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Stanford University School of Medicine

Hunter-gatherers’ seasonal gut-microbe diversity loss echoes our permanent one, study shows

The Hadza, a small group of hunter-gatherers inhabiting Tanzania's Rift Valley, number just over 1,000 people, fewer than 200 of whom still adhere to the traditional hunter-gatherer lifestyle.

This tiny remnant population, one of just a few scattered hunter-gather populations in Africa and South America, will probably have completely abandoned this way of life within a few decades. But for now it serves as the closest available proxy to a time machine we in the modern industrialized world can climb into to learn about the ways of our remote human ancestors.

In a recent study published in Science, Stanford microbiologist Justin Sonnenburg, PhD, and several colleagues analyzed the seasonal variation in diversity of the trillions of microbes that inhabit the typical Hadza gut.

As I wrote in a news release:

For more than 15 million years, human beings have co-evolved with thousands of microbial species that take up residence in the lowermost part of the intestine, earning their keep by helping us digest food components we're unable to break down by ourselves, chiefly dietary fiber; manufacturing vitamins and other health-enhancing molecules; training our immune system and fostering the maturation of cells in our gut; and guarding our intestinal turf against the intrusion of all-too-eager competing microbial species including pathogens.

As many studies have shown, we moderns are hosts to substantially fewer gut-microbial species than hunter-gatherers are. Most likely, that's due to the vast difference in traditional versus modern peoples' dietary-fiber intake. "Dietary fiber" is a fancy term for the complex carbohydrates we consume but can't digest -- which is fortunate for our intestine-dwelling microbes, because they can, do, and must digest fiber in order to survive. Fiber's our gut-microbes' version of fast food, and its constant availability in the lower intestine is, after all, why they moved into the neighborhood in the first place.

The typical Hadza eats more than 100 grams of fiber daily, on average. We urban dwellers are lucky to get 15 grams a day. Consequently, as many studies have shown (and as Sonnenburg has lamented), we're hosts to substantially fewer gut-microbial species than hunter-gatherers are.

The researchers found that Hadza individuals' resident gut-microbial ecosystem (or microbiota, as scientists refer to it) also varies seasonally, just as their diet does. Again, from my release:

[T]he Hadza lifestyle doesn't include refrigerators and supermarkets. So the population's diet fluctuates according to the season, of which there are two ...: dry, when meat, baobab and tuber consumption play a relatively larger role; and wet, during which berries, tubers, honey and baobabs prevail.

Here's the study's really intriguing finding: It's precisely the microbial species whose populations wax and wane most intensely throughout the year among the Hadza that seem to have gone AWOL in the our own modernized guts. Might the last several sequential generations of virtually fiber-free food intake in the industrialized world have so radically altered our gut microbiota that once-flourishing cyclical species have died out entirely? And if so, with what consequences for our health, and our grandchildren's? Stay tuned.

Previously: The die-off within us: Are our low-fiber diets ruining our descendants' lives?, Can low-fiber diets' damage to our gut-microbial ecosystems get passed down over generations?, Getting to the good gut: How to go about it and Civilization and its dietary (dis)contents: Do modern diets starve our gut-microbial community?
Photo by Jeff Leach

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