Ever felt butterflies before a big moment, or lost your appetite under stress? That’s the gut–brain axis at work: a two-way communication network between your digestive system and your brain.
How they communicate
- The vagus nerve runs from the brainstem to the gut and carries signals in both directions — most of them traveling from gut to brain.
- The enteric nervous system, sometimes called the “second brain”, is a network of around 500 million neurons lining the gut.
- Chemical messengers. Gut cells and microbes produce compounds that act on the nervous and immune systems, including short-chain fatty acids from fiber fermentation.
- The immune system. Most of the body’s immune cells live in or near the gut, and inflammation is one route by which gut and brain influence each other.
The serotonin myth, clarified
You’ll often read that “90% of serotonin is made in the gut.” That’s broadly true — but gut serotonin doesn’t cross into the brain. It mainly regulates gut movement. The connection between gut and mood is real, but it runs through nerves, immune signalling and other molecules, not a direct serotonin supply.
What’s established
Stress clearly affects gut function: it can speed or slow digestion and heighten pain sensitivity, which is why conditions like irritable bowel syndrome often flare with stress. Gut-directed therapies such as CBT and gut-directed hypnotherapy have good evidence for IBS.
What’s promising
Animal studies show gut microbes can influence anxiety-like behaviour. In humans, studies have linked differences in the microbiome with depression, but whether the microbiome causes mood changes or reflects them isn’t settled. Trials of “psychobiotics” for mood are small and mixed.
What helps either way
Regular meals rich in fiber, fermented foods, sleep, exercise and stress management support both sides of the axis. If anxiety or low mood shows up alongside gut symptoms, it’s worth treating both — they often improve together.