Immigrants to Canada see MS risk rising with time in country: Study

Among immigrants to Canada, the risk of multiple sclerosis (MS) is higher for those who have spent a greater portion of their life in Canada, a new study reports.

The study, “Proportion of Life Spent in Canada and the Incidence of Multiple Sclerosis in Permanent Immigrants,” was published in Neurology.

In wealthy countries such as Canada and the U.S., it’s well established that immigrants are less likely to develop MS than people born in the country. This is at least partly due to the so-called healthy immigrant effect, the idea that people who are physically able to move to a new country tend to be healthier.

Still, other factors ranging from lifestyle habits to sunlight exposure and vitamin D levels have been thought to play a role in the lower MS risk among immigrants.

In theory, environmental and cultural differences would be expected to have less of an impact for immigrants who have spent more of their lives in the new country and have adapted to its culture, which is known as acculturation. For example, a 20-year-old who immigrated as a one-year-old would be expected to have a higher MS risk, more similar to the general population, than a 20-year-old who immigrated at 19.

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Researchers used a large dataset from Ontario to analyze MS risk among more than 1.5 million people who immigrated to Canada between 1985 and 2003. These individuals were followed for an average of almost 14 years, during which time 934 developed MS.

The scientists used statistical tests to compare rates of MS based on the proportion of each individual’s life spent living in Canada. Using the above example, the 20-year-old who immigrated at age 1 would have spent 95% of their life in Canada, whereas for the 20-year-old who immigrated at age 19, the figure would be 5%.

“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,” study co-author Manav V. Vyas, PhD, a neurologist and clinician-scientist at St. Michael’s Hospital in Toronto, said in a press release from the American Academy of Neurology. “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.”

Among the entire group, immigrants had lived in Canada for about 20% of their lives. But results showed that immigrants with the highest portion of life lived in Canada tended to have higher MS risk.

“Compared with the median value of proportion of life in Canada, we observed a higher [risk] of MS with greater proportions of life spent in Canada,” the researchers wrote.

In subgroup analyses, the same trend was seen irrespective of biological sex or immigration status, and it was especially pronounced among individuals who immigrated in childhood or their early teens.

“This acculturation effect was similar across sex and immigration class and most pronounced for those who immigrated at or before the age of 15 years,” the scientists wrote.

The researchers speculated that factors ranging from sunlight exposure to dietary habits to environmental shifts in gut bacteria may help explain the findings, but they stressed that this study was designed to see if there was a meaningful association, not to assess cause and effect.

They highlighted a need for more investigation into factors that could contribute to the trend.

“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.

“Social determinants of health,” such as income, education, neighborhood, and access to healthy food may make some immigrants more susceptible to those risk factors than others, Vyas added.