Unfortunately it's not an improvement.
As soon as immigrants arrive in the United States, their gut microbiomes start to change. The native strains of bacteria that they bring from other continents are replaced by those found more commonly in Westernized bodies, and overall diversity is lost.
This interesting discovery was made by scientists at the University of Minnesota and published earlier this month in Cell. The scientists wanted to understand how immigration affects gut health, so they embarked on a study of Hmong and Karen refugees living in Minnesota. Epidemiological evidence has shown that living in the U.S. increases risk of obesity in immigrants relative to people of the same ethnicity who stay in their native country, and that refugees are particularly vulnerable to rapid weight gain, with Southeast Asians showing the highest increases after moving.While there are a number of factors that contribute to weight gain, such as adopting a Western diet, feeling stressed, becoming more sedentary, taking antibiotics or antiparasitics, drinking new water, and coping with an abundance of food for the first time, the researchers say these don't fully explain the risk of obesity.
They reached out to Hmong and Karen refugees living in Minnesota, who have fled from China and Burma respectively, and took samples of microflora from adults and children. They tested individuals living in Hmong and Karen communities in Thailand and even tracked 19 Karen refugees who relocated from Thailand to the U.S., offering a rare look at longitudinal changes to their gut microflora over the first nine months in the U.S. Samples from Caucasian Americans were used as a control.
What the researchers found was that changes to gut microbiomes happen surprisingly fast. From Science Daily,
"In those first six to nine months, the Western strain Bacteroides began to displace the non-Western bacteria strain Prevotella. But this Westernization also continued to happen over the course of the first decade in the U.S., and overall microbiome diversity decreased the longer the immigrants had been in the U.S."
Prevotella is used to break down and digest traditional Southeast Asian foods, like wild greens, coconut, and tamarind; and once lost, it is not replenished. Diversity in microbial communities decreased, as did all native strains and a person's ability to digest fibrous foods. NPR's The Salt reported,
"By tracking everyone's food logs, the researchers found that an Americanized diet — featuring less fiber and more processed sugars — played a role in disrupting immigrants' microbiomes. Some of the bacteria in our guts feed, and survive, on particular fibers found in grains and greens — and die off when they don't get enough."
The study authors hope that their findings can guide healthcare providers in dealing with refugee and immigrant populations. Until a probiotic is developed that helps immigrants to compensate for the lost microbes, the best we can do is educate. As co-author Pajau Vanjay told NPR, the study may not offer an obvious solution to obesity, but it sends a clear message about the importance of hanging onto traditional ways of cooking and eating – and perhaps to the rest of us to start eating as others in the world do.