Immigrants to Canada who have spent a greater proportion of their lives in Canada have a greater risk of developing multiple sclerosis (MS) than people who have spent a smaller proportion of their lives there, according to a new study. The study does not prove that an increased proportion of life in Canada causes MS; it only shows an association.

“Other studies have shown that immigrants tend to have better health than long-term residents, which is thought to be because healthy people are more likely to choose to immigrate,” said study author Manav V. Vyasy. “We wanted to see if the lower risk of MS declines over time as people adopt some of the unhealthy lifestyles of their new country or are exposed to other environmental factors that increase their risk.”

The study involved 1.5 million immigrants who arrived in Canada between 1985 and 2003 and were covered by health insurance for at least two years with no diagnosis of MS. The people were then followed through 2016.

During that time, 934 people were diagnosed with MS, a rate of 0.44 cases per 100,000 person-years. Based on previous research, the overall rate of MS in Canada is estimated to be 15 to 17 cases per 100,000 person-years. Person years represent the number of people in the study and the amount of time each person spends.

The person’s age at arrival in Canada and the amount of time since they immigrated determined the proportion of life spent in Canada. Overall, people have spent an average of 20% of their lives in Canada.

Researchers found that people who had spent 70% of their lives in Canada were 38% more likely to develop MS than people who had spent 20% of their lives there. This result took into account other factors that could affect the risk of MS, such as sex, age and other health conditions.

The researchers did not find any differences between men and women or between people belonging to one of Canada’s immigration classes: family, refugee, or economic.  

“Our data did not include information on various environmental factors associated with MS, but our theories include that this increase in the risk of MS over time may be due to lifestyle factors such as higher rates of smoking and changes in diet, environmental factors such as sunlight exposure and biological factors such as the composition of the gut microbiome that have been previously associated with an increased risk of MS,” Vyas said. “Some immigrants may be more susceptible to these risk factors due to social determinants of health such as income, education, neighbourhood and access to nutritious food.”