Two men returning from a hunt. Photo: Andreas Lederer/Wikimedia Commons

A new study has found striking differences in the microbiome of traditional hunter-gatherers compared to the bacterial composition in the guts of populations from industrialized nations.

The findings could help explain why those with a Western diet have a less diverse set of gut microbes, and an increased risk of chronic disease.

The gut microbiome has been credited with assisting in food digestion, vitamin building, and enhancing the immune system to protect against pathogens.

A team of researchers, led by Justin Sonnenburg, associate professor of microbiology and immunology at Stanford University, set out to determine the link between a shift in diet and changes in the gut microbiome.

The Hazda population, located in Tanazania, is among the last remaining populations in Africa to live a solely hunter-gatherer lifestyle. Their food intake is reliant on the season and what nature provides them. For example, the Hazda will forage berries and honey more frequently in the wet season, and focus on hunting efforts during the dry season. According to the Stanford researchers, there are only about 200 people left that adhere strictly to a hunter-gatherer diet.

“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,” said Sonnenburg.

The team collaborated with members of the Human Food Project, who had already been living with the Hazda group. They collected 350 fecal samples from 188 Hazda people over the course of about a year to document any gut microbe changes throughout the seasons.

“Between seasons, striking differences were observed in their gut microbial communities, with some taxa apparently disappearing, only to reappear when the seasons turned. Further comparison of the Hadza microbiota with that of diverse urbanized peoples revealed distinctly different patterns of microbial community composition,” wrote the authors, in Science.

The team identified four families of bacteria that were particularly variable across the wet and dry seasons. Additionally, seventy percent of Bacteroidetes disappeared between the end of the dry season and beginning of the wet season.

Sonnenburg and colleagues then compared the microbiomes of Hazda people to that of 18 populations across 16 countries. Overall, the researchers concluded that the more distant a population’s diet was from the Western diet, the greater variety of microbes they had in their guts.

Two prevalent bacterial families within Hazda and other traditional groups analyzed were either rare or undetectable in people with non-traditional (Western) diets. The Hazda also possessed more enzymes to process plant carbohydrates than people with Western diets. But during the dry season, the Hazda shifted and had more enzymes to break down animal-made carbohydrates, as a result of eating more meat from hunting.

The Hazda microbiome somewhat resembled the gut composition of a Western diet during the dry season, when the hunter-gatherers eat the most meat. Some of the bacterial species that were prevalent during the wet season did disappear – showing similarities of a Western diet microbiome – but when the dry season ended, the missing microbes returned. This suggests that the evolution of the microbiome of industrialized nations can be reversed with a shift in diet.

But which specific foods or nutrients are altering the microbiome and bringing missing bacteria back is still a mystery.  The study authors hypothesize that a boost in fiber is needed, or cutting down on fat. A major stable for the Hazda are tubers and fruit from baobab trees, which provide about 100 to 150 grams of fiber per day. As a comparison, Americans typically consume about 15 grams of fiber per day.

The Western diet, which is considered to be in low in fiber and high in refined sugars compared to other cultures, is wiping out species of bacteria from our intestines, Sonnenburg believes.

Another find that wasn’t too surprising, but still interesting – U.S. populations exhibited substantially more antibiotic-resistant genes that the microbiota of the Hazda, according to the study.

But the most impactful result of the study was that the same microbial species that fluctuate the most throughout the year among the Hazda is completely missing in industrialized populations’ guts.