with the disease have a higher than normal level of anti-EBV antibodies. Finally, a recent meta-analysis, bringing together 18 studies and more than 19 people, concludes that contracting infectious mononucleosis increases the risk of suffering from multiple sclerosis.¹⁷.

• Smoke cigarettes. People who smoke 20 to 40 cigarettes a day are about twice as likely to have multiple sclerosis than non-smokers³⁴. In addition, smoking appears to worsen symptoms in sufferers and accelerate the progression from relapsing-remitting forms to progressive ones.¹⁸. 

• Eat lots of saturated fat, salt, and ultra-processed foods. Multiple sclerosis is more common in populations whose diet is rich in animal fats, industrial foods and salt and lower in those who consume mainly polyunsaturated fatty acids^{5 48}. Since northern populations generally eat a diet richer in animal fat, it is difficult to isolate the impact of diet from that of geographic location. As mentioned earlier, multiple sclerosis is 5 times more common in northern or temperate regions than in tropical and southern climates.

• Being in contact with chemical solvents in the workplace³⁵

Notes. There is data in the scientific literature showing that wearingmercury dental amalgam increases the risk of multiple sclerosis and also worsens its symptoms^7,8. However, the majority of these data come from studies whose scientific quality is considered to be low. Having had multiple mercury amalgams over the years could increase the risk of developing the disease, but this has not been clearly demonstrated.¹⁰. Thus, doctors generally consider that dental amalgam is not a problem. 

In France, after a massive campaign of hepatitis B vaccination in 1994, a study suggested a possible cause and effect link, which was never subsequently confirmed. Epidemiological studies have not shown no link between vaccination against hepatitis B and the occurrence of multiple sclerosis. Since then, scientists have concluded that there is no link between vaccination against hepatitis B and the occurrence of multiple sclerosis.