You know that feeling you get – you might be really stressed or nervous about a job interview or final exam and you had “butterflies” in your tummy? This is actually our brain talking to our gut! New research has discovered that this communication also works in reverse, and the gut can influence the brain too. Gut microbiota (the thousands of microorganisms living in our intestines) can impact how people feel, having a significant role in anxiety, depression and other mood related disorders.
The gut-brain axis
Now that research is showing that there is a two-way signalling pathway, it has been referred to as the gut-brain axis. With gut microbiota prompting central nervous system (CNS) function, and CNS function controlling microbiota composition, this communication happens in both directions, each having an influence on the other. The interaction occurs through the autonomic and enteric nervous systems, and through endocrine, immune and neural pathways. This bidirectional communication was reported after finding a link between gut microbes and the hypothalamic-pituitary adrenal (HPA) axis, finding that depressive episodes have been connected to the impairment of the HPA axis. This link has also shown that stress impacts gut microbiota, and the bidirectional communication between the CNS and microbiota also impacts stress reactivity. During times of stress, our brains can influence the composition of the gut microbiota through the release of cortisol. Cortisol can modify gut permeability, barrier function and ultimately variants of microbiota composition. Additionally, gut microbiota can modify circulating cytokines along with pre- and probiotics, which can control numerous brain functions. Overall this bidirectional communication is having a notable role in the progression of mental disorders.
In a recent study from 2015, researchers have discussed the latest evidence that subconscious inputs from microbes can actually influence emotions and behaviours. These researchers have found that gut microbiota impacts the development of emotional behaviour, stress modulation and brain neurotransmitter systems.
Gut microbiota, depression and anxiety
In a review published late last year, researchers noted that major depressive disorder is disproportionately dominant in people with gastrointestinal illnesses such as inflammatory bowel disease, which has already been associated with gut microbiota function. They explain further that links found between major depressive disorder, immune response and inflammation have been found, specifically, that people with major depressive disorder have shown changes in proinflammatory and cell-mediated immune cytokines. Further, it has been shown that people who suffer from inflammatory bowel diseases, which are characterised by an altered microbiome, have poorer emotional function such as anxiety or depression. Again, this is linked to cytokines. studies suggest that altered microbiome influences the host behaviour, by releasing host immune factors like cytokines, which have neuronal targets within the central and enteric nervous systems respectively. With several studies noting a cytokine response particularly in response to irritable bowel syndrome or functional dyspepsia, though not linked to all gut disorders. Another train of thought is around the immune response and looking at inflammatory pathways, particularly around intestinal endothelium and the blood-brain barrier, this has stemmed from work in mice but further research is needed. Ultimately whilst we know now this bidirectional communication is occurring, an overarching mechanism to explain for all types of gut disorders remains elusive. With new research coming out, hopefully this answer is in the pipeline.
A 12 year prospective study found that participants who were anxious but not depressed in initial stages of the study later developed functional gastrointestinal disorders. Conversely, in the other direction, participants who experienced gut dysfunction first also were predicted to develop anxiety. This ultimately highlights this bidirectional communication, with influences in both pathways brain to gut and gut to brain. Last year another study looked at microbiota in major depressive disorders, looking at fecal samples from both patients with major depression and from healthy controls. It was shown that patients with depression could be identified based on their microbiota, as they had higher levels of bacteroidetes, and proteobacteria, than those without depression.
There are many influences on gut microbiota including environmental, infection, medications and diet. These things are often beyond our control, including diet, but with diet, but we can certainly help steer things in the right direction. So whilst we don’t know the full implications of our gut microbiota on our brain as yet, it certainly doesn’t mean we can’t ensure a wholesome balanced diet for overall health and wellbeing. As we mentioned in our recent article, many supplements promoting gut health are often just gimmicks with no firm evidence base. Everyone’s gut is made up of millions of different microorganisms and there is no one magic pill that is going to work for everyone. Why not follow the Australian guidelines to healthy eating? It is far more beneficial to reduce the highly processed foods, those high in added sugar, carbonated drinks and alike and switch to a more plant based whole food, high fibre diet, incorporating wholegrains, fruits and vegetables. The overall benefits go beyond that of our gut!
Whilst we aren’t there yet, it is hoped that this emerging area will deliver new perspectives around management of mental health. Gut microbiota it is a topic that is growing momentum, particularly around mental health as technology improves and emerging research is coming out in this vastly complex area. Whilst we still have a way to go, and we need more high quality research to continually be produced, it is certainly an exciting development and I for one will be interested to see where it leads into the future!