Beyond Probiotics and Prebiotics
The gut health conversation has focused heavily on probiotics (beneficial bacteria) and prebiotics (bacterial food). The next frontier is postbiotics — the bioactive compounds produced by gut bacteria that have direct effects on human health. These include short-chain fatty acids (SCFAs) like butyrate, propionate, and acetate; bacterial lysates; enzymes; vitamins; and the signalling molecules bacteria produce from the conversion of indigestible dietary fibre. Postbiotics are the mechanism through which the microbiome exerts its effects on distant organs and systems, including the brain, liver, and immune system.
Butyrate: The Master Short-Chain Fatty Acid
Butyrate is produced when gut bacteria ferment dietary fibre in the colon. It is the preferred energy source for colonocytes (colon lining cells), supporting gut barrier integrity and maintaining the colonic mucus layer. Butyrate also has systemic effects: it inhibits histone deacetylases (HDACs), the enzymes that control gene expression in cells throughout the body. By inhibiting HDACs, butyrate acts as an epigenetic regulator, changing the expression of genes involved in inflammation, metabolism, and cell differentiation. Low butyrate production is associated with inflammatory bowel disease, metabolic syndrome, and colorectal cancer.
The butyrate-producing bacteria require specific substrates to do their work. Resistant starch, inulin-type fructans, and arabinoxylan — types of dietary fibre found in potatoes, legumes, oats, and whole grains — are the preferred substrates for butyrate production. A diet low in these fibres produces a microbiome that is less capable of butyrate production, with downstream effects on colonic health and systemic inflammation. The Western diet, characterised by high fat and low fibre, predictably reduces butyrate-producing bacteria.
Propionate and Its Metabolic Effects
Propionate is absorbed into the portal circulation and reaches the liver, where it inhibits cholesterol synthesis while also reducing hepatic gluconeogenesis. Population studies consistently find inverse associations between propionate-producing bacteria and metabolic syndrome markers. This is not surprising given propionate’s hepatic effects — it reduces the liver’s output of cholesterol and glucose simultaneously, addressing two of the core components of metabolic dysfunction.
What You Can Do Today
Postbiotic optimisation is primarily a fibre story — feed the bacteria that produce beneficial postbiotics. Include resistant starch (cooked and cooled potatoes, rice, legumes), inulin ( Jerusalem artichokes, garlic, onions), and arabinoxylan (oats, whole wheat) in your daily diet. These are the preferred substrates for butyrate and propionate production. If you are taking probiotics, ensure you are also eating prebiotic fibres — without substrate, probiotic bacteria cannot produce the postbiotic compounds that mediate their beneficial effects.
The Gut-Brain Postbiotic Axis
Postbiotics produced by the gut microbiome exert effects on the brain through multiple mechanisms. The vagus nerve carries direct signals from gut to brainstem, transmitting the information content of the gut microbiome’s metabolic output. Butyrate, propionate, and acetate all influence brain function through this route — butyrate activating vagal afferents through free fatty acid receptor 2 (FFAR2), propionate influencing neurotransmitter synthesis through epigenetic modulation, and acetate crossing the blood-brain barrier to influence hypothalamic appetite regulation.
The implications for mental health are significant. Several studies have documented reduced SCFA production in people with depression and anxiety compared to healthy controls. This is not surprising given that butyrate’s HDAC inhibitory activity in the brain produces broad-spectrum anti-inflammatory and neuroprotective effects. The connection between gut SCFA production and mental health outcomes is now strong enough that probiotic and postbiotic interventions for mood disorders are being studied in randomised controlled trials, with promising early results for specific bacterial strains and SCFA preparations.
Testing Your Postbiotic Production
Direct measurement of postbiotics is not routinely available outside of research settings. The most practical proxy is a comprehensive stool test that includes microbiome sequencing and short-chain fatty acid measurement — available through functional medicine practitioners and some direct-to-consumer lab services. Even without testing, dietary behaviour is a reliable proxy: people eating 25-30g of fibre daily from diverse plant sources are almost certainly supporting adequate postbiotic production, while those eating low-fibre, high-processed-food diets are not.
Butyrate and the Gut-Brain Axis
Butyrate’s effects on the brain are mediated through multiple mechanisms. The vagus nerve carries butyrate-responsive signals from gut to brain, producing measurable effects on anxiety and social behaviour in animal models — mice given butyrate show reduced anxiety-like behaviours and improved social recognition. Butyrate also crosses the blood-brain barrier, where it inhibits histone deacetylases in neurons and glia, producing neuroprotective and anti-inflammatory effects. In human studies, butyrate supplementation has shown preliminary evidence for reducing food craving frequency and improving mood in people with mood disorders.
The connection between butyrate-producing bacteria and mental health outcomes is one of the most robust findings in the gut-brain axis literature. A 2019 meta-analysis found that faecal microbiota transplantation from lean donors into people with metabolic syndrome produced not only metabolic improvements but measurable improvements in anxiety and depression scores. The proposed mechanism involves the restored microbiome’s increased production of butyrate and other SCFAs, which then act on the gut-brain axis to improve mood and stress resilience.
How to Increase Your Butyrate Production
The two most effective dietary strategies for increasing butyrate production are resistant starch and arabinoxylan supplementation. Resistant starch (RS) is starch that resists digestion in the small intestine and reaches the colon intact, where bacteria ferment it to butyrate. RS supplementation at 15-30g daily — available as raw potato starch, hi-amylose maize starch, or commercial RS supplements — consistently increases butyrate production within days. Arabinoxylan, the main fibre in wheat bran and whole grains, produces similar effects. Combining both fibres produces greater SCFA production than either alone.
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