Within our guts are billions of bacteria (referred to as the microbiome) that stimulate digestion, encourage absorption of nutrients, and discourage ‘bad’ bacteria from causing infections. The composition of the gut microbiome is influenced by multiple factors, including diet and the environment. For this reason, it is difficult to conduct controlled studies to determine how any single factor influences a human microbiome. In an effort to determine how diet influences the microbiome, Davenport et. al. studied the microbiomes of the small Hutterite community (North America) in summer and winter, when their diets vary considerably.
The Hutterite community is a ‘living laboratory’, geographically and genetically isolated from the outside world. Hutterites eat communal meals, and the variation in their food is mainly due to seasonal availability of fresh produce. This figure shows the differences in food intake during the summer and winter months for the community.
Stool samples were taken from 60 members of the community during the winter and summer for over one year. They found that most of the population had similar microbiomes, but diversity of gut bacteria changed from winter to summer. In the summer, the most abundant gut bacteria were Bacteroidetes. These bacteria are able to degrade plant cell walls which helps maximize absorption of plant nutrients, both by the bacteria and by the human cells that line the gut. An increased intake of plant material during the summer months likely permits Bacteriodetes bacteria to flourish. Another group of bacteria, Actinobacteria, were found to dominate the gut microbiome in the winter months. Numbers of these bacteria increase upon consumption of fat, and decrease upon consumption of plant fiber. Accordingly, Actinobacteria dominate the gut microbiome during the winter months, when fat consumption is relatively high and plant fiber is scarce. Interestingly, there was an overall decrease in the diversity of the gut microbiome in the summer compared to the winter, even though the diet is more varied in the summer. The working hypothesis to explain this trend is that certain types of bacteria, like the Bacteroidetes, which can flourish during the summer months, outcompete the other bacteria when excess complex carbohydrates are available.
Changes in Hutterite gut microbiomes over the year were relatively subtle compared to previous studies of populations from around the world. Nevertheless, it is clear that diet effects the composition of the human gut microbiome. There are many factors that change from season to season that could have an effect of the microbiome in ways we do not yet understand, including sun exposure, temperature, or day length. As the microbiome plays such an intricate role in our health, it is important to understand how to influence the communities living inside of us for the better.
Summary written by: Serena Drouillard
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Seasonal Variation in Human Gut Microbiome Composition
Emily R. Davenport, Orna Mizrahi-Man, Katelyn Michelins, Luis B. Barreiro, Carole Ober, Yoav Gilad