American diet changes gut bacteria of immigrants

Immigrants coming to the United States from Southeast Asia experience a rapid "Westernizing" of the bacteria in their guts that could explain their rising rates of obesity and related diseases.

University of Minnesota researchers discovered the pattern by analyzing the digestive bacteria of 514 Hmong and Karen women — some still living in Southeast Asia, some who recently arrived in the Twin Cities, and some who were U.S.-born children of immigrants — and comparing them with the bacteria of 36 white Minnesota women.

Six to nine months after arriving in the U.S., the immigrant women had a drop in the number and diversity of their gut bacteria — which are essential for digestion, and for immune system health — according to the study, which was published in Cell, an influential scientific journal.

"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," said Dan Knights, a lead author of the study and a quantitative biologist at the U.

The change was more significant among immigrants who were obese, and among immigrants' U.S.-born children. The dominant species of bacteria in their guts changed from Prevotella to Bacteroides, which are more common in Americans.

The diversity of their bacteria declined as well, which is significant because other studies have linked a lack of diversity to a greater risk of obesity, Knights said.

The microbiome of the gut is a new frontier in medicine, with recent studies making remarkable findings about its diversity and influence on the rest of the body.

"[The study] came at a time when diabetes and obesity rates were just at an extreme high," said Houa Vue-Her, who served as a community adviser to the study. She also has tackled Hmong health issues through her work at the Center for Prevention at Blue Cross and Blue Shield of Minnesota, which is supporting Hmong farmers in their efforts to introduce native vegetables into schools.

"We have been seeing [obesity] pick up and, in our community, we just want to know more about what's causing it," she added.

The study included testing of a handful of Karen women who were in refugee camps in Thailand, and then testing them again in Minnesota.

Most obesity surveys only ask people if they are Asian, so there is little data to prove a rising problem among Hmong Americans. The Minnesota Student Survey, conducted by the state Department of Health, only started asking students in 2016 if they were Hmong. Results showed that 35 percent of Hmong high school juniors in Minnesota were overweight or obese, compared with 24 percent of white juniors.

There is little doubt of an obesity problem, though, because southeast Asian immigrants often switch from diets heavy in vegetables and rice to higher-fat American diets. They also adopt more sedentary lifestyles, research has shown.

"As people come here, very few are overweight, let alone obese," she said. "The longer they're here, the more obese they become. So conversations I have with them at 10 years, 20 years, are much more similar to conversations I'm having with Americans about diabetes and hypertension."

Given these realities, Knights said he expected that his study would find a change in the gut bacteria of arriving immigrants. But he was surprised by the rapid pace of change, and that the children had less diverse microbiomes than their immigrant parents.

"It was striking to see this loss of diversity actually happening in people who were changing countries or migrating from a developing nation to the U.S.," he said.

Dietary change doesn't appear to be the only explanation, because immigrants didn't entirely abandon their native foods. The researchers tracked their eating habits and found they continued to consume more rice and vegetables than the Americans in the comparison group. And yet their changes in gut bacteria were substantial anyway.

The U's finding shows how little, even now, is fully understood about the gut microbiome, Selameab said. Stress, air quality and other factors are probably playing a role.

More research is needed to substantiate if one strain of bacteria common in Southeast Asia is protective against obesity, or whether one strain in Americans helps cause it.

"You could imagine actually isolating and growing some microbes from immigrants that are typically present in healthy individuals in their group," Knights said. "Those microbes could potentially be used as therapeutics — sort of as medical grade probiotics.

"Before we get to that, we have to find out whether the change in microbes is actually causing obesity in this group, rather than just responding to obesity."