In this lesson, we will focus on how the gut microbiome affects neurotransmitters and mental health.
We will discuss four key neurotransmitters:
✔ Serotonin which regulates mood and sleep.
✔ Dopamine, responsible for motivation and pleasure.
✔ GABA, which has a calming effect and helps manage stress.
✔ Glutamate, which influences memory and concentration.
In addition, we will examine four mental health disorders in which the role of the microbiome is particularly noteworthy: schizophrenia, depression, anxiety disorders, and autism.
Did you know that your gut and brain are connected and constantly communicating with each other?
This connection is called the gut-brain axis, and it operates in several ways: through nerves, the immune system, and chemical substances produced by gut bacteria. One of the most important components of this axis is the vagus nerve, which functions like an “information highway,” transmitting signals between the gut and the brain.
The gut and brain also influence the immune system. Bacteria in the gut help regulate immune function, which in turn can affect brain activity. Additionally, certain chemical compounds – such as short-chain fatty acids (SCFAs) and neurotransmitters – are produced by gut microbes and have a direct impact on mood and cognitive functions (Mhanna et al., 2024).
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How do gut microbiome imbalances affect mental health?
Research shows that disruptions in the composition of gut microbiome may contribute to the development of various mental disorders, such as depression, anxiety, autism, and schizophrenia. Animal studies have demonstrated that changes in the gut microbiome can influence behaviour and emotions. Scientists are now exploring whether improving the microbiome composition could help treat these conditions (Mhanna et al., 2024).
There is growing evidence that a healthy gut can support mental health. Probiotics, prebiotics, faecal microbiome transplantation (FMT), and appropriate dietary interventions are being tested as ways to enhance brain function. For example, studies by Kang et al. found that children with autism who received probiotic therapy showed improvements in both gut function and behaviour – and these effects lasted even two years after the treatment ended (Mhanna et al., 2024).
What is gut dysbiosis?
In a healthy body, the gut microbiome is in balance – a state known as eubiosis. In this condition, bacteria support digestion, protect against infections, and help regulate immune function. However, when this balance is disrupted, a condition called dysbiosis occurs, meaning an abnormal composition of gut microbiome. This can lead to a variety of health problems, including:
- Increased intestinal permeability (commonly referred to as “leaky gut”),
- Chronic inflammation,
- Damage to the blood–brain barrier, which allows harmful substances to reach the brain (Mhanna et al., 2024).
Researchers have found that gut dysbiosis may be linked to numerous diseases, such as diabetes, obesity, asthma, and disorders of the digestive, cardiovascular, and nervous systems. Moreover, increasing evidence shows that people suffering from depression, autism, anxiety, or schizophrenia often also experience gastrointestinal problems. However, it is still unclear exactly how microbiome changes contribute to the development of these conditions (Mhanna et al., 2024).
The gut microbiome remains a fascinating and actively studied field that may help us understand how to support mental health by taking care of the gut. Scientists are increasingly emphasizing that gut health plays a key role in brain function, mood regulation, and cognitive performance (Puri et al., 2023).
5.1. How do the gut and brain communicate?
The gut and brain communicate through several main mechanisms:
Vagus nerve – this is the primary pathway connecting the gut to the brain. It transmits signals about the state of the digestive system and influences mood and emotional responses. Studies have shown that stimulating the vagus nerve can reduce symptoms of depression and anxiety (Puri et al., 2023). Neuropod cells, a type of specialized enteroendocrine cell, play a key role in rapidly transmitting signals to the brain using glutamate. This allows the body to respond almost instantly to gut-related stimuli (Chen et al., 2021).
Hormonal system – the gut microbiome influences the production of hormones such as cortisol (the stress hormone), as well as ghrelin and leptin, which regulate hunger and satiety (Puri et al., 2023). Certain gut bacteria also affect serotonin levels by regulating the tryptophan metabolism pathway. Consumption of specific bacterial strains can increase serotonin production by activating the enzyme tryptophan hydroxylase 1 (TPH1) in enterochromaffin cells (Chen et al., 2021).
Immune system – gut bacteria modulate inflammatory responses in the body, which may influence the development of neurodegenerative diseases and mood disorders (Puri et al., 2023). The gut microbiome can activate the immune system through lipopolysaccharides (LPS), leading to increased blood–brain barrier permeability and elevated inflammation in the central nervous system (Chen et al., 2021).
- Neurotransmitter production – gut bacteria produce chemical substances essential for brain function, such as:
- Serotonin – known as the “happiness hormone,” about 90% of it is produced in the gut (Puri et al., 2023). Its synthesis can be stimulated by Clostridia species through the release of specific metabolites (Chen et al., 2021).
- Dopamine – responsible for motivation and the feeling of reward (Puri et al., 2023). Staphylococcus bacteria can convert the precursor L-DOPA into dopamine using the enzyme aromatic amino acid decarboxylase (Chen et al., 2021).
- GABA – influences stress levels and relaxation (Puri et al., 2023). Its production in the gut is supported by bacteria such as Bifidobacterium, Parabacteroides, and Eubacterium (Chen et al., 2021).
Impaired gut–brain communication can lead to problems with concentration, depression, and even neurodegenerative diseases (Puri et al., 2023).
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5.2. Gut microbiome and its role in brain function
The microbiome refers to the billions of bacteria residing in our intestines. These microorganisms play a key role in:
- Neurotransmitter production – certain bacteria (e.g., Lactobacillus and Bifidobacterium) produce serotonin, while Bacillus produces dopamine. They not only synthesize neurotransmitters but also regulate their transport to the brain. An example is tryptamine, produced by Clostridium sporogenes and Ruminococcus gnavus, which stimulates serotonin release in the gut and indirectly affects the nervous system (Chen et al., 2021).
- Protection against inflammation – beneficial bacteria reduce oxidative stress and help fight neurodegenerative diseases. In particular, short-chain fatty acids (SCFAs) such as butyrate can reduce neuroinflammation by acting on microglial cells and strengthening the blood–brain barrier (Chen et al., 2021).
- Mood regulation – microbiome imbalances may lead to anxiety and depression. Research shows that the microbiome affects GABA levels in the brain. Bifidobacterium and Lactobacillus synthesize GABA, which modulates neuronal excitability and has a calming effect (Chen et al., 2021).
- Stress regulation – gut bacteria can lower cortisol levels, reducing the risk of depression and anxiety disorders. Certain bacteria, such as Bacteroides fragilis, play a crucial role in balancing the body’s stress response by modulating GABAergic receptors in the gut (Chen et al., 2021).
5.3. Microbiome alterations in schizophrenia, depression, and anxiety disorders
Studies show that individuals with schizophrenia, depression, and anxiety disorders have a different composition of gut bacteria compared to healthy individuals.
Alterations in the gut microbiome may affect brain function through neurotransmitter production, inflammation, and the integrity of the blood–brain barrier. Researchers have found that certain bacterial strains are more prevalent in people with these conditions, while others are less common.
The table below presents the most significant changes in the gut microbiome observed in patients with schizophrenia, depression, and anxiety disorders (Mhanna et al., 2024).
Table: Microbiome alterations in mental disorders. Based on Mhanna et al. (2024).
| Disorder | Increased | Decreased | Impact on mental health |
|---|---|---|---|
| Schizophrenia | Enterococcus faecium, Lactobacillus fermentum, Cronobacter sakazakii, Alkaliphilus oremlandii | Ruminococcus, Roseburia | Reduced production of neurotransmitters (e.g. GABA, serotonin), increased inflammation, higher blood–brain barrier permeabilit |
| Depression | Bacteroides, Alistipes, Oscillibacter | Faecalibacterium, Coprococcus, Bifidobacterium | Increased production of pro-inflammatory cytokines, decreased SCFAs, lower serotonin levels |
| Anxiety disorders | Escherichia/Shigella, Enterobacteriaceae, Bacteroides | Prevotella, Lachnospiraceae, Ruminococcaceae | Elevated inflammation, reduced production of calming neurotransmitters (e.g. GABA), elevated cortisol levels |
The gut microbiome can have a significant impact on brain function and the development of mental disorders. In individuals with schizophrenia, depression, and anxiety disorders, there is a disruption in the balance between beneficial and harmful gut bacteria. This imbalance may lead to inflammation, impaired neurotransmitter production, and weakening of the blood–brain barrier. Further research may help develop new treatments aimed at improving mental health by regulating the gut microbiome.
Table: What should patients know about the gut microbiome?
| Remember to tell your client… | Why it matters |
|---|---|
| Healthy gut = better mood and less anxiety. | Gut microbiome produce neurotransmitters like serotonin and GABA, which affect emotional well-being. |
| Avoid excessive processed food and sugar. | Highly processed foods can increase gut inflammation and negatively affect the microbiome. |
| Include probiotics and prebiotics in the diet. | Probiotics (yogurt, kefir, fermented vegetables) and prebiotics (plant fiber) help restore and nourish healthy gut bacteria. |
| Get regular, quality sleep. | Lack of sleep disrupts gut microbiome balance and increases stress levels. |
| Manage chronic stress – find ways to relax. | Stress weakens the microbiome and increases cortisol, which may worsen symptoms of depression and anxiety. |
| Stay physically active. | Regular exercise supports a healthy gut microbiome and helps regulate neurotransmitter levels. |
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5.4. How does diet affect the microbiome and the brain?
Our diet has a major impact on gut health, and therefore on brain function. A healthy diet can improve memory, concentration, and mood. What we eat influences the bacteria in our gut, and in turn, those bacteria can affect our mood and mental health.
Good for the gut and brain:
✅ Fibre (vegetables, fruits, whole grain products) – supports the growth of beneficial bacteria.
✅ Probiotics (yogurt, kefir, fermented foods) – provide beneficial gut bacteria.
✅ Prebiotics (garlic, onion, bananas) – nourish probiotic bacteria and improve the gut microbiome.
✅ Omega-3 fatty acids (fish, flaxseeds) – protect neurons and enhance memory.
Negative effects on the gut and brain (in excess):
🚫 Simple sugars and processed foods – disrupt microbiome balance and may promote inflammation.
🚫 Trans fats – impair cognitive function and increase the risk of depression.
🚫 Excessive alcohol consumption – damages the gut microbiome and negatively affects memory (Puri et al., 2023).
An imbalanced gut microbiome can lead to increased production of pro-inflammatory cytokines, which negatively affect brain function and accelerate neurodegenerative processes.
Healthy gut = healthy brain!
The gut–brain axis plays a key role in memory, mood, and concentration, and maintaining a balanced gut microbiome may help prevent neurodegenerative diseases.
A diet rich in fibre, probiotics, and healthy fats is the best way to support both the digestive and nervous systems!
| References |
| Briguglio, M., Dell’Osso, B., Panzica, G., Malgaroli, A., Banfi, G., Zanaboni Dina, C., Galentino, R., & Porta, M. (2018). Dietary neurotransmitters: A narrative review on current knowledge. Nutrients, 10(5), 591. https://doi.org/10.3390/nu10050591 Mhanna, A., Martini, N., Hmaydoosh, G., Hamwi, G., Jarjanazi, M., Zaifah, G., Kazzazo, R., Haji Mohamad, A., & Alshehabi, Z. (2024). The correlation between gut microbiome and both neurotransmitters and mental disorders: A narrative review. Medicine (Baltimore), 103(5), e37114. https://doi.org/10.1097/MD.0000000000037114 Puri, S., Shaheen, M., & Grover, B. (2023). Nutrition and cognitive health: A life course approach. Frontiers in Public Health, 11, 1023907. https://doi.org/10.3389/fpubh.2023.1023907 World Health Organization. (2019). Risk reduction of cognitive decline and dementia: WHO guidelines. Geneva: World Health Organization. |
