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Immigration to US Quickly Westernizes Gut Microbiome

NEW YORK (GenomeWeb) – A new study has found that individuals who immigrate to the US start to experience changes to their gut microbiomes soon after they arrive.

Individuals living in developing nations tend to have a more diverse assortment of bacteria within their gut microbiomes than do individuals in the US. Many immigrant and refugee populations in the US like the Hmong and Karen develop metabolic diseases like obesity, prompting concern among community members.

Researchers from the US and Thailand, in conjunction with Hmong and Karen community members, developed a study to examine the gut microbiomes of individuals still living in Thailand, individuals who have immigrated to the US, and the children of immigrants to examine how migration affects the gut microbiome. They also sampled 19 Karen individuals before and after they moved to the US. As they reported today in Cell, the researchers observed a loss of gut microbiome diversity and changes to its function soon after individuals arrived in the US.

"We found that immigrants begin losing their native microbes almost immediately after arriving in the US and then acquire alien microbes that are more common in European-American people," senior author and University of Minnesota computer scientist and quantitative biologist Dan Knights said in a statement. "But the new microbes aren't enough to compensate for the loss of the native microbes, so we see a big overall loss of diversity."

The researchers put together a cohort of 514 healthy Hmong and Karen women who lived in Thailand, were born in Southeast Asia but moved to the US, or were born in the US to immigrant parents, as well as 36 US-born women of European descent as controls. They collected 550 stool samples from these women for 16S rRNA gene sequencing.

No matter what country they lived in, the Hmong and Karen populations had distinct gut microbiomes. However, after immigrating to the US, both populations' microbiomes began to resemble those of European Americans, the researchers found.

In particular, Hmong and Karen immigrants to the US experienced a loss of bacterial diversity that increased with each generation, a loss the researchers noted was linked to increased obesity. The Western-associated genus Bacteroides increasingly displaced the non-Western-associated genus Prevotella. A deep shotgun metagenomic analysis of 55 samples found that the US controls had varied Bacteroides­ strains and those who did harbor Prevotella generally had one species, while individuals in Thailand harbored up to four Prevotella species. Long-term Hmong residents in the US had a profile that fell in between.

Length of residence in the US was also associated with decreased microbiome diversity, the researchers said, adding that the longer immigrants spent in the US, the more their microbiomes diverged from populations in Thailand to be more similar to those of European Americans.

These microbial shifts corresponded with changes in the functional capabilities. First-generation Hmong in the US had increased capacity for sucrose, glycerol, and glucose/xylose degradation, but lost carbohydrate-degrading enzymes that confer the ability to break down plant fibers.

While individuals' dietary profiles were significantly associated with their overall microbiome profiles, the researchers found that they only explained about 17 percent of the variance in microbial profiles, suggesting that diet was not the only factor at play.

These changes to immigrants' gut microbiomes also started quickly upon arrival in the US, the researchers found. They profiled the gut microbiome of 19 Karen individuals before and after they immigrated to the US and found that Prevotella began to be replaced by Bacteroides­ within six to nine months.

"When you move to a new country, you pick up a new microbiome. And that's changing not just what species of microbes you have, but also what enzymes they carry, which may affect what kinds of food you can digest and how your diet interacts with your health," Knights said.

While he noted said such a change "might not always be a bad thing," Knights added that "Westernization of the microbiome is associated with obesity in immigrants." 

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