Recent findings from mouse studies have concluded that genetics may have a more important role than environmental factors in the composition of the microbiome (Lennon: 2019). Yet, although it is true that humans and mice share certain genetic loci, researchers warn that their findings may not be completely relevant to humans. Thus, are genetics really more significant in the make-up of the microbiome?
A study conducted in Israel analyzing blood and stool samples of 1,046 Israeli adults of mixed origins demonstrated that their ancestry had little impact on the composition of their microbiomes (Rothschild: 2018). Comparing host genetic profiles and the diversity between microbiome samples, they found that environmental factors instead played a larger role in the make-up of each person’s microbiome.
The same researchers also investigated the impact of the environment on the microbiome by looking at the microbial compositions of related individuals who did not live together alongside those of unrelated couples who did. They found that the microbiomes of relatives who did not live together had few similarities, whereas those of unrelated couples who lived together bore many (Daley: 2018).
These findings are consistent with other studies too. For example, a study on the microbiome composition of 1,126 pairs of twins in the UK found that heritable bacterial taxa are only temporarily stable (Goodrich: 2016). Analysing their data, they also found that only between 5.3%and 8.8% of gut taxa is inherited. According to Emily Davenport, one of the study’s coauthors: “Certain bacteria are heritable, but it’s a very small portion of the microbiome, and even if we do identify them as heritable, it’s very moderate. (Daley: 2018)”
But how much can environmental factors shape microbiota composition over generations? A 2016 study researched the reasons behind reduced biodiversity in gut microbiota in Western populations, leading to a higher risk for inflammatory diseases, compared to those with more traditional diets and lifestyles. They found that microbiota-accessible carbohydrates (MACs) found in dietary fibre play a fundamental role in producing a healthy microbiome, and that these are significantly reduced in Western diets, where they are substituted by more fats and simple carbohydrates (Sonnenburg: 2016).
They then conducted experiments on several generations of mice to understand how reversible these changes in the microbiota are. Feeding the mice a low-MAC diet, they found that if changes in the microbiota were only experienced over one generation, they are largely reversible by dietary changes alone. However, if a low-MAC diet is maintained over several generations, certain taxa are driven to low abundance, meaning that they are less efficiently transmitted between generations, and thus become extinct. The only way to regain more healthy microbiota in such cases, is via both the reintroduction of dietary MACs and the administration of certain taxa (ibid.).
To conclude, although genetics certainly seem to play some role in shaping the human microbiome, to insist that they are mostly responsible for its composition would be to discount other research that expresses otherwise. Thus, even if genetics provide a baseline for the microbiome, its operational composition is otherwise open for modification by diet and/ or administration of certain taxa.
Lennon, Annie: LabRoots
Rothschild, Daphna, Nature
Goodrich, Julia K., Science Direct
Daley, Jim: The Scientist
Sonnenburg, Erica D. et al. : Nature