Gut–brain communication

The Nervous System Living in Your Gut

This is well-established science, built on decades of neuroscience and, more recently, microbiome research layered on top of it.

The gut has its own dense, semi-independent nervous system, one large enough that researchers have nicknamed it the second brain.

That system is in constant, two-way contact with the brain in the head, and gut bacteria appear to influence what gets communicated.

A genuine second nervous system, wired into the gut wall

The enteric nervous system is a network of roughly 500 million neurons embedded in the walls of the gut, from the oesophagus down to the rectum. That's around five times more neurons than the entire spinal cord contains.

This network can coordinate digestion largely on its own, controlling muscle contractions and enzyme release without needing constant instructions from the brain. It's a genuinely independent, functional nervous system, not just a bundle of cables relaying messages.

The main line connecting this system to the brain is the vagus nerve, a large nerve running between the brainstem and the gut that carries signals in both directions.

Source: Unexpected Roles for the Second Brain: Enteric Nervous System as Master Regulator of Bowel Function

Most of the body's serotonin is made in the gut, not the brain

Serotonin is best known as a brain chemical involved in mood, but around 90% of the body's serotonin is actually produced in the gut, by specialised cells in the gut lining called enterochromaffin cells.

Gut bacteria influence how much serotonin these cells produce. Certain bacterial byproducts, including short-chain fatty acids made by fermenting fibre, have been shown to boost serotonin synthesis in the gut.

That gut-produced serotonin then activates nearby vagus nerve endings, which relay the signal onward to specific regions in the brainstem that help regulate mood, stress responses, and immune activity.

Source: Interaction of the Vagus Nerve and Serotonin in the Gut-Brain Axis

The signal travels by diffusion, not a direct wired connection

A 2024 study answered a basic, previously unresolved question about how this actually works physically. Do the gut's serotonin-producing cells make direct, wired contact with vagus nerve endings, the way two connected neurons normally would?

The answer was no. Researchers found the distance between these serotonin-producing cells and the nearest vagus nerve endings was consistently too great for a direct synaptic connection. Instead, serotonin released from these cells has to diffuse, drifting through the surrounding tissue, before reaching and activating nearby nerve endings.

This is a genuinely different signalling method to how most of the nervous system operates internally, and it helps explain why gut-brain signalling can feel diffuse and slow-building rather than instantaneous, unlike a reflex.

Source: Gut-brain communication turned on its axis

What this means for you

There's no single habit that directly "optimises" this communication system, since it depends on so many interacting parts working normally together. What the research does show clearly is that gut bacteria are genuinely woven into how this signalling works, shaping serotonin production at the source, not just reacting to whatever the brain sends downward. A varied, plant-rich diet supports the same gut bacteria this research ties to healthy serotonin production and steady gut-to-brain signalling. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.