Diarrhea getting you down? Tummy pain cramping your social life? Turns out, the influence of the gut microbiome on mental health could be more than symptomatic.

For sufferers of digestive disorders, such as irritable bowel syndrome (IBS), it makes sense that ongoing digestive symptoms may impact mood by reducing quality of life. However, researchers and clinicians have long noted that many people with mental health or neurological conditions also report co-occurring digestive issues^1-4.

Interestingly, research has linked changes in the gut microbiome to conditions such as IBS⁵, depression and anxiety^{6, 7}, and even disorders such as autism, Alzheimer’s and schizophrenia^{8, 9}.  So, it seems our gut bacteria may be playing a role in these conditions^9-11.

‘But how on earth can our gut bacteria affect our mental health?’

The nature of this interaction between our gut and brain, also known as the ‘microbiome-gut-brain axis’, is not fully understood and is still an active area of research.

Scientists have known for a long while that the gut and the brain communicate with each other, but it has only been recently that scientists discovered the microorganisms in our gut also influence this communication. The back and forth exchange between the gut and brain was highlighted in a 12-year population-based study, investigating the relationship between mood and digestive disorders¹².

The researchers found that participants with anxiety and/or depression at the beginning of the study were significantly more likely to have digestive disorders by the end of the study¹².

This relationship was also found in the reverse direction, with digestive disorders at baseline being a significant predictor of anxiety and/or depression at the study’s conclusion¹².

It is also clear that stress impacts on the gut microbiome^13,14

When we are stressed or anxious, our body activates our stress response system, producing hormones such as cortisol¹³. Cortisol can impact gut function, increase gut permeability and change gut microbiota composition¹⁵. Therefore, stress management techniques such as exercise or meditation, may play an important part in maintaining gut health.

A major finding putting gut bacteria in the mental health spotlight was the discovery that, like our brains, some gut microbes stimulate and produce neurotransmitters, also referred to as ‘chemical messengers’, which transmit signals between neurons in the brain^2,3 . Two neurotransmitters of particular interest are serotonin and gamma-aminobutyric acid (GABA)^2,3 which have both been implicated in mood disorders¹⁴.

Our gut cells produce up to 90% of our total serotonin¹⁶ and consequently changes to the gut microbiome have been associated with issues in the serotonin system¹⁴. One pathway to stimulate serotonin production by our gut cells is through bacterial by-products, known as metabolites. These metabolites include short chain fatty acids (SCFAs)^2,17,18, which are produced when our gut bacteria feed on fibre.

Unlike serotonin, the majority of GABA production occurs within the brain and central nervous system, with smaller amounts being produced and consumed by gut bacteria¹⁹. Lactobacillus and Bifidobacteria are examples of some of the gut microbes which have been shown to produce GABA in a laboratory setting²⁰. While this bacterially produced GABA has been shown to improve anxiety-like behaviour in animal studies²¹, it is still not clear if the same connection exists in humans. That is, it is unknown if our brains can use GABA produced by bacteria, or if it is only used within the microbial community of the gut.

The connection is complex

As our understanding of the connection between our gut bacteria and brain remains incredibly intricate and continues to evolve, it likely includes multiple factors and pathways.

What is clear is that diet is fundamental to determining the species that make up our gut microbiome and the metabolites those species produce (including SCFAs, serotonin and GABA)^{2, 22-24}. This makes sense as our gut bacteria eat what we eat.

Therefore, microbial manipulation through diet could provide promising treatment options for those with mental health conditions.

In fact, there is evidence that plant-based diets and Mediterranean diets could be beneficial for mental health issues^{23, 25, 26}. These diets are naturally rich in fibre and low in fat, patterns of eating that have been associated with increased microbial diversity and anti-inflammatory, SCFA-producing bacteria²⁷.

The potential of your gut microbiome to produce health promoting metabolites can be measured with metagenomic sequencing. Currently, Microba is the only Australian based company to provide this technology to the public. The results of your personalised test can be used to adapt your diet to improve your overall health which may include benefits to your mental health.

About the Author

Leanne Mitchell

Leanne is an Accredited Practising Dietitian and works as one of the microbiome coaches at Microba. Leanne has a developing interest in the clinical application of the gut-brain axis in gastrointestinal disorders, mental health and neuro-developmental conditions.