While it may initially seem absurd to think that the brain and microbiome of the gut could have an intimate connection ( I was skeptical coming in to the idea, to say the least), there are actually many different connections that allow this relationship to be possible, as the gut is considered to be the “second brain” and is wrought with neural connections.
Looking more towards the practical applications of this axis, the microbiome affects brain function by:
- Triggering the release of cytokines by affecting immune cells
- The direct production of corticosteroids and neurotransmitters
- Other small materials, and enzymatic products may affect the brain via nerves
In return, the brain can adjust the microbiota via the release of hormones and steroids capable of affecting it. For example, during times of stress, the environment of the stomach may become much more acidic, and this could ultimately result in a population shift along the microbiome towards more acid-tolerant microbes.
In terms of mental health, these brain affecting capabilities can actually have quite the effect. For example, the addition of Lactobacillus rhamnosus to the microbiome was shown to reduce the observable effects of depression in mice, but only when there was an appreciable nerve connection between the gut and brain. Research has also shown that the presence of a microbiota may have a hand in reducing the effects that stress exerts on a body.
With all of that said, turnabout is fair play, and mental health can be readily affected by the microbiome as well. Besides the simple case of having the brain and autonomic responses themselves adjusting microbiota, there are multiple examples of mental conditions which adjust behavior. Lesch-Nyhan syndrome is a particularly devastating example which compels an inflicted individual to eat his own fingers and lips. Conditions such as this could readily introduce new organisms into a microbiome, and could also upset the relatively well maintained environment found therein.
All of these effects come together in a roiling stew of situations and conditions that can dramatically affect human health as a whole, in both positive and negative ways.
