The microbes living in our gut could vary with the seasons, according to evidence from one of the few remaining groups of hunter-gatherers.

 at the Human Food Project, a non-profit looking at the role of the microbiome in health, and his team spent more than a year collecting stool samples from 350 Hadza people, living in Tanzania. They found that the Hadza gut microbiome is about 30 per cent more diverse than what we find in the residents of Western nations. In fact, the Hadza’s gut flora is about as diverse as that of some Yanomami people in Venezuela, previously described as having the world’s most diverse microbiome.

The diversity in both groups isn’t that surprising, given that they hardly, if ever, consume antibiotics and processed food. But Leach’s team also discovered that the Hadza microbiome is seasonal, changing in a cycle throughout the year. Diversity peaks in the dry season, when Prevotella species become particularly abundant, and the bacteria that showed the greatest annual fluctuations generally tended to be strains not present in the gut of people with Western lifestyles.

These annual changes in the gut microbiome are probably caused by cyclical shifts in the Hadza diet. During Tanzania’s dry season, the Hadza eat a lot of meat plus tubers and fruit from the baobab tree, but in the wet season they eat more honey and berries. Prevotella species are particularly good at breaking down plant material, so may be particularly useful during the dry season.

Leach also thinks seasonal changes in the landscape may affect the microbiome. “It may sound a little hippy,” he says, “but it’s the environment, stupid!”

Human guinea pig

Researchers had observed seasonal gut microbiome shifts in , but the Hadza finding is the first detection of an an annual cycle of changes, says of Wageningen University in the Netherlands. He predicts that microbiome probably cycles in other groups with seasonal diets – including those of us who manage to eat seasonal produce.

Leach also adopted the Hadza lifestyle for several months to see if his intestinal microbiome would resemble theirs, and even gave himself a Hadza faecal transplant. The results remain unpublished, but Leach, now back in the US, says that some of the Hadza gut bacteria have survived in his body.

“Leach has been bold, being a guinea pig,” says his colleague at Stanford University in California.

It is thought that certain health problems may arise because our bodies may not be well adapted to the microbiomes associated with Western diets and lifestyles. However, many studies will have to be done to see whether transplanting bacteria as Leach has is a good idea, says Sonnenburg.