For more than 15 million years, humans have co-evolved with thousands of microbial species that take up residence in the intestine, earning their keep in many ways. They help us digest food components we’re unable to break down by ourselves, chiefly dietary fiber.
They manufacture vitamins and other health-enhancing molecules. They train our immune system and foster the maturation of cells in our gut. They guard our intestinal turf against the intrusion of competing microbial species, including pathogens.
Scientists who study these communities of microbes have noted that the species mix has changed over the centuries, becoming markedly less diverse. They had a hunch that the radical alteration in diet over the millennia has been a major factor in these changes. But how to test that hypothesis?
That’s where members of a small group of hunter-gatherers inhabiting Tanzania’s Rift Valley enter the story. This group, known as the Hadza, number just over 1,000 people, fewer than 200 of whom adhere to the traditional hunter-gatherer lifestyle, which includes a diet composed mainly of five items: meat, berries, baobab (a fruit), tubers and honey. While Western diets are pretty much the same throughout the year, the Hadza lifestyle doesn’t include refrigerators and supermarkets.
So the population’s diet fluctuates according to the season, of which there are two in the Rift Valley: dry, when meat, baobab and tuber consumption play a relatively larger role; and wet, during which berries, tubers, honey and baobabs prevail. (Tubers and baobab are available year-round.)
A team of researchers led by Justin Sonnenburg, PhD, associate professor of microbiology and immunology at Stanford, collected 350 stool samples over a one-year period — a full seasonal cycle — from 188 Hadza people, and analyzed their microbiota.
“Surviving hunter-gatherer populations are 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,” says Sonnenburg, who is a senior author of the study, published Aug. 25 in Science.
“The 100 to 200 Hadza sticking to this routine will possibly lose it in a decade or two, maybe sooner. Some are using cellphones now. We wanted to take advantage of this rapidly closing window to explore our vanishing microbiota,” he says.
The research confirmed that the Hadza microbiota is more diverse than, and substantially different from, that of industrialized countries’ urban-dwelling denizens.
Analysis of their stool samples also showed that fewer of a subset of microorganisms are present during the wet seasons, but the levels increase during the dry seasons when consumption of fiber-rich tubers peaks. It also shows the microorganisms that increase in the Hadza and other hunter-gatherer groups when more high-fiber foods are added are the same ones missing from the guts of most people in the industrialized world.
So, yes, though other factors could be at play — for instance our increasingly sedentary existence and the introduction of antibiotics — the change in diet seems to be a major factor in the reduced microbial diversity in the guts of those of us who are not hunter-gatherers.