Published in the journal Cell, the study links the act of immigration with a loss of gut microbiome diversity that could play role in obesity that is compounded across generations.
In particular, new arrivals appear to lose the bacterial enzymes associated with plant fibre degradation with bacteroides strains displacing Prevotella strains according to time spent in new surroundings.
“We found that immigrants begin losing their native microbes almost immediately after arriving in the US and then acquire alien microbes that are more common in European-American people," said senior study author Dan Knights, a computer scientist and quantitative biologist at the University of Minnesota.
"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."
Gut microbe diversity is considered key in regulating host metabolism and is heavily influenced by an individual’s long-term diet as well as dramatic changes to diet.
Rural indigenous populations have been found to harbour substantial biodiversity in their gut microbiomes, including novel microbial taxa not found in industrialised populations.
Refugees, in particular, appear to be more vulnerable to rapid weight gain with Southeast Asian refugees exhibiting the highest average increases in body mass index (BMI) after relocation to the United States.
Researchers at the University of Minnesota and the Somali, Latino, and Hmong Partnership for Health and Wellness began measuring gut microbiomes and dietary intake from Hmong and Karen immigrants and refugees.
One of the groups included individuals whose stay in the US ranged from a few days to more than 40 years, allowing changes in the gut microbiome associated with long-term US residence to be noted.
Second-generation immigrants (born in the US to first-generation Hmong immigrants) were also included to determine whether the effects of US immigration were compounded across generations by birth in the US.
Finally, the research team followed a group of 19 Karen refugees for up to nine months beginning immediately before or after arrival in the US in their first six to nine months to measure the short-term effects of US immigration.
In that period, the team found the Western strain Bacteroides began to displace the non-Western bacteria strain Prevotella.
However, this Westernisation was also observed in the first decade in the US as overall microbiome diversity decreased the longer the immigrants had been in the US.
Food logs kept by the subjects suggested that eating a more Western diet played a role in disturbing the microbiome but could not explain all the changes.
Diversity loss across generations
“We don't know for sure why this is happening,” said Knights. “It could be that this has to do with actually being born in the US or growing up in the context of a more typical US diet.”
”But it was clear that the loss of diversity was compounded across generations. And that's something that has been seen in animal models before, but not in humans."
While the study could not establish a cause-and-effect relationship between the microbiome modifications in the gut of newcomers and the rise in obesity amongst this population, it did suggest that greater microbiome westernisation was linked to greater obesity.
Knights described the changes observed when moving to a new country, commenting that it was not just the species of microbes that were affected but also the enzymes carried, which could affect what kinds of food were digested and how diet interacted with health.
"This might not always be a bad thing, but we do see that Westernisation of the microbiome is associated with obesity in immigrants, so this could an interesting avenue for future research into treatment of obesity, both in immigrants and potentially in the broader population."
Published online: doi: 10.1016/j.cell.2018.10.029
“US Immigration Westernizes the Human Gut Microbiome.”
Authors: Pajau Vangay et al.