European Journal of Nutrition & Food Safety

Fructooligosaccharides (FOS) are fermentable, low-digestible carbohydrates that combine moderate sweetness with bulking, fibre-like and prebiotic functions. These attributes have encouraged their use in reduced-sugar foods, but the description of FOS as a low-calorie alternative sweetener can obscure important distinctions between ingredient energy value, sweetness potency, product-level calorie reduction and health effects. This critical narrative review evaluates FOS as sweetening ingredients across carbohydrate chemistry, digestive fate, sensory properties, production and processing stability, food-matrix performance, metabolic outcomes and gastrointestinal tolerability. Literature was selected from multidisciplinary food-science and biomedical sources, with emphasis on verified human interventions and directly relevant food-processing studies. The evidence most consistently supports three propositions. First, FOS largely escape small-intestinal digestion and provide less metabolisable energy than conventional digestible sugars because their major energetic contribution arises after colonic fermentation. Second, FOS can reduce added sugar while contributing bulk, yet their sweetness and technological behaviour vary with degree of polymerisation, purity, residual mono- and disaccharides, processing conditions and food matrix; they therefore do not behave as universal one-for-one sucrose replacements. Third, bifidogenic effects are comparatively reproducible, whereas improvements in glycaemic control, blood lipids, satiety, body weight or mineral absorption are inconsistent and should not be assumed from prebiotic activity alone. Gastrointestinal effects are dose- and host-dependent, with higher intakes increasing flatulence, bloating and other fermentation-related symptoms, especially in susceptible individuals. The most defensible use of FOS is consequently as a multifunctional ingredient for targeted partial sugar replacement where energy dilution, soluble fibre, prebiotic activity and matrix compatibility can be jointly demonstrated. Future studies should quantify actual FOS composition before and after processing, use sucrose-replacement designs that preserve product comparability, and connect sensory and technological outcomes with clinically meaningful endpoints rather than surrogate microbiota changes alone.