Immigrating to the U.S. leads to loss of microbiome diversity

When people move to another country, they must make many adjustments, such as learning a new language or adapting to new customs and traditions. Now, a study suggests they also may face a biological change: The assortment of microbes inhabiting immigrants’ guts is altered when they move to the U.S. (Cell 2018, DOI: 10.1016/j.cell.2018.10.029). This shift may predispose them to obesity and its associated metabolic ills, the study’s authors suggest.

Almost immediately upon their arrival in the country, immigrants “begin losing their native microbiome strains and taking up more typical U.S.-associated strains,” says Dan Knights, a computational biologist at the University of Minnesota who led the study. What’s more, his team found that immigrants picked up fewer strains than they lost, resulting in an overall loss of diversity in the types of microbes living in their guts. “This effect was even stronger in people who had obesity,” Knights notes.

Knights and his colleagues sequenced microbial genes in stool samples to identify the line-up of gut microbes in members of two immigrant communities from Southeast Asia, the Hmong and the Karen. Both groups have particularly high rates of obesity after arriving in the U.S. It was the first large-scale study to look at how moving from a developing region to a developed one affects the microbiome.

The researchers compared the microbiome composition of four groups of participants: Hmong and Karen people living in Thailand, Hmong and Karen people who immigrated to the U.S., U.S.-born children of Hmong immigrants, and U.S. Americans with European ancestry. The team also measured participants’ body mass index and asked them to report what they had eaten in the past 24 hours. In a subset of participants, they tracked changes in microbiome composition, diet, and weight at several time points before and after the people arrived in the U.S.

The longer the immigrants in the study lived in the U.S., the more they lost microbes belonging to the genus Prevotella. These microbes were replaced by those of the genus Bacteroides, which are prevalent in European Americans. Prevotella strains produce carbohydrate-degrading enzymes that help break down and digest plant fiber, so losing these strains led to changes in how people’s guts could extract nutrients from their food.

The researchers found that immigrants who had lived longer in the U.S. or who were U.S.-born children of immigrants were more likely to be obese. Obesity also tracked with less diverse microbiomes.

Immigrants’ microbiomes probably changed in large part because their diet changed. “It’s kind of a use-it-or-lose-it paradigm,” Knight says. In the absence of certain types of fiber in an immigrant’s new U.S. diet, the gut’s ability to digest that type of fiber atrophies.

But diet changes didn’t fully explain the results, Knights points out. Other factors, such as exposure to different medicines, like antibiotics; changes in water composition; or a difference in lifestyle and stress levels could also contribute, he says.

“It’s a very well conceived and well conducted study,” says Martin Blaser, a microbiologist at New York University who was not involved in the work. The findings support the idea that modernization and industrialization have gone hand in hand with a loss of microbial diversity, he adds.

Knights and colleagues plan to use mouse models to test whether the changes they observed are directly linked to obesity and diabetes. If so, it may be possible to use Prevotella and other beneficial types of microbes therapeutically to stave off these conditions, Knights says.