Table of Contents
Introduction
Anxiety disorders are among the most prevalent mental health conditions globally, with rising rates placing increasing pressure on already stretched primary care services.[1]
From 1990 to 2021, the number of young people worldwide aged 10-24 diagnosed with anxiety disorders rose by 52%.[2]
While pharmacological treatments and cognitive behavioural therapy remain first-line interventions, these approaches often have limitations - ranging from side effects and long waiting lists to partial or non-response in many patients.
This has prompted interest in novel, holistic strategies that address underlying contributors to mental health.
Emerging evidence suggests the gut microbiome, a diverse community of trillions of microorganisms residing in the human gastrointestinal tract, plays a key role in regulating mood and behaviour via the gut-brain axis.[3] This bidirectional communication system may hold the key to low-risk, accessible interventions for anxiety.
This essay explores how modulation of the gut microbiota via diet and lifestyle may offer a promising adjunctive approach for managing anxiety in primary care.
The Gut-Brain axis
The gut-brain axis is a complex, bidirectional communication network linking the gut microbiota, the central nervous system (CNS), and the enteric nervous system (ENS) within the gastrointestinal tract. This axis allows the brain and gut to influence each other’s function, playing a crucial role in maintaining mental and physical health.[4]
Several mechanisms mediate this interaction. The vagus nerve serves as a major communication highway, transmitting signals between the gut and brain.
The gut microbiota also modulates the immune system, influencing inflammation, which impacts brain function. Microbial metabolism produces short-chain fatty acids (SCFAs) such as butyrate, propionate, and acetate, which can affect neuronal signalling and maintain gut barrier integrity.
Additionally, gut bacteria contribute to the production and regulation of key neurotransmitters, including serotonin and gamma-aminobutyric acid (GABA), both essential for mood regulation.
Two dominant bacterial phyla in the gut are Firmicutes and Bacteroidetes. An imbalance in these populations, known as dysbiosis, has been linked not only to mood disorders like anxiety, but also to function gastrointestinal conditions, most notably irritable bowel syndrome.[3],[4]
Evidence linking gut health and anxiety
A growing body of research supports the link between gut microbiota and mental health, notably anxiety and depression.
Patients with mental health disorders often show altered gut microbial compositions compared to healthy individuals. [4]
For example, Mason et al. (2020) found that individuals with depression and anhedonia had significantly reduced levels of anti-inflammatory gut bacteria, suggesting that microbial imbalance may contribute to poor mental health outcomes.[5]
These findings support the idea that disruptions in the gut microbiome could play a role in the pathophysiology of anxiety, as well as depression.
In animal models, David et al. (2019) demonstrated that mice fed a high-sugar, high-fat diet or given antibiotics experienced drastic shifts in gut microbiota, including a decline in Bacteroides and an increase in Clostridium species, alongside anxiety-like behaviours.[6]
Another study using germ-free mice colonized with human gut microbiota found that switching from a low-fat, plant-rich diet to a high-fat, high-sugar Western diet altered microbiota composition and gene expression within just one day, highlighting the microbiome’s rapid responsiveness to dietary changes.[7]
Certain pathogenic bacteria such as Campylobacter jejuni may trigger anxiety symptoms via activation of the c-FOS proto-oncogene, while beneficial strains like Lactobacillus and Bifidobacterium have been shown to alleviate symptoms by modulating neurotransmitter pathways and inflammation.[8]
Particular strains of bacteria, such as Lactobacillus helveticus R0052 and Bifidobacterium longum R0175, have shown efficacy in reducing anxiety and depression symptoms after short-term supplementation.[9]
However, not all findings are consistent, and the strains used are critical.
For example, Pinto-Sanchez et al. found that Bifidobacterium longum improved depression and quality of life in IBS patients but did not significantly affect anxiety, highlighting that probiotics do not always consistently alleviate anxiety symptoms.[10]
Conversly, Miyaoka et al. found that supplementing with Clostridium butyricum MIYAIRI 588 led to over a 50% reduction in anxiety and depression scores in individuals with treatment-resistant depression after eight weeks.[11]
Despite promising results, this field is still in its infancy. Most studies are small, short-term, and vary in probiotic strains and dosages, making comparisons difficult.
Larger, well-controlled trials are needed to determine the specific mechanisms, effectiveness, and long-term safety of probiotics as adjunctive treatments for mental health conditions.
Implications for primary care
Growing evidence of the gut-brain axis has important implications for managing anxiety in primary care. Recognising the gut microbiota’s role in mental health enables low-risk, complementary strategies alongside conventional treatment.
Preventively, GPs can promote dietary changes that support a healthy microbiome. Diets rich in prebiotic fibre, vegetables, and fermented foods (like kefir or sauerkraut) enhance microbial diversity and reduce inflammation, while cutting back on ultra-processed foods may help stabilise gut-brain signalling.[12]
For symptomatic patients, brief nutrition-focused interventions could be integrated into consultations, especially for those with coexisting gastrointestinal symptoms.
Probiotics or prebiotics may offer further support- particularly strains like Lactobacillus and Bifidobacterium, which show promise in reducing anxiety.
However, due to variable product quality and limited large-scale trials, such supplements should be seen as adjunctive rather than primary treatments. Educating patients about the gut-brain link can empower lifestyle changes that benefit both mental and physical health.
Still, barriers remain. Appointment time constraints limit diet discussions, most clinicians receive minimal nutrition training, and few guidelines exist on microbiota-targeted approaches for anxiety. More RCTs are needed to clarify which interventions are most effective.
Ultimately, addressing anxiety via the gut-brain axis requires a multidisciplinary approach. Collaboration between GPs, dietitians, and mental health professionals can ensure holistic, personalised care that targets both psychological symptoms and biological contributors.
Conclusion
The gut-brain axis offers a promising perspective for managing anxiety in primary care.
Growing evidence suggests that gut microbiota influences mood, with diet and lifestyle playing key roles. Though research is ongoing, interventions like dietary changes, probiotics, and patient education are low-risk and may enhance current treatments.
To harness this potential, more robust studies are needed. Integrating nutritional psychiatry into medical training and practice could help GPs deliver holistic mental health care addressing both mind and microbiome.
References
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2.Bie, F., Yan, X., Xing, J., Wang, L., Xu, Y., Wang, G., Wang, Q., Guo, J., Qiao, J. and Rao, Z. (2024). Rising global burden of anxiety disorders among adolescents and young adults: trends, risk factors, and the impact of socioeconomic disparities and COVID-19 from 1990 to 2021. Frontiers in Psychiatry, [online] 15. doi:https://doi.org/10.3389/fpsyt.2024.1489427.
3.Clapp, M., Aurora, N., Herrera, L., Bhatia, M., Wilen, E. and Wakefield, S. (2017). Gut microbiota’s effect on mental health: The gut-brain axis. Clinics and Practice, [online] 7(4). Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5641835/.
4.Carabotti, M., Scirocco, A., Maselli, M.A. and Severi, C. (2015). The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems. Annals of Gastroenterology : Quarterly Publication of the Hellenic Society of Gastroenterology, [online] 28(2), p.203. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC4367209/.
5.Mason, B.L., Li, Q., Minhajuddin, A., Czysz, A.H., Coughlin, L.A., Hussain, S.K., Koh, A.Y. and Trivedi, M.H. (2020). Reduced anti-inflammatory gut microbiota are associated with depression and anhedonia. Journal of Affective Disorders, 266, pp.394–401. doi:https://doi.org/10.1016/j.jad.2020.01.137.
6.David, L.A., Maurice, C.F., Carmody, R.N., Gootenberg, D.B., Button, J.E., Wolfe, B.E., Ling, A.V., Devlin, A.S., Varma, Y., Fischbach, M.A., Biddinger, S.B., Dutton, R.J. and Turnbaugh, P.J. (2013). Diet rapidly and reproducibly alters the human gut microbiome. Nature, [online] 505(7484), pp.559–563. doi:https://doi.org/10.1038/nature12820.
7.Turnbaugh, P.J., Ridaura, V.K., Faith, J.J., Rey, F.E., Knight, R. and Gordon, J.I. (2009). The Effect of Diet on the Human Gut Microbiome: A Metagenomic Analysis in Humanized Gnotobiotic Mice. Science Translational Medicine, [online] 1(6), pp.6ra14–6ra14. doi:https://doi.org/10.1126/scitranslmed.3000322.
8.Goehler, L.E., Gaykema, R.P.A., Opitz, N., Reddaway, R., Badr, N. and Lyte, M. (2005). Activation in vagal afferents and central autonomic pathways: Early responses to intestinal infection with Campylobacter jejuni. Brain, Behavior, and Immunity, 19(4), pp.334–344. doi:https://doi.org/10.1016/j.bbi.2004.09.002.
9.Messaoudi, M., Violle, N., Bisson, J.-F., Desor, D., Javelot, H. and Rougeot, C. (2011). Beneficial psychological effects of a probiotic formulation (Lactobacillus helveticusR0052 andBifidobacterium longumR0175) in healthy human volunteers. Gut Microbes, 2(4), pp.256–261. doi:https://doi.org/10.4161/gmic.2.4.16108.
10.Pinto-Sanchez, M.I., Hall, G.B., Ghajar, K., Nardelli, A., Bolino, C., Lau, J.T., Martin, F.-P., Cominetti, O., Welsh, C., Rieder, A., Traynor, J., Gregory, C., De Palma, G., Pigrau, M., Ford, A.C., Macri, J., Berger, B., Bergonzelli, G., Surette, M.G. and Collins, S.M. (2017). Probiotic Bifidobacterium longum NCC3001 Reduces Depression Scores and Alters Brain Activity: A Pilot Study in Patients With Irritable Bowel Syndrome. Gastroenterology, 153(2), pp.448-459.e8. doi:https://doi.org/10.1053/j.gastro.2017.05.003.
11. Miyaoka, T., Kanayama, M., Wake, R., Hashioka, S., Hayashida, M., Nagahama, M., Okazaki, S., Yamashita, S., Miura, S., Miki, H., Matsuda, H., Koike, M., Izuhara, M., Araki, T., Tsuchie, K., Azis, I.A., Arauchi, R., Abdullah, R.A., Oh-Nishi, A. and Horiguchi, J. (2018). Clostridium butyricum MIYAIRI 588 as Adjunctive Therapy for Treatment-Resistant Major Depressive Disorder: A Prospective Open-Label Trial. Clinical Neuropharmacology, 41(5), pp.151–155. doi:https://doi.org/10.1097/wnf.0000000000000299.
12. Zhou, P., Chen, C., Patil, S. and Dong, S. (2024). Unveiling the therapeutic symphony of probiotics, prebiotics, and postbiotics in gut-immune harmony. Frontiers in Nutrition, [online] 11. doi:https://doi.org/10.3389/fnut.2024.1355542
Author: Jack Topliss lives in the UK and is a fourth-year medical student at the University of Sheffield.
