The evidence showed that the longer immigrants spent in the USA, the less diverse their bacteria became, and that this was linked to rising obesity. 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).
The changes were even more pronounced in their children.
People switching from a Thai diet to an American diet exhibit a drastic reduction in microbiome diversity. More specifically, that included the Hmong and Karen people groups - ethnic minorities originally from China and Burma. "But it was striking to see this loss of diversity actually happening in people who were changing countries or migrating from a developing nation to the US", he says.
"Obesity was a concern that was coming up a lot for the Hmong and Karen communities 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", explained Pajau Vangay, first author of the study.
The human gut is home to hundreds of different species of bacteria known collectively as the "gut microbiome". Ground-breaking studies from the lab of United States biologist Jeff Gordon first found a link between obesity and the gut microbiome in 2006, when they showed mice gained weight when they were given gut bacteria from obese humans.
These are, essentially, "good" microbes - at least some of them.
These two bacteria are not necessarily good or bad; they're simply dominant members of the gut microbiomes in different populations around the world.
For the study, the researchers collected stool samples as well as asked for dietary recalls from 514 Hmong and Karen individuals living in Thailand and the United States. The study also featured the children of those immigrants, as well as Caucasian American controls.
The researchers found significant and lasting changes to the gut microbiome that started right away-in the first six to nine months after the Karen refugees immigrated-according to the analyses of the established immigrants. "Having the wrong gut microbes has now been associated with nearly every major human disease and has been found to have a causative role in a wide variety of chronic diseases", they said.
For newcomers to the United States, the downside of immigration may be a rapid change in gut bacteria, researchers say. Most notably, a Western strain of bacteria (Bacteroides) began to displace the non-Western bacteria strain (Prevotella).
Knights pointed out that changes were sharper and more distinct in kids. They suspect that diet might have something to do with it. "However, it is clear in our study that when people lose certain species of microbes, they also lose the ability to produce the enzymes that those microbes carry, and that is probably affecting the types of foods they can digest". In recent years, it's become clear that our microbiome responds to things like diet and lifestyle, and that an unhealthy microbiome may play a role in metabolic diseases like obesity and diabetes, as well as neurological disorders. "It could be that this has to do with actually being born in the United States of America or growing up in the context of a more typical US diet", Knights said in a journal news release. But in this case, "Westernization of the microbiome is associated with obesity in immigrants, so this could be an interesting avenue for future research into treatment of obesity, both in immigrants and potentially in the broader population", Knights concluded. Indeed, obese study participants had the lowest levels of diversity in their microbiomes, and as Knights tells Gizmodo, "there seems to be a universal association with loss of diversity with disease". "And that's something that has been seen in animal models before, but not in humans", he said.