We've probably all experienced that feeling in our stomach when we're nervous or excited, right? Those butterflies demonstrate quite nicely how our brain and gut interact. Although when we need a nervous poop or have stomach cramps before public speaking, we might not be thinking it's so nice!
If you think about it this makes perfect evolutionary sense, when we see or smell food (or even just think about it) this readies our gut to begin digestion, i.e. by releasing more saliva and gastric juices. When we experience stress many of us lose our appetite, as our blood and attention is focused on our muscles incase our fight or flight response is triggered. So we've established that our brain and gut are connected... now what?
What is the gut-brain axis then?
Put (kind of) simply, the gut-brain connection or axis is the complex set of communications which go between our brain and our gut, in both directions. More specifically these connections go between our central nervous system (CNS), in the brain, and our enteric nervous system (ENS), in the gut, but it also involves our immune and hormonal systems as well as our autonomic nervous system which unconsciously controls many functions in our body such as digestion, breathing and heart rate. Okay, already this is quite a lot so I'll try and keep this as simple and 'need-to-know' as possible.
Conversations between our brain and gut happens in different ways via:
The vagus nerve - the long nerve that runs from our brain to our colon (and also into many other crucial organs)
The gut microbiome* - the ecosystem of microbes (bacteria, viruses, fungi, archea, protozoa and bacteriophages) which live predominantly in our large intestine
Neurotransmitters - chemical messengers which transmit signals between nerves and from nerves to muscles.
Mediators of the gut-brain axis allow the brain to influence intestinal function in terms of smooth muscle action and what the epithelial (gut lining) cells and immune cells are doing. But the gut microbiome has also been shown to influence gut cells.
Serotonin is largely produced in our gut. The gut microbiome can regulate this too. In the gut serotonin is involved in digestion and in the brain, it's involved in mood and sleep.
The gut microbiome can help to produce other substances, like the neurotransmitter GABA, which can influence mood or reduce levels of stress hormones; this is thought to be mediated by the vagus nerve.
What does this mean for our health?
There's some emerging research to suggest that the health of our gut (read gut microbiome and by this we think health means diverse, balanced array of microbes) can have an affects on symptoms of mental disorders like depression, but we're only just beginning to understand why this might be.
Much research looking at our gut and brain has been done in animal models of human disease, which helps to give us ideas for possible interactions; for example germ-free mice have difficulty with spatial awareness and memory tasks as well as exaggerated stress responses. However, this has to be shown in humans too to know for sure.
Research using faecal matter transplants (FMT) has shown some promising indications in helping to improve symptoms of depression and anxiety when taken from healthy donors. This is a technique where we take poop from a healthy donor and quite literally transplant this into the colon of the patient, to help make use of 'healthy' gut microbes which might repopulate the patient with symptoms. Now this is quite an extreme kind of treatment that wouldn't be for most of us and in many of the studies to date symptoms return some months after treatment has ended.
However, there is some indication that improving the diversity in our gut microbiota can help improve symptoms of mental health conditions like anxiety. For example, a new study looking at fermented foods and effects on stress and sleep found that eating these foods which contain live microbes and also providing food for them in the form of prebiotics could be helpful. In another recent study greater intake of fermented foods was associated with reduced social anxiety. However, this type of study doesn't demonstrate causality and relies on self-reporting of symptoms and food intake.
Although we don't think that foods which contain live cultures become established and permanent part of our gut microbiome they can clearly still have effects. This is why it's recommended to eat foods which contain live cultures regularly, and incorporate them as part of our daily diet when possible.
Some research suggests that although probiotics can help in some cases, dietary changes might be more helpful. This is often seen in health research where making changes to the diet to improve it has a better effect than taking supplements to correct a poor diet.
It's important to remember that there are lots of ways we can help improve both our gut and mental health. Diet plays a role but it's not the be all and end all. Things like stress management, moving our bodies more and creating better social connections can all have much bigger impacts on our mental wellbeing.
References
Berding, K., Bastiaanssen, T.F.S., Moloney, G.M. et al. Feed your microbes to deal with stress: a psychobiotic diet impacts microbial stability and perceived stress in a healthy adult population. Mol Psychiatry (2022). https://doi.org/10.1038/s41380-022-01817-y
Hilimire MR, DeVylder JE, Forestell CA. Fermented foods, neuroticism, and social anxiety: An interaction model. Psychiatry Res. 2015 Aug 15;228(2):203-8. doi: 10.1016/j.psychres.2015.04.023. Epub 2015 Apr 28. PMID: 25998000.
Yang B, Wei J, Ju P, et al. Effects of regulating intestinal microbiota on anxiety symptoms: A systematic review General Psychiatry 2019;32:e100056. doi: 10.1136/gpsych-2019-100056