Serotonin, the gut-brain axis, the microbiome and mood. A plain-language look at what the research actually says in 2026, and what it does not. Why we call it a second brain The gut has a nervous system of its own, the enteric nervous system, with hundreds of millions of neurons. It runs much of digestion without checking in with the brain, and stays in constant contact with it the rest of the time. So the phrase second brain is not marketing. It is closer to anatomy. The lin