How the gut talks to the brain: the gut-brain axis in plain English
Your gut and your brain are in constant conversation. That is not a metaphor. It runs through the vagus nerve, through hormones, and through the immune system, and the trillions of bacteria living in the gut take part. Scientists call it the microbiota-gut-brain axis. Here is what is actually established.
Serotonin starts in the gut
Serotonin is best known as a brain chemical, but the gut makes and handles a large share of the body's supply. Specialised cells in the gut lining called enterochromaffin cells release it to drive the movement of the bowel and to send signals to the brain along the nerves. Gershon and Tack laid this out in Gastroenterology in 2007: two different enzymes make serotonin, one in the gut's enterochromaffin cells and one in neurons. (PMID 17241888.)
In 2015 a team at Caltech showed that gut bacteria help regulate this. Indigenous spore-forming bacteria from mouse and human microbiota promoted serotonin production by the gut's own cells, which in turn affected gut motility and platelet function. Germ-free mice made less; adding specific microbial metabolites raised it again. Most of this work was in mice. (Yano et al., Cell, 2015, PMID 25860609.)
The vagus nerve
The single most cited experiment in this field is from 2011. Mice fed Lactobacillus rhamnosus JB-1 showed changes in GABA receptors across brain regions, lower stress hormone, and less anxiety- and depression-like behaviour. When the vagus nerve was cut, the effects disappeared. The nerve was the wire. (Bravo et al., PNAS, 2011, PMID 21876150.)
Bacteria, mood and behaviour
Cryan and Dinan's 2012 review in Nature Reviews Neuroscience pulled the animal evidence together: germ-free animals, animals given probiotics, antibiotics or infections all showed changes in anxiety, mood, cognition and pain, and the authors proposed the gut microbiota as a target for the brain. (PMID 22968153.) The following year the same group defined a "psychobiotic": a live organism that, when ingested in adequate amounts, produces a health benefit in patients with psychiatric illness, acting through the vagus nerve, the spinal cord or hormones, and producing neuroactive substances such as GABA and serotonin. (Dinan, Stanton and Cryan, Biological Psychiatry, 2013, PMID 23759244.)
What people with mood conditions have in common
A 2021 meta-analysis in JAMA Psychiatry pooled 59 case-control studies of gut microbiota in adults with psychiatric diagnoses, including ADHD. Across 34 studies with usable data (1,519 patients and 1,429 controls), patients had lower microbial richness, and a shared pattern showed up across depression, bipolar disorder, psychosis and anxiety: fewer of the bacteria that make anti-inflammatory butyrate (Faecalibacterium and Coprococcus) and more pro-inflammatory genera (Eggerthella). The authors called it a transdiagnostic pattern. It is an association in adults, not a cause. (Nikolova et al., JAMA Psychiatry, 2021, PMID 34524405.)
What this does and does not mean
It means the gut is part of how the brain feels. It does not mean any food fixes a mood, and the human trials that test that idea are the subject of the next article, on probiotics.
This article reports published research for parents and adults who want to read it for themselves. It is not medical advice, it does not describe or recommend any product, and it never replaces a conversation with your GP or your child's clinicians.