Seasonal Eating and the Microbiome

For millennia, humans have enjoyed a symbiotic relationship?with our gut-dwelling microbes. In exchange for the food and shelter of living in our bodies, our microbiota evolved to help keep us healthy. Our friendly bacteria help us digest food, protect our intestines from invading pathogens, and send important signals to the rest of the body.

Just as the planet benefits from large-scale biodiversity, a more diverse gut microbiome can influence our health. Unfortunately, a Westernized diet is associated with a loss of microbiome diversity. This loss of diversity is likely due to differences in how we eat. One of the main differences is that the modern Western diet tends to be low in dietary fiber. This means we’re missing out on many of the prebiotics that promoted bacterial growth in the guts of our great-great-grandparents. Research suggests that these microbiome changes may contribute to soaring rates of obesity in the United States.

The standard Western diet has another key feature: a lack of seasonality. From bread and bacon to lettuce and tomatoes, most of our modern staple foods can be found year-round at the supermarket. People who farm, hunt, or gather most of their food themselves have diets that vary widely depending on what is available at a given time of year.

The Western diet’s shift away from seasonal staple foods may have led to a loss of seasonality in our microbiomes, too.


The diets of our ancestors

We can’t know exactly what microbes our ancestors carried in their guts. Some groups, however, still eat seasonal or hunter-gatherer diets, similar to our ancestors. Recent studies have shown that this type of eating may support a more diverse microbiome that changes with the seasons.

A 2014 study looked at the gut bacteria of the Hutterites, a group of people who live in farming communities across the northeastern US and Canada. Hutterites grow all of their own food, and the researchers noted that study participants showed “significant population-wide shifts in microbiome composition across seasons.” For example, the Hutterites consume a lot of fresh produce during the summer. The researchers found that this high-fiber, high-carbohydrate summer diet was associated with higher levels of carbohydrate-digesting Bacteroidetes. At the same time, study participants had decreased levels of Actinobacteria, which is associated with high-fat diets. One finding took the researchers by surprise: despite the Hutterites’ summer diet having more variety and more produce, overall microbial diversity decreased in summer.

In 2017, a study reported similar seasonal microbiome changes among the Hadza, a hunter-gatherer group living in Tanzania. The Hadza diet undergoes an even more drastic seasonal variation than the Hutterite diet. The Hadza have one year-round staple food, which is a tuber rich in dietary fiber. During the dry season when diverse food sources are not available, the Hadza diet is restricted to meat and tubers. Tubers make up a high proportion of their dry-season diet. During the wet season, the Hadza primarily eat baobab fruits, berries, and honey — high-fiber tubers make up less of their diet.

The differences in the Hazdas’ gut microbe profiles during wet and dry seasons are striking. The changes also appear to serve a digestive purpose. During the wet season, when fiber intake is lower and most calories come from honey, their microbiome diversity crashes. Despite diversity being lower, their microbiomes become enriched with microbial enzymes that digest the plant sugar fructan. The microbes that are there are working overtime to maximize energy out of all of those berries and baobabs.


Is seasonal eating good for our microbiomes?

The researchers found that these dramatic changes in microbial diversity aren’t permanent. The Hutterites’ microbial balance shifts seasonally but is stable over the course of the year. When the Hadza change their diet in the dry season, the bacterial species whose numbers sank during the wet season quickly bounce back.

If modern Western microbiomes are missing certain flora, however, that biodiversity could be hard to replace. A 2016 study found that mice put on a low-fiber diet for just three or four generations could not recover lost microbial diversity when switched to a high-fiber diet. Simply eating seasonally probably won’t be enough to make your gut more like a hunter-gatherer’s.

Researchers don’t yet know if seasonal cycling of the microbiome — or the lack thereof — has a significant impact on human health. That said, if shopping for produce in season encourages you to eat more servings of fruits and vegetables, that’s a definite win.