Much like the weather, some human stomachs change throughout the year. The gut microbes of the Hadza, a hunter-gatherer group in Tanzania, shift dramatically as their diet changes with the seasons, according to new research from Stanford University. When applied on a longer timescale, these trends could explain why industrialized populations have a less diverse set of gut microbes and more chronic disease relative to hunter-gatherer populations.
“For much of [human] evolution and migrations, it appears that we harbored certain types of bacteria commonly and then many of those bacteria have gone missing over the course of modernization of society,” said Stanford microbiologist Justin Sonnenburg, who led the study published Thursday in Science.
In an ideal world, Sonnenburg said he would have a time machine, so he could study how the human microbiome evolved to influence human health over eons. The gut microbiome is an army of small organisms, like bacteria, that occupy the lower intestine. Mainly feeding on dietary fiber, these tiny troops assist in food digestion, build vitamins, train our immune system and guard against pathogens.
Without a time machine, Sonnenburg’s next best option was ostensibly the Hadza, one of the world’s few remaining populations of hunter-gatherers, with about 200 still adhering to the lifestyle. Much of their traditional way of life, Sonnenburg said, “gives a window into humanity’s ancient past.”
During the dry season, Hadza consume more tubers, potato-like crops, and wild game like zebra or giraffe, while in the wet season, their diets favor berries and honey. Sonnenburg wondered if Hadza microbiomes, like their diets, would also flux with the seasons. So he partnered with Jeff Leach, founder of the Human Food Project, who had been living with and studying the Hadza for years.
The team collected 350 stool samples from 188 Hadza individuals over the course of a year. After analyzing the microbes in each sample, the researchers found that the gut microbes did vary seasonally, which aided the stomach’s ability to digest food.
Hadza had more enzymes for breaking down animal-made carbohydrates during the dry season, and more enzymes to target plant-based carbohydrates in the wet season.
“Given that the changes are so quick and happen every year, [the study] provides support to the claim that a [gut microbiome] change is dependant on the diet,” said University of Minnesota geneticist Ran Blekhman, who was not involved in the study. While previously documented in multiple animal studies, “this is the first study to document the seasonal effect in hunter gatherers.”
But the “jaw-dropping moment,” according to co-author Sam Smits, happened when the team found the biggest fluctuations in the Hadza involve the same bacterial species that are absent in the guts of people living in industrialized societies.
“These bugs are virtually undetectable in Western populations,” Smits said. Given the Hadza lost microbiome diversity over seasons as their dietary fiber decreased, the team argues the same may have happened in Western populations over longer timeframes.
“The Hadza eat 100-150 grams of dietary fiber per day. We Americans eat about 15 grams per day. So I think in the Western world what we are doing is starving our microbial cells,” Sonnenburg said.
But nutritional anthropologist Alyssa Crittenden doesn’t think plant consumption is the best explanation for these seasonal differences. She wished the study had collected more dietary data at the same time the samples were collected. Instead, the researchers relied on diet patterns previously documented by other studies.