Do you know that our gut microbiome actually changes with each season? How can natural temporal variation in diet can affect the microbiome?
We all know that our gut bacteria are determined by available foods, genetics and, of course, the environment we live in. But there is also a cyclic reconfiguration of this complex universe living within our digestive system. As our diet changes with each season, some taxa (microbes) become more abundant than others. Moreover, certain types of gut bacteria may be completely undetectable for a period only to reappear in a subsequent season, as if the gut has a memory. Such findings have only emerged in the last 10-15 years thanks to research on the gut ecosystem. By studying the gut microbiome of traditional tribes, in comparison to those modern western lifestyles, we start better understanding this complex gut ecosystem and its connection with health risks such as diabetes and obesity.
The cyclical dynamics of gut bacteria were uncovered in a fascinating study of the Hadza people of Tanzania – a traditional hunter-gatherer society.
This study identified that certain bacteria, associated with processing fat and lower fibre content, seem to increase in winter. In the summer, they found a rise in bacteria for digesting complex carbohydrates, which help to process copious amounts of plants. It does indicate that the gut ecosystem is not static, but shows significant patterned changes through the course of a year.
Well okay, you might say, the gut microbiome adapts to its environment, but the Hadza people’s diet is not very diverse; it’s essentially based on five main groups: berries, honey, baobab, tubers and meat. They live in a climate with two main seasons: wet and dry, so the availability of certain food groups in each season will determine bacterial diversity. So shouldn’t this traditional Hadza diet be associated with low microbiome diversity? This hypothesis is also very interesting, but contradicted by what we actually know about the diversity and composition of gut bacteria in traditional diet populations.
It’s been found that Hadza gut flora is quite diverse, estimated to have 30% more diverse bacteria than Western people, with diversity peaking during dry seasons with high meat intake.
There is some evidence suggesting that the diversity and composition of gut ecosystems are much higher when exposed to the traditional diet of hunter-gatherers than in city dwelling populations. In addition, there are also differences in specific bacteria. The Hadza microbiome seems to possess more genes encoding plant-degrading enzymes, while an American cohort seems to have more genes encoding enzymes for animal and mucin degradation (bacteria consuming our own gut mucus which protects intestinal epithelium).
On the other hand, a recent 2017 study on an Inuit population, living in the Canadian Arctic, found no differences in diversity between the Inuit and urbanised Montreal population. Isn't it interesting? For thousands of years, the Inuit’s diet has been low in carbohydrates but very rich in animal fats and proteins from land and marine mammals, so we would expect them to have a completely distinct microbiome. And as you know, the Inuit had a low incidence of cardiovascular disease. However, their dietary habits are currently changing, increasingly influenced by western diets. I even came across a massive debate triggered by harsh criticism of the Inuit diet by different vegan groups, and Inuit representatives trying to defend their traditional lifestyle. The world becomes a really strange place in these dietary spaces, but I understand the opposing ethical considerations.
Bottom line – according to this study, Inuit microbiome was broadly similar to Western microbiome, even after controlling for age, gender, BMI, ethnicity. In fact, Inuit population is currently transitioning to western-like diet, like many other indigenous tribes, although they still consume a high amount of raw sea mammals. Sadly, this transition is happening with an increase in the prevalence of obesity in the Inuit.
But how about the seasonality factor? Unfortunately, this study was conducted between July and August, when most Inuit consume a mix of traditional and market food. But another paper actually identified seasonal shifts in both diversity and composition of the Inuit microbiome over time as opposed to much more stable westernised microbiome.
The takeaway from Hadza and Inuit studies: the microbiome changes over time and it interacts with diet and human genetics, affecting health and disease. The Inuit, like other native populations, have their own health risks, many of which may be shaped by the microbiome. Yes, people practicing hunter-gatherer traditions are not exposed to the same amounts of antibiotics, domestic chemicals, and probably have different types of stress. All of which may explain a much more diverse gut microbial system, but we also know that diet has a huge impact on our gut health.
It’s absolutely amazing that our bodies (and our digestive system) are designed to naturally change and adapt with environment fluctuations. The human microbiome is a dynamic ecosystem, where some bacteria temporarily take the place of others to help us efficiently extract the maximum nutrients from changing diets. But in our current modern lifestyle with less pronounced seasonality of the diet (all year around available products from all over the world) perhaps we continue to lack in diversity because we focus on the wrong choices? Perhaps we should be more in sync with the local environment and consume more locally produced seasonal foods to encourage seasonal shifts in our gut microbiome? Maybe one day we will routinely discuss gut plasticity, as we do brain plasticity!