An empire of germs dwells inside the human body, trillions strong. About a half-pound of bacteria plus their genes make up our microbiome. Though each microbe is small, a healthy and diverse microbiome is mighty. Its influence, studies suggest, spans the human condition, from mood swings to weight gain.
The microbiome begins at birth, but many factors alter its composition. Growing evidence shows location has a profound impact on the diversity of microbes, and some places are much worse than others. A new study just published in the journal Cell follows multigeneration immigrants from Southeast Asia to the United States. As they moved, their microbes responded. Once in the United States, the immigrants' gut diversity dropped to resemble the less-varied microbiomes in European Americans. At the same time, obesity rates spiked.
"We found that moving to a new country changes your microbiome," said Dan Knights, a computational microbiologist at the University of Minnesota and an author of the paper. "You pick up the microbiome of the new country and possibly some of the new disease risks that are more common in that country."
In the United States, immigrants in the study ate foods richer in sugars, fats and protein. Microbiomes changed within months of moving. "People began to lose their native microbes almost immediately after arriving in the U.S.," Knights said. "The loss of diversity was quite pronounced: Just coming to the USA, just living in the USA, was associated with a loss of about 15 percent of microbiome diversity."
Obesity rates among many of the study immigrants increased sixfold. Those who became obese also lost an additional 10 percent of their diversity. "And the children of immigrants," Knights said, "had yet again another 5 [percent] to 10 percent loss."
As microbial diversity decreases, the risk of diseases like obesity and diabetes increases. "It's been known from previous studies that people in developing nations tend to have more gut microbiome diversity and lower risk of metabolic diseases," Knights said. "It was also known that moving from a developing nation to the U.S. increases your risk of those diseases." But no one had tested whether the microbiome changed after immigration, too.
"The association made between changes in dietary factors, toward a more 'westernized' nutritional diet, and the loss of bacterial diversity" was "particularly striking," said Eran Elinav, who studies the human microbiome at the Weizmann Institute of Science in Israel. (In 2014, Elinav and his colleagues reported that traveling to distant time zones by plane alters the microbiome, as though the germs were jet-lagged.)
But changes in diet were slower than changes in microbiome, suggesting factors beyond American food were at play. "We found that diet alone wasn't enough to explain the rapid westernization of the microbiome," Knights said. Possibly differences in drinking water and antibiotics also contributed.
The new study supports hypotheses that Western lifestyle influences the microbiome. Industrialization is correlated with a drop-off. Indigenous South American people, for instance, have about twice as many species in their guts compared with a person in the United States.
"We have known from some small, not well controlled studies that the microbiome does change, and we have known for many years that adopting a Western lifestyle is associated with an increase in disease," said microbial ecosystem expert Jack Gilbert, director of the University of Chicago's Microbiome Center, not involved with the current study. "This brings those two concepts together."
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Print Headline: Life in U.S. found to wreck immigrants' thriving gut bacteria