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Residents of developing nations tend to have more diverse gut microbiomes than Americans. Now, a new study has found that people who have emigrated from Southeast Asia to the US shed some of that variety when they enter the country.

“We found that immigrants begin losing their native microbes almost immediately after arriving in the U.S. and then acquire alien microbes that are more common in European-American people,” coauthor Dan Knights, a computer scientist and quantitative biologist at the University of Minnesota, says in a press release. “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 results were published today (November 1) in Cell.

Knights and his colleagues obtained their results by comparing the microbes in the stool of Hmong and Karen immigrants living in Minnesota with people of these communities still residing in Thailand. They also followed 19 Karen refugees in the process of moving from Thailand to the US and examined the gut microbiota of immigrants’ children who were born in the US as well as of Caucasian Americans.

The researchers found significant and lasting changes to the gut microbiome that started right away—in the first six to nine months after the Karen refugees immigrated—according to the analyses of the established immigrants. Specifically, a non-Western bacteria strain known as Prevotella was displaced by Bacteroides, a common member of Western microbiomes, and overall microbial diversity in the gut decreased with time. 

The microbiome was already known to change as people moved from one type of society to another, Stanford University microbiologist Justin Sonnenburg, who was not involved in the study, tells The Atlantic, but “to watch it happen six to nine months after people moved is startling.”

Changes in diet appeared to play a role, but couldn’t account for all the differences, the researchers report. “We don’t know for sure why this is happening,” Knights says in the release. 

Knights and his colleagues suspect the changes may help explain the high rates of obesity and diabetes among immigrant populations in the US. Indeed, obese study participants had the lowest levels of diversity in their microbiomes, and as Knights tells Gizmodo, “there seems to be a universal association with loss of diversity with disease.” Moving from correlation results to determine causation “is an obvious direction for future work,” he adds.