Fructooligosaccharides (FOS) are a prebiotic fiber, well known and with a consolidated use in food supplements and in fortified fodds. Chemically they are short chain (3-5 units) oligosaccharides in which the terminal sugar is a glucose and the remaining chain is made of fructose moyeties.

FOS are not digested by human gastro-intestinal tract because of beta bond configuration of anomeric C2 (while human digestive enzymes are specific for alfa bonds); some species of the resident microbiota, instead, are capable to hydrolize and ferment FOS that can thus exert a prebiotic and, particularly, bifidogenic function.

FOS can be naturally found in different foods such as Jerusalem artichokes, tubers, onions, leeks, some cereals and in honey.