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The first study, published in Nature Medicine in August, compared the gut microbiomes of adults from Amsterdam’s six largest ethnic groups. A team led by Mélanie Deschasaux, an epidemiologist at the Academic Medical Center in Amsterdam, assessed stool samples from 2,084 individuals who were ethnically Dutch, Ghanaian, Moroccan, Turkish, African Surinamese or South Asian Surinamese. Most of the non-Dutch participants had immigrated to the Netherlands as adults.

Between ethnic groups, the researchers discovered significant differences in overall gut microbe composition. Of the various factors studied, ethnicity was the strongest determinant of gut microbial makeup.

Across the Atlantic, Pajau Vangay and Dan Knights, of the University of Minnesota, worked with two local communities to study how migration alters the human gut microbiome. They published their results in Cell last week.

One community, the Hmong, began arriving in Minnesota in the 1970s as refugees from the CIA-backed Secret War and Vietnam War, which ravaged their communities in Laos. The second group, the Karen, arrived in Minnesota in larger numbers in the past decade, fleeing human rights abuses in Myanmar.

Stool samples and other data from more than 500 women revealed that immigrants from these groups began losing their native microbes almost immediately after resettling. They picked up American microbes, but “not enough to compensate for the loss of native strains, so they end up losing a substantial amount of diversity overall,” Dr. Knights said. Furthermore, losses were greater in obese individuals and children of immigrants.