There is a rather unexpected side effect of moving to the United States. Scientists have found that it is not just an immigrant's place of residence that changes, but something even more personal.

In a paper published in the journal Cell, researchers at the University of Minnesota and the Somali, Latino, and Hmong Partnership for Health and Wellness have shown that the gut microbiome of Hmong and Karen immigrants (ethnic minorities originally from Southeast Asia) radically shifts upon coming to the US, becoming a lot less diverse and a whole lot more American. This, they say, may at least partly explain the rise in obesity rates among immigrant communities. 

"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," senior author Dan Knights of the University of Minnesota said in a statement.

"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 team studied the composition of microbiota (collected from stool samples) of more than Hmong and Karen women, some living in Thailand, some who had moved the US, and some second-generation immigrants, as well as 36 white American women, the control group. They also tracked changes in the microbiome of 19 Karen refugee women, starting before they immigrated and ending one year after their move to the States. 

"Obesity was a concern that was coming up a lot for the Hmong and Karen communities here,” first author Pajau Vangay explained.  

“In other studies, the microbiome had been related to obesity, so we wanted to know if there was potentially a relationship in immigrants and make any findings relevant and available to the communities.”

They found that levels of a particular bacteria called Prevotella, responsible for producing some of the enzymes that break down plant fiber, declined as the Hmong and Karen women spent more time in the US. At the same time, those of Bacteroides, a type associated with a westernized diet, shot up.

Overall, their microbiota started to look a lot more western – and like the white American controls'. This shake-up of bacteria in the gut was noticeable after just nine months on US soil but the change was most apparent among the children of immigrants. 

Knights described this radical loss of diversity as "striking". “We don't know for sure why this is happening. It could be that this has to do with actually being born in the USA or growing up in the context of a more typical US diet,” he continued.

"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."

This would not be the first time diet (and obesity) has been associated with changes to the microbiome, particularly a microbiome that is less diverse – studies have linked microbiota to our waistlines, food cravings, and the food we eat as well as to changes in our mood, sleep, and behavior. But it is important to remember all this is showing is a correlation, it is not proving causation.

So, is the rise in obesity due to the change in microbiota? Or is a third factor – diet, environment, or both – affecting both microbiota and obesity rates? To find out, more research is needed.