How colonic bacteria turn fiber into acids
In the oxygen-poor colon, resident bacteria ferment fibers and other carbohydrates that escaped digestion. The main products are short-chain fatty acids: acetate, propionate, and butyrate, molecules with just two to four carbons. Cross-feeding is common, where the byproducts of one species, such as lactate or acetate, become substrate for butyrate producers downstream.
This cooperative network means SCFA output reflects the whole community, not any single organism.
Local fuel and systemic signals
Butyrate is the dominant energy source for colonocytes and supports the barrier and local immune balance. Acetate and propionate are absorbed and travel to the liver and peripheral tissues, where they participate in lipid and glucose metabolism and act as signaling molecules at specific receptors.
These roles are well established mechanistically, but translating them into specific dietary prescriptions for healthy people is still an open question. This is general educational information, not medical advice.