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esearchers studying traditional hunter-gatherer societies had already known that they have much more diverse microbiomes — the collection of bacteria that live in our guts — than those of people living in industrialized societies.

But this week, researchers reported a new discovery: The composition of the hunter-gatherers’ microbiomes changes seasonally, apparently driven by the regular shifts in their diets. And interestingly, their guts look a lot more like those of people who have Western diets during parts of the year.

For the study, published in the journal Science, researchers focused on the Hadza, a group living in Tanzania and one of the few that still practices hunter-gatherer lifestyles. They collected 350 stool samples over a year from 188 Hadza people, and then compared the bacteria found among them to samples from 18 other populations living in 16 countries.

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The Hadza diet goes through two distinct seasons. During the wet season (November to April), the Hadza rely more on berries and honey, but they eat more meat during the dry season. (They eat fiber-rich tubers and baobab year-round.)

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Those cyclical changes, the researchers discovered, correspond to shifts in the composition of the Hadza’s microbiomes.

There was a greater variety of bacteria present during the dry season, but certain types of bacteria then dwindled during the wet season, the researchers found. But come the next dry season, those bacteria reappeared.

It turns out that the type of bacteria that fluctuated seasonally is the type of bacteria that are not found often in people who eat Western diets, or, as the researchers put it, the species that are “rare or absent, regardless of season, in industrialized populations.”

Scientists have pinpointed several reasons why microbiome diversity has been falling for thousands of years, and more recent developments — including births by caesarean section and the use of antibiotics — have only contributed to that. But the new research adds to the evidence that diet is a major factor in determining the composition and variety of the microbes that live in our guts.

The new study shows that microbes that disappear for some amount of time can be revived. But past research in mice from some of the same scientists suggests that might not be so easy.

For a study last year, they found that the lack of fiber among mice eating Western diets weakened the diversity of their children and grandchildren’s microbiomes. Worse, they found that the loss of diversity couldn’t be reversed easily as it was passed down over more generations.

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