A new study comparing the gut microbiome of four distinct, but geographically close, Himalayan populations has revealed a fascinating insight into how dietary lifestyle can dramatically alter gut bacterial composition. The research uncovers how the shift from a hunter-gatherer lifestyle to an agricultural society can alter microbiome compositions within a single person's lifetime.

The research looked at four tribal populations: the Tharu, the Raute, the Raji and the Chepang, all of which live in rural villages in Nepal, at the foothills of the Himalayas. However, despite similar ancestries and cultural practices, each population's dietary lifestyle has diverged over the past few hundred years.

The Tharu present as the most agricultural of the groups, transitioning into an agrarian lifestyle late in the 18th century. As of today, the Tharu have completely disengaged from historical foraging practices. At the other end of the spectrum, the researchers examined the Chepang, a traditionally nomadic group who today inhabit a series of remote villages and primarily subsist on a daily diet of foraged plants from nearby forests.

In between these two extremes, the Raute and the Raji are two formerly semi-nomadic groups that have recently abandoned their foraging lifestyles and are transitioning to a lifestyle of substance farming. Both the Raute and the Raji began this lifestyle, and dietary transition, in the 1980s.

The research set out to examine several members of each population in order to identify what specific differences the transition from a traditional to an agrarian lifestyle has on the composition of the gut microbiome. The results revealed not only did each population display a distinct type of gut microbiome, but a gradient between all four populations could be identified, mirroring the populations' transition from hunter-gathers to farmers.

"This study indicates that human microbiomes may have changed gradually as human lifestyle changed, and those changes can happen within a human's lifetime," says lead author on the study, Aashish Jha.

Comparing the four distinct microbiomes to fully industrialized populations, such as those found in North America, the research saw distinct subdivisions of bacteria, apparent in hunter-gatherer groups, begin to diminish as farming and mass agriculture influences diet.

For example, high levels of Ruminobacter and Treponema, which are commonly associated with metabolizing plant products, were found in the microbiomes of the Chepang foragers, yet were increasingly rare in the other groups, relative to the influence of farming on their diets. On the other hand, bacterial strains including Actinobacteria and Verrucomicrobia, which are found in abundance in the microbiomes of North Americans, were extremely rare in the Chepang group, and intermediate in the Tharu farmers.

One of the compelling insights illustrated by this new research is just how rapidly a specific population's microbiome can change as the collective's dietary lifestyle shifts.

"We have always thought of humans as human DNA and the collection of humans cells that we walk around with," says Justin Sonnenburg, senior author on the new research. "But now we know that we have this microbial identity, and that microbial portion of our biology is malleable. It can change over really short time periods."

At this stage, the research has not delved into what specific environmental or dietary factors are associated with the microbiome variations, or if they have any potential health consequences. However, further work is moving to home in on identifying these factors, and the researchers note a degree of urgency in gathering this data as soon as possible.

"As the world is urbanizing rapidly, our microbiomes are also changing rapidly," says Jha. "So, if we don't study the traditional societies today, 20 years down the road we may be too late."

The study was published in the journal PLOS Biology.

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