The Gut–Brain Connection
January 10, 2026 · 5 min read
The enteric nervous system contains more neurons than the spinal cord, and communicates bidirectionally with the brain via the vagus nerve. What happens in the gut does not stay in the gut.
The gut has been called the "second brain" — a description that, while slightly reductive, points to something real. The enteric nervous system (ENS) lining the gastrointestinal tract contains approximately 500 million neurons.
The Vagus Nerve as Information Highway
Roughly 80–90% of vagal nerve fibres run from gut to brain, not the reverse. The vagus nerve carries information about gut state — microbial activity, inflammatory signals, nutrient sensing — directly to the brainstem and from there to higher cortical and limbic structures.
The Microbiome's Role
The gut microbiome — the ecosystem of approximately 38 trillion microorganisms — produces many of the same neurotransmitter precursors the brain uses. Approximately 90% of the body's serotonin is produced in the gut.
Inflammation as a Shared Signal
Intestinal permeability allows bacterial products to enter systemic circulation, triggering immune responses. Systemic inflammation has direct effects on the brain: it upregulates inflammatory cytokines that cross the blood-brain barrier, contributing to fatigue, social withdrawal, low mood, and cognitive slowing.