When you feel butterflies in your stomach before an important event, your gut and brain are communicating in real time. This bidirectional network – neural, hormonal, and immunological – is known as the gut-brain axis. Discover what the science says about this connection and how you can strengthen it. Before changing your diet or lifestyle, consult your doctor.
Key facts about the gut-brain axis:
- Over 90% of serotonin is made in the gut – the mood neurotransmitter is produced mainly in the digestive tract, not the brain
- The vagus nerve as a motorway – it connects the gut and brain through a bidirectional stream of signals
- Gut microbiome and depression – the composition of gut bacteria correlates with the risk of mood disorders
- Dysbiosis and intestinal permeability influence inflammation that extends to the nervous system
- Psychobiotics – probiotics with proven effects on cognitive function and well-being
Why is the gut called the “second brain”?
The enteric nervous system – the gut’s neural network – contains between 200 and 600 million neurons distributed along the entire digestive tract. That’s more than in the human spinal cord. This extensive system operates autonomously. It regulates peristaltic movements, digestive secretions, and the local immune response without direct commands from the brain. This independence is precisely what led scientists to adopt the term “second brain”.
The gut and brain develop from the same embryonic material and share neurotransmitter pathways. When the brain experiences stress, the gut responds with cramps or bloating. When inflammation develops in the gut, the brain responds with fatigue, irritability, and difficulty concentrating.
How much serotonin does the gut produce?
As much as 90-95% of all serotonin in the body originates in the gut – produced by specialised enterochromaffin cells lining the intestinal mucosa. Gut serotonin doesn’t cross the blood-brain barrier, but it influences mood indirectly – through the vagus nerve and humoral signals. Its production is influenced by fermented products in the diet and the overall composition of the gut microbiome.
Gut-brain axis – do probiotics really change brain activity?
They do – and we have hard data to prove it. A landmark 2013 study published in Gastroenterology by a UCLA team led by Dr Kirsten Tillisch found. That women consuming fermented dairy for four weeks showed measurably altered activity in the brain’s emotional centres in response to stress. FMRI imaging revealed reduced reactivity in the amygdala. The structure responsible for fear and anxiety. This was the first evidence that microbiome changes translate into brain function in healthy individuals.
The mechanism works on multiple levels. Gut bacteria produce short-chain fatty acids. Substances that nourish the gut wall – butyrate, propionate, acetate – which reach the brain and influence gene expression in neurons. They also produce precursors of neurotransmitters: tryptophan (needed for serotonin synthesis), GABA (gamma-aminobutyric acid), and dopamine.
Gut microbiome and depression – how does diet affect mood?
The link between microbiome composition and mood disorders is one of the fastest-growing areas of neuroscience. In 2022, a large study published in Nature Mental Health – encompassing over 18,000 participants – found that specific differences in gut microbiome composition were statistically associated with depressive episodes. People with depression had reduced levels of Lactobacillus and Bifidobacterium (key types of gut bacteria). Alongside an excess of bacteria linked to inflammatory processes.
The MIND diet demonstrated a 35% reduction in depression risk among older adults following it for five years in the Rush Memory and Aging Project. The Mediterranean diet supplies the polyphenols and prebiotic fibre essential for nourishing beneficial bacterial strains.
Foods that support the gut microbiome and mood:
- Fermented foods – yoghurt, kefir, kimchi, sauerkraut (live bacterial cultures)
- Prebiotics: garlic, onion, leek, Jerusalem artichoke – inulin as fuel for Bifidobacterium
- Berries and dark fruits – anthocyanins reduce intestinal inflammation
- Oily fish – omega-3 fatty acids reduce intestinal permeability
- Walnuts – polyphenols and alpha-linolenic acid support microbiome diversity
What are psychobiotics and how do they act on the gut-brain axis?
Psychobiotics are probiotics or prebiotics that produce a measurable positive effect on mental health – through the gut-brain axis. The term was introduced by Professor Ted Dinan of University College Cork. In a review published in Nutrients (2021), he stated: “The evidence is clear: specific strains of gut bacteria produce neurologically active substances. This is biochemistry.” Psychobiotics don’t replace psychiatric treatment, but they may support it.
Which bacterial strains have psychobiotic effects?
The best-studied psychobiotic strains:
- Lactobacillus rhamnosus JB-1 – in animal models reduced anxiety and lowered cortisol through modulation of GABA receptors
- Lactobacillus helveticus R0052 + Bifidobacterium longum R0175 – the combination demonstrated psychological stress reduction in a 30-day randomised controlled trial (RCT) with healthy volunteers (Messaoudi et al., 2011)
- Lactobacillus plantarum 299v – improved cognitive functions and working memory in clinical trials
- Bifidobacterium longum NCC3001 – reduced depressive symptoms in patients with irritable bowel syndrome
How does the vagus nerve connect the gut and brain?
The vagus nerve – the tenth cranial nerve. Runs from the brainstem through the neck and chest to the abdomen. Crucially, as many as 80-90% of its fibres carry signals in the ascending direction. From the gut to the brain, not the other way around. The gut speaks to the brain far more than the brain speaks to the gut.
The vagus nerve and the gut form an early-warning system. Gut bacteria activate nerve endings that relay signals to the limbic system and prefrontal cortex. Areas managing emotions and stress. Disruption of this pathway (dysregulation) is observed in anxiety disorders, depression, and irritable bowel syndrome.
How to support the gut-brain axis in daily life?
Three types of action have the strongest and best-documented impact. Shifting towards a fibre-rich diet with fermented foods, regular physical activity, and reducing chronic stress. Gut dysbiosis (a disruption in the composition and diversity of the microbiome) reverses slowly. But a consistent dietary change produces measurable effects on microbiome composition within as little as two to four weeks.
What to eat to feed good bacteria?
The key is plant diversity. Research shows that people eating more than 30 different plants weekly have a significantly richer microbiome (one apple, a handful of nuts, and a four-ingredient salad already amounts to seven plants). Daily one to two portions of fermented foods, 150-300 minutes of movement, and seven to nine hours of sleep strengthen the microbiome synergistically. Cutting back on simple sugars and managing chronic stress completes the picture – cortisol shifts the microbiome towards inflammation.
FAQ: Frequently asked questions about the gut-brain axis
How do the gut and mood connect?
The gut produces over 90% of the body’s serotonin and numerous other neurotransmitters, and through the vagus nerve sends a continuous stream of signals to the brain that modulate mood, anxiety levels, and stress reactivity.
Which bacterial strains support the brain?
The best-studied are Lactobacillus rhamnosus JB-1, Lactobacillus helveticus R0052 with Bifidobacterium longum R0175, and Bifidobacterium longum NCC3001, all showing effects on anxiety, depression, and conditions such as Alzheimer’s and dementia.
Can dysbiosis cause mood problems?
Gut dysbiosis – a disruption in microbiome composition and diversity – is linked to increased intestinal permeability, inflammation, and changes in neurotransmitter production, which together raise the risk of depression and anxiety disorders.
What do studies tell us about diet and mood?
The SMILES trial (2017, BMC Medicine) found that switching to a Mediterranean-style diet in people with depression produced significant symptom improvement within 12 weeks. A greater effect than social support alone, suggesting a direct link between diet, the microbiome, and mood.
References:
- Tillisch, K., et al. (2013). Consumption of fermented milk product with probiotic modulates brain activity. Gastroenterology. https://doi.org/10.1053/j.gastro.2013.02.003
- Jacka, F. N., et al. (2017). A randomised controlled trial of dietary improvement for adults with major depression (the SMILES trial). BMC Medicine. https://doi.org/10.1186/s12916-017-0791-y
- Dinan, T. G., et al. (2013). Psychobiotics: a novel class of psychotropic. Biological Psychiatry. https://doi.org/10.1016/j.biopsych.2013.05.001
