What is already known on this topic
Diet and geographical location are the main determinants of our gut microbiota, and rural indigenous populations have been shown to harbor a much greater diversity of bacteria in their gut compared to industrialized populations. But how human migration influences gut microbiota structure and diversity is not well understood.
What this research adds
After arriving in the United States, the gut microbiota of immigrants from non-Western countries rapidly changes, decreasing in diversity. Lower bacterial diversity is associated with higher rates of obesity.
The study shows how migration from a non-Western country to a Western country alters the gut microbiome. It also offers an insight into metabolic diseases such as obesity that affect immigrants to the US.
Soon after immigrants arrive in the United States, their gut microbiota becomes more “Westernized”, with Western bacterial strains displacing non-Western strains, according to a study led by Pajau Vangay of the University of Minnesota in Minneapolis. The research was published in November in the journal Cell.
Previous studies have shown that mostly diet and geography determine the composition of the gut microbiota, with rural indigenous populations having a much greater bacterial diversity than industrialized populations. However, it’s not yet clear how human migration affects gut microbiota composition.
To address this question, the researchers conducted a study on refugees and immigrants that moved to the US from Southeast Asia. The team focused on the Hmong and Karen peoples, two ethnic groups from China and Burma that have communities in Thailand.
US immigration changes the gut microbiota
The team recruited 514 healthy Hmong and Karen women aged 18–78, of whom 179 were living in Thailand, 281 were born in Southeast Asia and had moved to the US (first-generation immigrants), and 54 were born in the US but had parents born in Southeast Asia (second-generation immigrants). Thirty-six healthy European American women were used as US controls.
The analysis of stool samples revealed that Hmong and Karen women have two distinct gut microbiotas, but both these microbiotas become more similar to European American microbiota after a person’s arrival into the US. In particular, the genus Bacteroides, usually associated with Western gut microbiota, displaced the non-Western genus Prevotella.
The overall microbiome diversity decreased the longer the immigrants had been in the US, with first- and second-generation immigrants showing a microbiota composition that was more similar to European Americans than to those living in Thailand.
Diet explains in part the changes in gut microbiota composition
When comparing Hmong women living in Thailand, first-generation Hmong women living in the US, and European American women, the researchers also observed that microbiota-derived β-glucan-targeting glycoside hydrolases, which contribute to the metabolism of plant fibers, were abundant in the Thailand group but almost absent in the US groups. These glycoside hydrolases are mainly produced by a Prevotella strain, suggesting that the loss of these bacteria after US immigration resulted in the inability to degrade plant-fibers.
The participants’ food recalls revealed that a Western diet contributed to alter their gut microbiota, but it couldn’t explain all the changes.
Microbiota Westernization begins soon after immigration and lasts
The researchers were also able to track the gut microbiota of 19 Karen refugees as they moved from Thailand to the US. The Westernization of their gut microbiota, with the displacement of Prevotella by Bacteroides, began within the first 9 months after they moved to the country.
What’s more, the researchers found that obesity was more prevalent in immigrants who had lived in the US for a decade, and that obesity was associated with lower bacterial diversity in all the study groups.
In conclusion, this research shows that US immigration is linked to a series of changes in the gut microbiota, including loss of microbial diversity, non-Western bacterial strains, and plant fiber degradation capability. By showing that greater Westernization of the microbiota is associated with greater obesity, the study also offers an insight into some of the metabolic diseases that have been shown to disproportionately affect immigrants to the US.