You've probably heard someone say "trust your gut" as a figure of speech. It turns out there's more literal truth to it than you might realise.
The gut and the brain are in constant communication. What happens in your digestive system directly affects how you think and feel, and vice versa. This isn't alternative medicine. It's an increasingly researched area of neuroscience and microbiology.
The Second Brain
Your gut contains roughly 500 million neurons (the fundamental building blocks of the brain and nervous system). The nervous system lines the walls of your digestive tract, and is so extensive and so capable of operating independently that scientists often refer to it as the "second brain" (Furness JB. 2012)
This system doesn't just manage digestion. It communicates directly with the central nervous system via the vagus nerve, a long, branching nerve that runs from the brainstem all the way down to the abdomen. The vagus nerve acts as a communication highway, carrying signals in both directions: from brain to gut and, significantly, from gut to brain.
A 2025 study examined this relationship in detail, finding that serotonin created in the gut activates vagal nerve fibres that transmit signals to the brain, influencing emotional regulation, stress responses, and immune function (Hwang, Y. K., & Oh, J. S. 2025)
The Serotonin Connection
Here's the statistic that tends to surprise people: approximately 90 to 95% of the body's serotonin is produced in the gut, not the brain.
Serotonin is one of the key neurotransmitters and hormone involved in mood regulation, sleep, appetite, and a sense of wellbeing. The version produced in the gut can't cross the blood-brain barrier, so it doesn't directly affect mood in the way brain serotonin does. But it plays a significant role in regulating gut movement, signalling to the vagus nerve, and influencing the broader gut-brain communication system. (Chen Yijing et al, 2021)
The gut microbiome plays a direct role in serotonin production, research found that gut bacteria influence how much serotonin the gut produces. In other words: the health of your gut microbiome affects serotonin production, which affects gut-brain signalling, which affects mood and mental wellbeing. The chain is real.
What the Research Shows About Gut Health and Mental Health
The link between the gut microbiome and mental health conditions is now well established in research, though the exact mechanisms are still being mapped.
A review in 2024 found that patients with depression and anxiety consistently show different gut microbiome profiles compared to healthy individuals: lower microbial diversity, reduced populations of beneficial species, and the function of their neurotransmitters where effected. (Rathore, Kanchanbala et al, 2025)
Another review in 2024 examined the gut-brain axis specifically in relation to anxiety disorders, post-traumatic stress disorder, and obsessive-compulsive disorder, finding evidence of microbiome imbalance across all three conditions. (MacKay, Marnie et al, 2024)
The direction of causality is still being investigated. Does poor gut health contribute to poor mental health, or do mental health conditions cause changes in the gut microbiome? The current understanding is that it runs both ways, which is why this system is described as two directional.
What You Can Do About It
The research on interventions is still developing, but a few things are consistently supported:
Eat more fermented foods. Probiotics found in fermented foods, particularly strains of Lactobacillus and Bifidobacterium, have been associated with improvements in mood and anxiety symptoms in multiple trials. A 2021 review found these strains had the most evidence for positive mental health impacts out of all probiotic interventions studied.4
Eat enough fibre. The beneficial bacteria that support gut-brain communication feed on fibre. A diet low in fibre starves the microbiome. Vegetables, legumes, whole grains, and fruit are all useful here.
Manage stress actively. The gut-brain axis runs both ways. Chronic stress triggers inflammatory responses that disrupt the gut microbiome. Things like exercise, sleep, and whatever works for you in terms of stress management are not optional extras; they're part of the picture.
Limit ultra-processed food. Emulsifiers, artificial sweeteners, and the additives common in ultra-processed foods have been shown to disrupt the gut microbiome even in relatively modest quantities.
Where Kombucha Fits In
Kombucha is a naturally fermented drink that contains live cultures and organic acids produced during fermentation. It won't rewire your brain chemistry. But as part of a diet that supports the gut microbiome, it's a sensible and genuinely enjoyable choice.
Our kombucha is brewed in small batches, raw and unfiltered, with no pasteurisation that would kill the live cultures. Try our Lemon, Ginger and Black Pepper or our Blackberry, Apple and Mint. A mixed case is a good way to try both.
If you're curious about the basics of how kombucha is made, our Kombucha 101 post is a good place to start.
The gut-brain connection is one of the more exciting areas of current health research. It's genuinely changing how scientists think about mental health, digestion, and the relationship between the two.
References
- Furness JB. The enteric nervous system and neurogastroenterology. Nature Reviews Gastroenterology and Hepatology, 2012. PubMed
- Interaction of the Vagus Nerve and Serotonin in the Gut-Brain Axis. International Journal of Molecular Sciences, 2025. PubMed
- Regulation of Neurotransmitters by the Gut Microbiota and Effects on Cognition. PMC, 2021. PMC8234057
- The Bidirectional Relationship Between the Gut Microbiome and Mental Health: A Comprehensive Review. PMC, 2024. PMC12007925
- The Gut-Brain Axis and the Microbiome in Anxiety Disorders, PTSD and OCD. PMC, 2024. PMC10845093