Mental health awareness is important for public health. This article discusses the gut-brain axis and its association with mental health.
Mental health problems and what it means
Mental health covers a broad category of mental health problems, with some of the common illnesses being anxiety, stress and depression. The personal impacts of mental health problems can be debilitating, affecting everyday life including, work, relationships and physical health.
In addition to the personal cost, mental health problems are one of the main causes of disease burden worldwide, with major depression thought to be the second cause of disability globally (1). Mental health disorders often co-occur with other non-communicable diseases, emphasising the disease burden for the individual (2). One in four people will suffer from a mental or neurological condition at some point in their lives (3) making this an important public health issue.
Enabling the awareness and support of mental health is necessary for everyone, especially health care professionals to help with the management of mental health patients to help them to maintain their positive psychological functioning. This emphasises the importance to not only treat mental health disorders but to protect and promote the mental health of everyone (4). There’s no wonder that with the rising prevalence of diagnosed mental health disorders that causes are often discussed in the media.
An area of increasing interest is the role of the gut microbiota on mental health. The gut microbiota is the total collection of microorganisms that live in our gut that have an influence of many physiological processes in our body, including metabolism, immunity and mood. The interaction between the gut and the brain is referred to as the gut-brain axis. There is a bidirectional communication between the central nervous system (CNS) and the gut microbiome through the vagus nerve (5). Gut microbes also communicate through at least another two pathways – endocrine and immune signalling mechanisms. Not only is this involved in the hunger-satiety signalling but it has become more apparent that the gut-brain axis is important in stress-response (6), social behaviour and fear-expression. Brain disorders have been identified to have associations with dysbiosis in the gut microbiome, including Alzheimer’s disease, Parkinson’s Disease (PD), Major Depressive Disorder, Autism Spectrum Disorder (ASD) (7) and Schizophrenia (5). A particular strain of bacteria, Bacillus subtilis has recently been associated with protecting against the build-up of a protein, alpha synuclein, which form toxic clumps in PD (8).
Image 1. Gut-brain axis via the vagus nerve.
An absence of some bacteria has been shown across cohorts where patients with depression had lower levels of Coprococcus and Dialister (9). The increase of negative bacteria such as Enterobacteriaceae has been reported in depression, this is associated with the inflammatory environment associated with depression causing a decrease in some beneficial bacteria and rise in bacteria that has negative effects (10). Furthermore, Veillonellaceae and Lachnospiraceae have been correlated with schizophrenia severity, along with the presence of a panel of specific microbes enabling researchers to differentiate individuals with schizophrenia from healthy subjects (11).
Can diet influence mental health?
A higher adherence to a Mediterranean Diet and lower adherence to a pro-inflammatory diet is associated with a lower risk of depression. This could be due to diet-induced damage to the brain linked with inflammation and oxidative stress and the link to depression. A Mediterranean diet is high in fibre, omega-3s and antioxidants (12). A diet rich in vegetables and other sources of prebiotic fibre will increase good bacteria in the gut microbiome which in turn may influence mental wellbeing via the gut-brain axis.
The use of commercial prebiotic supplementation has been reported to alleviate stress caused by stressors that act on the hypothalamus-pituitary-axis (HPA). Prebiotic supplementation of Bimuno® Galactooligosaccharides (GOS) intake is associated with decreased waking-cortisol activity and altered attentional bias compared to placebo. This indicates that prebiotics interact with HPA activity. The research showed a key role for the gut microbiome in the regulation of stress (6). In a clinical trial using Bimuno, it was found to lower anxiety scores in irritable bowel syndrome (IBS) patients, improving their quality of life (13). In a cohort of children with ASD, behavioural improvements were seen when Bimuno was added to a restrictive diet (7).
There appears to be a place for the use of probiotics in dietary techniques to ameliorate depression or low mood states. In a double blind, randomised control trial, the use of Lactobacillus casei in a milk drink led to mood score improvements in an elderly cohort. Additionally, a Bifidobacterium longum strain has been shown to lower depression scores in a depressed cohort (14).
Although there is evidence for targeting the gut microbiome to alleviate stress and anxiety, it remains to be determined if the gut microbiome plays a causal role in their development. The gut-microbiome-brain communication needs further human research, as it has mostly been explored in animal models where the gut-microbiome dysbiosis does not just appear to be a secondary effect (15). However, a diet rich in fermentable fibre and low in processed foods, sugar and fat should be recommended to help the good bacteria such as Bifidobacterium and Lactobacillus proliferate. As the gut microbiome clearly plays an important role in cognitive function, a dietary intervention would be important in controlling environmental factors in mental health. Dietary interventions to target the gut microbiome are an area of interest for future research.
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2. Stein et al., 2019 https://www.bmj.com/content/364/bmj.l295.full
5. Long-Smith et al., 2020 https://www.annualreviews.org/doi/pdf/10.1146/annurev-pharmtox-010919-023628
6. Schmidt et al., 2015 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4410136/
7. Grimaldi et al., 2018 https://microbiomejournal.biomedcentral.com/articles/10.1186/s40168-018-0523-3
8. Goya et al., 2020 https://www.sciencedirect.com/science/article/pii/S2211124719317437
9. Valles-Colomer et al., 2019 https://www.gwern.net/docs/biology/2019-vallescolomer.pdf
10. Macedo et al., 2017 repositorio.ufc.br/bitstream/riufc/29673/1/2017_art_dmacedo.pdf
11. Zheng et al., 2019 https://advances.sciencemag.org/content/advances/archive/5/2/eaau8317/1.full.pdf
12. Lassale et al., 2019 https://www.nature.com/articles/s41380-018-0237-8
13. Silk et al., 2009 https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2036.2008.03911.x
14. Benton et al., 2007 https://www.nature.com/articles/1602546
15. Martin et al., 2018 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6047317/