The Gut-Brain Axis: How Your Microbiome Affects Everything

The Four Communication Routes

The vagus nerve. Roughly eighty percent of vagal fibres are afferent, carrying information from gut to brain rather than the reverse. Vagal signalling transmits information about distension, nutrient content and inflammatory state. Animal work shows that some behavioural effects of specific bacteria disappear when the vagus is severed, which establishes the route as functional rather than theoretical.

Immune signalling. The gut houses the largest concentration of immune tissue in the body. Barrier integrity determines how much bacterial material reaches systemic circulation, and bacterial components crossing a compromised barrier activate inflammatory signalling that affects the brain through cytokines and through effects on the blood-brain barrier.

Microbial metabolites. Gut bacteria ferment fibre into short-chain fatty acids, principally butyrate, propionate and acetate. Butyrate is the preferred fuel of colonocytes and supports barrier integrity; propionate and acetate enter circulation and have systemic effects including on appetite signalling and on microglia. Bacteria also produce and modify neurotransmitter precursors, bile acids and tryptophan metabolites, the last of which links directly to serotonin and kynurenine pathways.

The enteric nervous system. The gut contains hundreds of millions of neurons, capable of local reflex activity independent of the brain. It produces the majority of the body's serotonin, though that serotonin acts locally on motility rather than crossing into the brain, which is a distinction routinely misrepresented in popular accounts.

What Is Well Established

• Microbiome composition differs between health states. Reproducible differences exist in composition and diversity between groups with and without various conditions. Whether these differences are cause or consequence is frequently unresolved.
• Diet changes composition rapidly. Controlled feeding studies show measurable shifts within days of a major dietary change, and reversal when the diet reverts.
• Fibre diversity predicts composition diversity. The number of distinct plant fibre sources in a diet associates with microbial diversity more reliably than total fibre grams.
• Short-chain fatty acid production is fibre-dependent. Without fermentable substrate, butyrate production falls and bacteria shift toward degrading the mucus layer, which affects barrier integrity.
• Antibiotics cause lasting change. A course of broad-spectrum antibiotics reduces diversity for months, and some taxa do not recover.
• Microbiome changes with age. Diversity patterns shift, and dysbiosis was added as a hallmark of aging in 2023.
• Faecal microbiota transplant works for one specific indication. It is established for recurrent Clostridioides difficile infection, which is the strongest proof that microbiome manipulation can be clinically decisive.

What Is Overstated

Three claims circulate far ahead of the evidence and are worth naming.

"Ninety percent of serotonin is made in the gut, so gut health determines mood." Gut serotonin is real and acts on motility. It does not cross the blood-brain barrier. The gut-brain connection to mood runs through vagal signalling, immune signalling and tryptophan availability, not through gut serotonin travelling to the brain.

"Leaky gut causes most modern illness." Barrier permeability is a genuine and measurable phenomenon, and it is elevated in several conditions. The leap from measurable permeability to a general explanation for unrelated symptoms is not supported, and the commercial ecosystem built on it outpaces the science substantially.

"A microbiome test tells you what to eat." Consumer microbiome testing has poor reproducibility between providers, unstable results on repeat sampling, and no validated framework linking composition to individual dietary recommendations. The tests measure something real and the interpretations offered are largely not validated.

What Actually Changes the Microbiome Favourably

Ordered by strength of evidence.

• Plant fibre diversity. The most consistent finding. Aiming for a wide range of distinct plant sources weekly, including legumes, whole grains, vegetables, fruit, nuts and seeds, supports diversity better than a single fibre supplement.
• Fermented foods. A controlled trial comparing high-fibre and high-fermented-food diets found the fermented food arm increased microbial diversity and reduced inflammatory markers, which is a notable and specific result. Yoghurt, kefir, kimchi, sauerkraut and similar foods deliver live organisms plus fermentation metabolites.
• Polyphenol intake. Many polyphenols reach the colon largely unabsorbed and act as substrates for bacterial metabolism, and specific bacteria are required to produce active metabolites such as urolithins from pomegranate ellagitannins.
• Adequate protein without excess. Very high protein with very low fibre shifts fermentation toward less favourable products.
• Exercise. Associated with greater diversity independently of diet in several studies.
• Avoiding unnecessary antibiotics. Straightforward and consequential.
• Alcohol moderation. Heavy intake affects barrier integrity and composition.
• Probiotics, strain-specifically. Effects are strain and indication specific rather than general. Specific strains have evidence for specific outcomes, including antibiotic-associated diarrhoea and irritable bowel symptoms. Colonisation is usually transient, so effects generally do not persist after stopping.
• Prebiotics. Inulin, galacto-oligosaccharides and resistant starch feed existing populations. They can cause substantial bloating, particularly at higher doses and in people with existing sensitivity, so gradual introduction matters.

The Gut and the Other Pillars

The gut-brain axis is one edge of a larger network, and several connections are directly relevant to other Pillars.

Inflammation. Barrier integrity determines systemic exposure to bacterial components, which is one of the recognised sources of chronic low-grade inflammation.

Metabolic health. Short-chain fatty acids affect appetite signalling, and bacterial bile acid modification affects lipid and glucose handling. Microbiome composition also affects how individuals respond to specific foods, which is part of why postprandial glucose responses differ substantially between people eating identical meals.

Sleep. Bidirectional. Circadian rhythms exist in microbiome composition, disrupted by irregular eating and sleep timing, and microbial metabolites influence sleep signalling.

Cognition. Through immune signalling, microglial regulation by short-chain fatty acids, and tryptophan metabolism.

Drug and supplement metabolism. Gut bacteria transform many compounds. Berberine is converted to a better-absorbed form by bacteria; ellagitannins become urolithins only in people carrying the right organisms; some medications are inactivated by bacterial enzymes. This is one concrete mechanism by which the same supplement produces different results in different people.

Reassessment in a System That Changes Weekly

The microbiome is the fastest-changing system in the Pillar Matrix, responding to diet within days, and that has a specific implication: composition is not a stable personal attribute to be measured once and optimised against.

The sensible objects of attention are inputs and function rather than composition. Inputs meaning fibre diversity, fermented food intake and antibiotic exposure. Function meaning stool regularity and form, bloating, tolerance of fibre, and where relevant inflammatory markers.

Specific review triggers matter here more than a schedule. A course of antibiotics is a reason to deliberately rebuild fibre and fermented food intake over the following months. Travel frequently changes composition and tolerance. A new persistent change in bowel habit is a clinical question rather than a supplement question, and this is a Pillar where that boundary needs stating clearly, because persistent symptoms attributed to dysbiosis are sometimes something else.