FOS (fructo-oligosaccharides)

FOS, or fructo-oligosaccharides, are prebiotic [fibres](/glossary/fibre) made of short chains of fructose that the small intestine cannot digest, so they arrive in the colon intact. There, they are selectively fermented by beneficial gut bacteria such as bifidobacteria, producing short-chain fatty acids that feed the cells lining the colon and help maintain a balanced flora. FOS are among the best-studied [prebiotics](/glossary/prebiotics) in dogs and cats, which is why they appear so often in premium digestive-care recipes. They occur naturally in several plants, especially chicory root, from which they are commonly extracted, and in sugar beet after processing. In a finished food they are added at a low inclusion rate, typically a fraction of a percent, to support gut health and bowel regularity. There is a catch: too much too fast ferments aggressively and can cause flatulence and loose stools, which is why careful dosing and a gradual introduction matter. FOS are frequently paired with [MOS](/glossary/mos-mannan-oligosaccharides), the two acting through different mechanisms, FOS by feeding good bacteria and MOS by binding undesirable ones, and the close cousin [inulin](/glossary/inulin) works the same way with longer chains. On a label, look for FOS or chicory among the ingredients. See the [Petipedia glossary](/glossary).

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