Fructo-oligosaccharides (FOS) are starches that the human body cannot fully digest. Inulin and galacto-oligosaccharides (GOS) are similar substances also discussed in this article.

When a person consumes FOS, the undigested portions provide nourishment for bacteria in the digestive tract. “Friendly” bacteria ( probiotics) may respond particularly well to this nourishment. Because FOS feed probiotics, they are sometimes called a “prebiotic.”

Low doses of FOS are often provided along with probiotic supplements to aid their growth. High doses of FOS (and related substances) have been advocated for a variety of health conditions. However, currently, the available scientific evidence for benefit remains more negative than positive.

There is no daily requirement for FOS. FOS and related substances are found in asparagus, Jerusalem artichoke, leeks, onions, and soybeans, among other foods.