Fructooligosaccharide

Fructooligosaccharide, or FOS, is a short-chain fiber made of fructose units, found in plants such as chicory and also produced commercially. Cats cannot digest it, so gut bacteria ferment it. A study in adult cats found that dietary FOS changed fecal protein breakdown products and microbial populations. For a cat with a digestive or other medical condition, ask your veterinarian.

Source: pubmed.ncbi.nlm.nih.gov

Fructooligosaccharide is a prebiotic fiber added in small amounts to feed bacteria in the gut. It is a short chain of fructose sugars that passes undigested into the large intestine. There, bacteria ferment it and produce short-chain fatty acids. Manufacturers add it as a fiber source, usually near the end of the ingredient list.

None in substance. Fructooligosaccharide is the singular form and fructooligosaccharides the plural, and both mean FOS. AAFCO's consumer page uses the singular. Most labels use the plural or just FOS. Pettings keeps two pages because labels spell it both ways, and both carry the same score.

Source: aafco.org

Both are fructans, chains of fructose that cats cannot digest. Inulin has longer chains, and FOS has shorter ones, often made by breaking down inulin or by building chains from sucrose. Gut bacteria ferment both. Shorter FOS chains tend to ferment faster and earlier in the colon than long-chain inulin.

Pettings gives fructooligosaccharide 4 out of 5 and lists it as a prebiotic fiber. This singular spelling appears on 1 of the 149 dry cat foods we list. The plural spelling and FOS are far more common and share the same score. Each food's page shows how it scores overall.