Fructooligosaccharides (FOS): Prebiotic Fiber Explained
Fructooligosaccharides, often shortened to FOS, are fermentable carbohydrates used as prebiotic ingredients. They can influence the gut microbial environment, but fermentability also explains why some people notice more gas or bloating.
Reviewed September 2026. This page is educational, not medical advice.
What FOS does
FOS resists digestion in the upper GI tract and can be fermented by intestinal microbes. That microbial utilization is the basis for its prebiotic role.
Why tolerance varies
Fermentation produces metabolites and gas. People differ in baseline diet, microbiome and visceral sensitivity, so the same prebiotic dose can feel fine to one person and uncomfortable to another.
FOS on a supplement label
If FOS appears inside a blend, the exact amount may not be obvious. That makes it harder to compare with research or to know whether a reaction is related to the prebiotic dose.