The idea that the gut and brain are connected is not a metaphor. There is a literal two-way communication highway between them — and the bacteria in your gut are sending signals to your brain right now.
The Second Brain
The enteric nervous system — the network of neurons lining your gastrointestinal tract — contains approximately 500 million nerve cells. That is more neurons than the spinal cord. It operates largely independently of the central nervous system, regulating digestion, immune responses, and the production of neurotransmitters without instruction from the brain. Neuroscientists call it the "second brain" — not as a metaphor, but as a literal description of its complexity and autonomy.
The gut and brain communicate via the vagus nerve — a bidirectional superhighway that runs from the brainstem to the abdomen. Crucially, approximately 90% of the signals travelling along the vagus nerve go from the gut to the brain, not the other way around. The gut is not just receiving instructions from the brain. It is sending them.
And the primary authors of those signals are the 38 trillion microorganisms that live in your gut.
The Serotonin Surprise
Serotonin is widely understood as the brain's "feel-good" neurotransmitter — the chemical that antidepressants like SSRIs are designed to increase. What is far less widely known is that approximately 90–95% of the body's serotonin is produced not in the brain, but in the gut — specifically by enterochromaffin cells in the intestinal lining, in response to signals from gut bacteria.
The gut bacteria that most strongly influence serotonin production are Lactobacillus and Bifidobacterium species — the same bacteria found in fermented foods like kimchi, sauerkraut, kefir, and yoghurt. A 2019 study published in Cell found that germ-free mice (raised without any gut bacteria) had significantly lower serotonin levels than normal mice — and that colonising them with specific Lactobacillus strains restored serotonin production. A 2021 study in Nature Microbiology found that people with depression had significantly lower levels of Lactobacillus and Bifidobacterium in their gut microbiome compared to healthy controls.
This does not mean that depression is simply a gut problem, or that eating kimchi will cure it. Mental health is complex, and the research is still developing. But it does mean that the gut microbiome is a meaningful contributor to mood regulation — and that feeding it well is not just about digestion.
The Short-Chain Fatty Acid Connection
When gut bacteria ferment dietary fibre, they produce short-chain fatty acids (SCFAs) — primarily butyrate, propionate, and acetate. These compounds have profound effects throughout the body: they reduce intestinal inflammation, strengthen the gut lining, regulate immune function, and — remarkably — cross the blood-brain barrier, where they directly reduce neuroinflammation and support the production of Brain-Derived Neurotrophic Factor (BDNF), a protein essential for the growth and survival of neurons.
Low BDNF levels are consistently found in people with depression, anxiety, and neurodegenerative diseases. High-fibre diets that feed SCFA-producing bacteria are associated with higher BDNF levels, better mood, and reduced risk of cognitive decline. The mechanism is now well understood: fibre → gut bacteria → SCFAs → blood-brain barrier → BDNF → neuronal health.
The Leaky Gut Problem
The gut lining is a single cell layer thick — the thinnest barrier between the outside world and your bloodstream. When this lining is healthy, it allows nutrients through while blocking pathogens and undigested food particles. When it is damaged — a condition called intestinal permeability, or "leaky gut" — those particles enter the bloodstream, triggering systemic inflammation that has been linked to depression, anxiety, autoimmune conditions, and metabolic disease.
The primary cause of leaky gut is dysbiosis — an imbalance in the gut microbiome, with too few beneficial bacteria and too many harmful ones. The primary causes of dysbiosis are a low-fibre diet, antibiotic use, chronic stress, and excess alcohol. The primary treatment is the same as the primary prevention: feeding the beneficial bacteria with fermented foods and prebiotic fibre.
The Gut & Brain Axis Box
The Root & Reason Gut & Brain Axis Box is built around this science. It combines live fermented foods (kimchi, sauerkraut, miso, kombucha — each providing different strains of beneficial bacteria) with prebiotic vegetables (leeks, garlic, asparagus — which feed the bacteria you are introducing) and specialist mushrooms (Lion's Mane for NGF, Reishi for cortisol reduction, Shiitake for immune modulation). It is the most comprehensive protocol in the Root & Reason range — and arguably the one with the broadest impact on overall health and wellbeing.
Your mood lives, in part, in your stomach. That is not a metaphor. It is biochemistry.
Explore the Bio-Box Range
Every Root & Reason Bio-Box is built around the science in this article. The ingredients, the protocols, and the activation keys — all in one weekly delivery.