Glossary · Microbiota
Gut-brain axis
In one sentence
The gut-brain axis is the two-way communication network between the digestive tract and the central nervous system, operating through the vagus nerve, the immune system, hormones and microbial metabolites.
Also known as: microbiota-gut-brain axis · brain-gut axis · gut–brain axis
What is the gut-brain axis?
The term captures the idea that the gut and the brain influence each other. The brain alters gut motility, secretion and sensitivity, as anyone who has felt “butterflies” before an exam will recognise. The gut, in turn, sends the brain signals about distension, pain, nutrients and inflammation.
When the microbiota is included in this network, researchers speak of the microbiota-gut-brain axis. A widely cited reference is the review by Cryan and colleagues in Physiological Reviews (2019), which summarises how gut microbes may take part in this communication.
How it works
Several pathways have been described, and they operate in parallel:
- Neural: the enteric nervous system and the vagus nerve connect the gut with the brainstem.
- Immune: the microbiota interacts with immune cells in the gut lining, which release cytokines with systemic effects.
- Endocrine: the hypothalamic-pituitary-adrenal axis and gut hormones modulate the stress response and satiety.
- Metabolic: short-chain fatty acids, tryptophan metabolism and some bacterially produced neurotransmitters, such as GABA, are being studied as possible messengers.
What the evidence shows
Much of the mechanistic evidence comes from animal models, such as germ-free mice or studies in which the vagus nerve is cut. In humans, most studies are observational: they find differences in the microbiota of people with depression, anxiety or irritable bowel syndrome, but cannot show which came first.
Trials of probiotics aimed at psychological symptoms use very different strains, doses and durations, and their results are not conclusive. The gut-brain axis is best presented as an active and plausible field of research, not as an established basis for treating mental health conditions.
Example
A psychiatrist and a gastroenterologist share the care of a patient with irritable bowel syndrome and generalised anxiety whose bowel symptoms flare during stressful periods. They use the gut-brain axis to explain the link to the patient and agree a combined plan: dietary measures, stress management and continued psychological therapy. A microbiota test, if requested, is treated as complementary information, not as an explanation for the anxiety.
Gut-brain axis at Xenogene
Xenogene’s Gut Microbiota test identifies the microorganisms in a stool sample and reports on microbiota functions. The Essential Microbiota analysis also includes functional profiles related to the gut-brain axis, such as capacity linked to serotonin and GABA. These describe composition and functional potential; they are not a neurological or psychiatric diagnosis, and interpretation rests with the professional who requests the test.
Frequently asked questions
Can gut bacteria cause anxiety or depression?
That cannot be stated. Differences in the gut microbiota have been observed in people with anxiety or depression, but these are associations and do not prove causation. Mental health depends on many factors and needs professional assessment.
What role does the vagus nerve play in the gut-brain axis?
It is one of the main routes, carrying information about the state of the gut to the brain and signals in the opposite direction. Much of what is known about its role in behaviour comes from animal studies.
Do probiotics improve mood?
Some trials of specific strains have reported changes in certain symptoms, but results are inconsistent and do not support recommending them as a general treatment. They are not a substitute for psychological or psychiatric care.
Related terms
- Microbiota
The microbiota is the community of living microorganisms (bacteria, archaea, fungi, protozoa) that inhabits a specific site of the body, such as the gut, mouth, skin or vagina.
- Short-chain fatty acids
Short-chain fatty acids (SCFAs) are fatty acids with one to six carbon atoms produced by colonic bacteria when they ferment fibre and other non-digestible carbohydrates. The main ones are acetate, propionate and butyrate.
- Dysbiosis
Dysbiosis is a change in the composition or function of the microbiota away from what is considered compatible with health: loss of diversity, loss of beneficial microbes or overgrowth of others.
- Probiotic
A probiotic is a live microorganism that, when administered in adequate amounts, confers a health benefit on the host. This is the FAO/WHO definition, retained by the ISAPP consensus panel in 2014.
Educational content for healthcare professionals and the general public. It does not replace assessment by a healthcare professional: ordering and interpreting any test is the responsibility of the requesting clinician.
Updated on