Introduction

The gut and brain axis is defined as a complex bidirectional network of communication pathways between the gastrointestinal (GI) tract and the central nervous system (CNS). This helps regulate bodily functions within neural, endocrine, immune, and microbial pathways that can have a knock-on effect on our emotions and cognition.  

Research has suggested that this pathway acts as a powerful mediator in mental health. However, if individuals experience stress or a decline in their mental health this can cause an imbalance in these bacteria, resulting in a ‘leaky gut’. Evidence has shown that such disruptions have been linked to psychiatric disorders, including depression and anxiety. Therefore in understanding this connection can provide insights into the therapeutic implications to support healthy microbiota bacteria and gut functioning, needed to help maintain the brain’s circuitry and overall mental wellbeing.¹

Biological Foundations of the Gut-Brain Axis

Within the gut-brain axis, there are several key components that work together in maintaining its connectivity, which include the following.²

Key Components

• Central Nervous System (CNS): The CNS is the body's main control system including the brain and spinal cord; working to process emotional and cognitive responses in the body. It plays a vital role in receiving information and monitoring this to process and store to the rest of the body. This is conducted via the peripheral nervous system that sends commands to the rest of the body
• Enteric Nervous System (ENS): Also known as the body’s "second brain" located within the gut, operates independently but communicates with the CNS. This system consists of various neurons within the gut wall helping to facilitate GI functions such as nutrient absorption and digestion
• Vagus Nerve: is a crucial nerve that connects the brain to the gut, transmitting signals in both directions. This helps facilitate neural pathways receiving sensory information about the current state of the gut and the brain, which are transmitted through this two-way communicative pathway, to promote healthy functioning of the systems
• Gut Microbiota: refers to the trillions of microorganisms that live in the gut that influence brain function through chemical signaling; such as brain chemistry and immune responses. This effect on immune pathways can lead to neuroinflammation, affecting mental health and well-being

Role of Gut Microbiota in Mental Health

Composition and Function of Gut Microbiota

It is well established that gut microbiota aids the functioning of mental well-being. Gut microbiota acts as a diverse microbial ecosystem that aids digestion and regulates immunity. Through the connections between the vagus nerve, it has been shown that having optimal levels of the microbiota bacteria living within our gut, can help regulate our mood through healthy production of neurotransmitters. This then allows chemical messengers to work together to transmit signals between neurons in regulating brain chemistry. 

However, if unhealthy bacteria enter the bloodstream, this can activate chronic inflammation. This can impact the functioning of connective pathways between the gut and brain and cause poor neurotransmitter production, ultimately triggering a mental health decline. Therefore, factors such as diet, stress, and infections are all implicated in influencing the composition of gut microbiota and mental wellbeing.³

Microbiota Influences on Neurotransmitters

There are several key neurotransmitters implicated in the gut and brain relationship. Neurotransmitters are chemical messengers within the body that allow neurons to communicate with each other. Doing so serves important functions such as mood regulation and supports brain activity in cognition. In particular, this influences tryptophan metabolism, a central pillar for neurotransmitter synthesis.

One key neurotransmitter is serotonin, which the gut makes up for over 90% and is influenced by microbial activity by enteroendocrine cells. Other transmitters such as GABA, dopamine, and norepinephrine are all further influenced by certain bacterial strains, which can produce or stimulate the production of these mood-regulating chemicals. Such neurotransmitters are also implicated in certain mental health and psychiatric conditions.⁴

Psychological Disorders

With the association between neurotransmitter production and modulation, this can influence the development of certain mental health and psychiatric conditions such as the following.⁵

• Depression and anxiety:  Research has shown that poor diversity in gut microbiota can impact short-chain fatty acids (SCFA) production, impacting neuroinflammation in the body. This causes changes in microbial composition and diversity have been associated with altered stress fight and flight responses leading to higher risks of depressive and anxiety symptoms^6,7
• Autism Spectrum Disorder (ASD): Studies have found a link between gut microbial imbalances and behavioural symptoms, presented in ASD. This highlights individuals with ASD often have a different gut microbiota composition and diversity profiles, compared to counterparts⁸
• Schizophrenia: Similarly, altered microbiota has been identified in individuals with schizophrenia compared to healthy controls. It has been shown both elevated and lowered levels of various microbiota strains such as proteobacteria and fusobacteria significantly more enhanced whereas firmicutes is reduced⁹

Key Research Findings

Human Studies

Research studies have used protocols implementing the use of probiotics, showing mild to moderate reductions in anxiety and depressive symptoms. A meta-analysis assessed 10 pre-clinical studies in humans, examining the effects of probiotics vs a control placebo group. A significant improvement was found in mild to moderately depressed patients who received probiotics compared to the placebo group, that were more tolerable compared to antidepressants of SSRIs. This highlights evidence to support the theoretical foundations of the gut and brain axis in mental health conditions such as depression.¹⁰

Similarly, another study recruited 40 patients with depression who were randomly allocated to either a probiotic group or a control placebo group over 8 weeks. Upon analysing symptom scores, patients who were in the probiotic group had a significantly reduced post-total symptom score compared to controls. Thus, providing additional evidence for using probiotics and improving gut diversity, acting as important therapeutic targets.¹¹

Further research has used faecal microbiota transplant (FMT) to examine the influence of gut microbiota. In a systematic review, an association was found between a decrease in depressive and anxiety-like symptoms and behaviours following the transplantation of healthy microbiota. This emerging evidence suggests FMT from healthy individuals may improve mood and behaviour in recipients.¹²

Animal Research

Similar results have been demonstrated using animal methodology paradigms. Utilising samples of germ-free mice (born without microbiota) shows that manipulating gut microbes can alter their stress response and depressive symptoms, which were reversible upon microbiota introduction and developing more diversity in their gut health ¹³. 

Limitations of Research

While research findings provide useful therapeutic targets and promising results, they should be taken with caution.¹⁴

• Correlational data: Most studies have been correlational; however, this does not imply causation between microbiota gut functioning and mental well-being, considering confounding wider factors that may impact findings. Such as isolating specific microbiome strains is challenging to know if assessing true association
• Need for further investigation: Methodologies with a longitudinal and larger sample size remain limited to establish long-term effects in a standardised manner. Therefore requiring a more diverse population group for more robust conclusions
• Animal samples: Likewise, many studies have initially been explored by altering the microbiota bacteria in mice, which led to decreases in depressive-like behaviours and inflammation, warranting further investigation into whether improvements transfer to human clients over a longer time

Therapeutic Implications

Probiotics and psychobiotics

Probiotics have been offered as a therapeutic, non-invasive intervention due to their anti-inflammatory effects, which deliver a dose of live microbiota bacteria to rebalance gut functioning. Specific strains such as Lactobacillus rhamnosus and Bifidobacterium longum show promise in modulating mood and reducing anxiety. In addition, findings have highlighted the use of psychobiotics, known as a specific type of probiotic proposed for its ability to influence mental well-being. They are thought to be effective in synthesising and transporting neurotransmitters, which are crucial for brain function.¹⁵

Dietary Interventions

There are different forms of probiotic and prebiotic foods that deliver a dose of diverse microbiota bacteria, containing anti-inflammatory properties. Incorporating a Mediterranean and high-fibre diet has been positively associated with microbiota diversity and brain function. For example, fermented foods such as yoghurt, kefir, bananas, oats and whole grains, contain live cultures that may support mental well-being.¹⁶

Personalised Medicine

With ongoing research incorporating new technology into its infrastructures, there are growing studies observing the use of gut microbiome profiling to customise interventions based on individual microbial composition. Using this profiling technique offers an individualised pathway with the potential for tailored psychobiotic therapies and engineered individual probiotics for specific mental disorders.¹⁷

Conclusion

Overall the gut and brain axis plays a significant role in regulating overall health and wellbeing, demonstrating important implications in supporting mental health through multiple interconnected pathways. Understanding these implications highlights key holistic mental health treatment options, opening doors to non-invasive, natural, and complementary therapies for mental health conditions. With this approach, it can provide useful collaboration across neuroscience, gastroenterology, nutrition and psychology to optimise mental health care with an integrative approach.