The Gut, the Brain and Mood

There have been multiple articles this week looking at the relationship of our microbes to mood.  I thought this research out of Oxford University[i] was particularly interesting in that, it looked at that relationship from an evolutionary point-of-view.

It’s accepted as fact now that our bacterial microbiome has tremendous effect on directing our social behavior, anxiety, stress and depression.  One of the researchers involved in this latest work asks the following:

“We know there are numerous possible mechanisms, including communication via the vagus nerve (major nerve linking the gut and brain), the immune system and hormonal changes, as well as the production of neuroactive chemicals by gut microbes. But why should we expect gut bacteria to affect behaviour at all?”[ii]

These scientists believe that it is unlikely that bacteria evolved to manipulate our brains by making neurotransmitters, as this “extra energetic cost” would make them weak and vulnerable to being out-competed by other species.  Instead, they suggest that it is our bodies which learned to live with a wide variety of flora, and we came to depend on their presence and their secretions, like short chain fatty acids.  These by-products, and the interaction of our immune systems with the microbes, directed natural selection of humans.  In other words, the strongest people adapted to the natural presence of their inevitable microbes, making our “invaders” into friends.  It is the alteration and depletion of this friendly biome that have effects on behavior.

“”Rather than viewing our microbial companions as puppeteers manipulating our behaviour, instead we suggest that the behavioural effects of gut microbes are more likely a result of natural selection on microbes to grow and compete in the gut, and natural selection on hosts to depend on their microbes. Microbial growth gives rise to metabolic by-products such as short-chain fatty acids known to affect brain function and microbial metabolites can also interact with our immune response. In addition, our physiology may have adapted to make use of our associated microbes.”

We evolved – not them.

I guess this suggests that as we further alter and/or deplete our biomes, we humans will also change to adapt.  Considering that the rates of mental illnesses like depression and anxiety disorders are skyrocketing I’m not sure that our evolution is moving ahead fast enough!


[i] Katerina V.-A. Johnson et al. Why does the microbiome affect behaviour?, Nature Reviews Microbiology (2018). DOI: 10.1038/s41579-018-0014-3


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