Short-Chain Fatty Acids: What Gut Bacteria Make From Fibre

**Short-chain fatty acids (SCFAs)** are fatty acids with fewer than six carbons, produced when gut bacteria ferment dietary fibre and resistant starch that human enzymes cannot digest. They are the main currency of the relationship between the gut microbiome and the host. The three that dominate, in roughly a 60:20:20 ratio, are **acetate** (C2), **propionate** (C3) and **butyrate** (C4). ## Where they come from Fibre reaching the colon undigested is fermented anaerobically by resident bacteria. Humans obtain no direct energy from that fibre; SCFAs are how some of its energy is recovered, supplying a modest share of daily energy intake — and, more importantly, acting as signalling molecules rather than merely as fuel. Production depends on **substrate**: more fermentable fibre means more SCFAs, which is the mechanism behind Prebiotics, Probiotics, Synbiotics and Postbiotics: The Terminology. Different bacterial groups specialise in different products, so community composition shapes the mix. ## What each does **Butyrate** is the primary energy source for **colonocytes**, the cells lining the colon — an unusual arrangement in which a tissue is fed chiefly by its resident bacteria rather than by the bloodstream. It supports the tight junctions of the gut barrier, and has anti-inflammatory effects mediated partly through inhibition of histone deacetylases, making it of interest in inflammatory bowel disease and colorectal cancer research. **Propionate** is absorbed and travels to the liver, where it is a substrate for gluconeogenesis, and it has been linked to satiety signalling and cholesterol metabolism. **Acetate** is the most abundant, enters the systemic circulation, and serves as a substrate for peripheral tissues including in lipogenesis. All three act on host G-protein-coupled receptors (notably FFAR2 and FFAR3) expressed on gut epithelial and immune cells, which is how they influence immune regulation and appetite signalling rather than only supplying calories. ## Why they appear in microbiome studies "Enrichment of SCFA-producing microbes" is one of the commonest positive findings in microbiome intervention trials, including work on fermented foods — see Is Kombucha a Probiotic? What the Clinical Trials Actually Show. It is worth reading such a finding carefully. A shift in the abundance of SCFA-producing taxa is a **compositional** result. It is not the same as demonstrating increased SCFA production, and certainly not the same as demonstrating a clinical benefit. Trials frequently report the compositional change while finding no movement in inflammation or metabolic markers — which is precisely the gap between a plausible mechanism and a demonstrated effect.