You know that feeling when you are nervous about something and your stomach flips? Or when stress hits and suddenly you need the bathroom? That is not coincidence. Your gut and your brain are in constant two-way communication — and it is a much more significant conversation than most people realise.
Your gut has its own nervous system
Your gut contains about 500 million nerve cells running through its walls from top to bottom. This network — called the enteric nervous system — is sophisticated enough to work independently of your brain. It manages digestion, senses what is in your gut, and communicates with your immune system, all without waiting for instructions from your head.
Some researchers call it the "second brain." The name sounds dramatic, but the biology backs it up.
Most signals go from gut to brain, not the other way
Your gut and brain are connected by the vagus nerve, a long nerve that runs from your brainstem down through your chest and into your abdomen. Here is the surprising part: roughly 80–90% of the signals on that nerve travel upward — from gut to brain. Your gut is constantly informing your brain and influencing how you feel.
Your gut makes most of your serotonin
Around 90–95% of serotonin — the chemical most linked to feeling good, calm, and stable — is made in your gut, not your brain. And your gut bacteria directly influence how much gets produced. When gut bacteria are out of balance, serotonin production can fall. This is one of the main reasons gut health and mood are so closely connected.
What the research shows
Studies have found that people with depression consistently have different gut bacteria compositions from those without it — lower levels of beneficial bacteria and less diversity overall. In animal research, transferring gut bacteria from depressed humans into mice with no existing gut bacteria caused the mice to develop anxious and depressed behaviour. Not because of anything psychological — purely because of the bacteria.
People with IBS have significantly higher rates of anxiety and depression than the general population. The gut-brain axis is powerful and runs in both directions.
What inflammation has to do with it
When your gut bacteria are out of balance, the gut lining can become more permeable — allowing things that should stay in your gut to leak into your bloodstream. This triggers body-wide inflammation. Those inflammatory signals reach the brain and directly disrupt the chemicals that regulate mood, motivation, and how you handle stress.
Your next steps: If you regularly experience both gut symptoms and low mood or anxiety, consider them connected rather than separate problems. Try three weeks of this: increase the variety of plants in your diet, add a daily fermented food, take a 20-minute walk after dinner each evening, and take three or four slow deep breaths before each meal to shift into a calmer eating state. Track both your gut and your mood across those three weeks. Most people are surprised by how much both improve at the same time.
This content is for educational purposes only and is not a substitute for professional medical advice.