A lot changed for Minnesota-based chef Yia Vang's family when they fled persecution in Laos and, in 1988, resettled in the American Midwest. For one, "I think my parents realized they don't have to go out and kill one every time we want to eat chicken," Vang says. "So Tyson chicken tenders were always in the freezer."

But it's not just the way they lived and ate that changed — the bacteria that lived alongside and inside them probably changed as well.

Moving to the U.S. can seriously mess with immigrants' microbiomes, according to a new study that tracked the digestive health of refugees coming to Minnesota from Southeast Asia. "We found that when people come to the U.S.A., they almost immediately begin losing some of their native microbes," says , a quantitative biologist at the University of Minnesota and the study's senior author. Some of the strains they lose are ones that help them break down and glean nutrients from fibers found in Southeast Asian staples like wild greens, coconut and tamarind.

The study, published in the journal Cell on Thursday, is "the first evidence that moving to another country — especially to the States from a developing nation — does cause you to acquire more of an American microbiome," says Knights — one that tends to be less diverse, featuring fewer strains of bacteria. "And while we don't know whether this loss of diversity is causing obesity, we know that it's at least associated with more obesity."

Knights and his colleagues arrived at these insights by analyzing the intestinal bacteria of about 500 ethnically Hmong and Karen women living in Thailand and in the U.S., including first- and second-generation immigrants. (The researchers focused on women because more Hmong women than men resettled in the U.S.) The scientists then compared the immigrants' microbiomes with those of Caucasian Americans. The researchers also followed a group of 19 Karen refugees as they relocated from Thailand to America, tracking how their gut microbiomes changed during the transition.

Among that group of 19 refugees, researchers noticed that a Western bacteria strain called Bacteroides began to displace the non-Western strain Prevotella within their first six to nine months in the U.S. But they lost more microbes than they gained — "so the diversity in their microbial communities decreased," Knights says. "And some of the Prevotella bacteria they lost were the ones that helped them digest fiber from plants and greens."

Among all the immigrants, it became clear that the more time they spent in the U.S., the more their microbiomes diverged from those of ethnically similar people living in Thailand. Second-generation Hmong- and Karen-Americans — born in the U.S. to parents who moved from abroad — had microbiomes that were most similar to those of Caucasian Americans.

By tracking everyone's food logs, the researchers found that an Americanized diet — featuring less fiber and more processed sugars — played a role in disrupting immigrants' microbiomes. Some of the bacteria in our guts feed, and survive, on particular fibers found in grains and greens — and die off when they don't get enough.

But changes in diet didn't explain all — or even most — of the change in immigrants' microbiomes. "It could be that other factors, like exposure to different medications, especially antibiotics or changes in the quality of water they're drinking, are also affecting their microbiomes," says Pajau Vangay, a researcher at the University of Minnesota who co-authored the study. In any case, Vangay notes, researchers are still untangling the influence of diet and microbiome on obesity. Doing so could help explain why immigrants and especially refugees are particularly vulnerable to rapid, unhealthy weight gain.

"In speaking with community members, we also realized that for them, the biggest concern was obesity," says Vangay. "Because they had observed in themselves and their relatives and friends that when they moved to the U.S., they gained a lot of weight. And in some cases, they hadn't really changed too much about their diet."

This new study is a good first step in solving that mystery, says Maria Gloria Dominguez-Bello, a microbiologist at Rutgers University in New Jersey who wasn't involved in the research.

"I strongly believe this whole topic needs to be studied more rigorously, in more immigrant groups and people all over the world," she says. "Because one thing is clear: Human populations are migrating, and they are increasingly Westernizing. We need to know how those changes are going to affect our health."

Further research could also reveal what immigrants — and all Americans — who are struggling with obesity can do to improve their health. In the future, perhaps researchers may be able to develop probiotics that immigrants could take to compensate for microbes they've lost, Dominguez-Bello suggests.

"For now, our study may not provide any obvious solutions for obesity," Vangay says. But rather than leaving the community members hanging, she's working with community leaders and local health care providers to educate people about the value of hanging onto traditional ways of cooking and eating.

Last year, she worked with chef Vang — who in his adulthood has laid off the frozen chicken tenders and embraced traditional Hmong cuisine at his pop-up restaurant — to teach a series of nutrition and Hmong cooking classes.

"Growing up, like when I was in high school, me and my siblings we'd be like 'Mom, we really want spaghetti, we really want that that American food,' " Vang says. "Now, it's Hmong food like steamed fish or braised vegetables — that's the stuff that revives my soul; it's my comfort food."

But unlike a lot of American comfort foods, he says, "I know it's good for me. I don't feel heavy when I'm done."

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