Fructooligosaccharides (FOS)

Not clearly. FOS feeds beneficial gut bacteria, but a study in cats found no change in the stool compounds linked to odor, so a milder smell is not proven.

FOS, short for fructooligosaccharides, is a prebiotic fiber that feeds gut bacteria such as bifidobacteria. AAFCO lists fructooligosaccharide among the pet food ingredients that often supply dietary fiber. Large amounts of fermentable fiber can cause gas or loose stool, though the amounts in dry food are small. For a cat with a medical condition, ask your veterinarian.

Source: aafco.org

FOS is added as a prebiotic, a fiber that the cat cannot digest but that gut bacteria can ferment. Bacteria such as bifidobacteria use it as food. A study in adult cats found that dietary FOS changed fecal microbial populations and protein breakdown products. Claims that it firms stool or supports immunity have not been clearly shown in cats.

Source: pubmed.ncbi.nlm.nih.gov

They are the same ingredient. FOS is the abbreviation for fructooligosaccharides, and some labels spell out the full name in the singular, fructooligosaccharide. Pettings has a separate page for the singular spelling, which appears on one food, while this page covers FOS and the plural name. Both pages give the same score.

No. FOS is a prebiotic, which means it is food for bacteria, not bacteria itself. Probiotics are live microorganisms, such as certain Lactobacillus or Enterococcus strains, added to some foods. Some cat foods include both, a pairing sometimes called synbiotic. On a label, FOS appears with the fiber ingredients, while probiotics are usually listed as dried fermentation products.

Pettings gives FOS 4 out of 5 and lists it as a prebiotic fiber. It appears on 27 of the 149 dry cat foods we list, also as Fructooligosaccharides. Foods with it average 51 out of 100 on our Ingredients score, compared with 59 for the 122 foods without it, though that average reflects the whole food. Each food's page shows how it scores overall.