As soon as a person immigrates to the US, their microbiome starts to change.

People switching from a Thai diet to an American diet exhibit a drastic reduction in microbiome diversity. Depicted here: a Thai dish (fish in chili sauce).

Immigration is a touchy subject in many parts of the world, but while some things are debatable, researchers found clear signs that the microbiome of immigrants drastically changes when they come to the US. Specifically, researchers from  the University of Minnesota and the Somali, Latino, and Hmong Partnership for Health and Wellness have studied communities migrating from Southeast Asia to the US, finding that their gut biome is immediately “Americanized.”

“We found that 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,” says senior 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.”

It has to be said that this isn’t really a good thing. Generally speaking, microbiomes from the Western world are associated with greater obesity, whereas people from developing countries tend to have more diverse and healthy microbiomes (particularly in areas where fruits and vegetables are more popular). It seems quite normal that a person’s biome would shift when the diet changes, but it’s striking to see how much diversity is lost, and how fast this happens — in only six to nine months.

“Obesity was a concern that was coming up a lot for the Hmong and Karen communities [from Thailand] here. 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. These are vulnerable populations, so we definitely try to make all of our methods as sensitive to that as possible and make sure that they have a stake in the research,” says first author Pajau Vangay.

Good microbes

The gut microbiota (also called the gut microbiome, and previously called the gut flora) is the name given to the microbe population living in our intestine. Essentially, our gut contains trillions of microorganisms, from hundreds of different species, containing 3 million genes — 150 times more than human genes.

These are, essentially, “good” microbes — at least some of them.

Increasingly, the gut microbiome has been shown to be important in a number of diseases, including your weight, general health, and even mental health.

The team compared the microbiome of Thai immigrants to people who were still living in Thailand. The study also featured the children of those immigrants, as well as Caucasian American controls. Researchers were also able to follow a group of 19 Karen refugees as they relocated from Thailand to the US, allowing them to see how the microbiomes were changing in time.

Significant changes took place quite fast. Most notably, a Western strain of bacteria (Bacteroides) began to displace the non-Western bacteria strain (Prevotella). The kids’ biomes changed significantly faster than those of the adults.

Overall, the changes seem to be a logical consequence of a change in diet, but they are still concerning.

“When you move to a new country, you pick up a new microbiome. And that’s changing not just what species of microbes you have, but also what enzymes they carry, which may affect what kinds of food you can digest and how your diet interacts with your health,” he says. “This might not always be a bad thing, but we do see that Westernization 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.”

While no direct cause-effect has been established between a Western diet and obesity and other health issues, there is a lot of correlation between the two. Migration from a non-western nation to the United States is associated with a loss in gut biome diversity, which may predispose individuals to metabolic diseases, researchers conclude.

“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. 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,” says Knights.

The study was published in Cell. http://dx.doi.org/10.1016/j.cell.2018.10.029

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