Galactooligosaccharides (GOS) and Fructooligosaccharides (FOS): Evidence and Products
Galactooligosaccharides (GOS) and fructooligosaccharides (FOS): what they are, what adult and infant-formula reviews found, how well adults tolerated them, and who should ask first.
Content last updated . This page has not been medically reviewed.
Quick answer
GOS and FOS are short chains of sugars that people cannot digest but gut bacteria can ferment. Reviews find that FOS raises Bifidobacterium counts, and one IBS review that included 2 beta-GOS trials reported improved overall symptoms with GOS. Evidence for relief of everyday symptoms is limited. In infant formula, a review found no safety concern for growth in healthy infants but no robust evidence to recommend routine use. This page is information only, not medical advice, and infant feeding decisions belong with a pediatric clinician.
What GOS and FOS are
A 2022 review describes GOS as a soluble, nonviscous, fermentable collection of oligosaccharides, often created from lactose using enzymes called beta-galactosidases. [1] FOS is a short-chain fructan (3 to 9 sugar units), related to inulin. [1] The review notes that isolated GOS is added to foods as a bulking agent and to improve texture, and FOS added to foods contributes sweetness. [1] Both are also used as ingredients in foods and infant formula. For inulin, see the inulin page.Evidence labels used here
Promising but limited, mixed, insufficient evidence, and Proper evidence not found (we searched and found nothing we could read). Counts of bacteria are markers, not symptom outcomes.What reviews and trials found in adults
| Question | What the source found | Label |
|---|---|---|
| FOS and gut bacteria | A 2022 meta-analysis of 8 trials (213 people on FOS, 175 controls) found Bifidobacterium counts rose, more in adults than infants; no differences from comparators in other bacteria or adverse gut symptoms. [2] | Promising but limited (marker) |
| GOS in IBS | A 2026 meta-analysis of fiber trials in IBS included 2 using beta-GOS; overall symptoms improved with them. [3] | Insufficient evidence (2 trials) |
| FOS in constipation | One crossover trial in 9 older adults on peritoneal dialysis with chronic constipation: 20 g of FOS daily for 30 days raised bowel movements per week versus placebo (study amount, tiny sample). [4] | Insufficient evidence |
| Prebiotics in IBS overall | A 2024 network meta-analysis found prebiotics and synbiotics did not show significant improvement in IBS. [5] | Mixed |
| GOS for constipation or other adult symptoms | Proper evidence not found. We searched PubMed for galactooligosaccharide with constipation, irritable bowel and adult trials and found no systematic review we could read beyond the above. | Not found |
Gas, bloating and tolerance
The 2022 tolerance review (103 trials in adults without gut disease) looked at both. For FOS and oligofructose, 7 studies using 10 to 40 g a day reported flatulence; one escalating-amount study reported excessive flatulence above 30 g a day, bloating and rumbling above 40 g, and diarrhea with cramps above 50 g. For GOS, 5 studies using 2.5 to 20 g a day found stool frequency and tolerance effects unchanged. [1] These are study amounts, not advice. In the FODMAP reintroduction trial of 117 people with IBS, galacto-oligosaccharides triggered symptoms in 35% and fructans in 56%. [6] See the low-FODMAP diet and bloating and gas.GOS and FOS in infant formula
An updated systematic review (2018, 41 publications) of prebiotic-supplemented formulas in healthy infants found no safety concern for growth or adverse effects, and possible stool softening. It also concluded there is no robust evidence to recommend routine use, and that each prebiotic-supplemented formula should be considered on its own. [7] A 2025 meta-analysis of 24 studies found no overall difference in weight, height or head-circumference gain; infants given prebiotics for 3 months or more gained less weight, and formulas with inulin-enriched oligofructose were linked to lower weight and height gain. [8] Choose or change formula with a pediatrician. See probiotics for infants and babies.Related
Prebiotic fibers overview and prebiotics vs probiotics.Frequently asked questions
What is the difference between GOS and FOS?
GOS is a collection of galactose-based oligosaccharides often made from lactose; FOS is a short-chain fructan. [1]
Do GOS and FOS cause gas?
FOS often did in trials at higher amounts. GOS did not change tolerance in 5 studies of 2.5 to 20 g a day. [1]
Are GOS and FOS proven prebiotics?
FOS raised Bifidobacterium counts in a meta-analysis, but ISAPP requires a documented health benefit, and symptom evidence is limited. [2][10]
Are GOS and FOS in infant formula necessary?
A systematic review found no robust evidence to recommend routine use of prebiotic-supplemented formula. [7]
Can GOS or FOS help IBS?
One review that included 2 beta-GOS trials reported improved overall symptoms, but a network meta-analysis found no significant improvement from prebiotics overall. [3][5]
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Safety and who should ask first
- Medicines. No interaction data for GOS or FOS in the sources we read. Ask a pharmacist; see how interactions work.
- IBS, Crohn's disease, ulcerative colitis, bowel blockage. Ask a clinician first. The NHS lists these among reasons to check before laxatives. [9] See fiber and prebiotic cautions.
- Lactose and milk allergy. The review says GOS is often made from lactose. [1] Whether a given GOS product contains lactose or milk residues: Proper evidence not found; check the label with a pharmacist, and see lactose intolerance.
- Pregnancy, breastfeeding, older adults, kidney, liver, surgery. The NHS says to check laxatives with a pharmacist or doctor if pregnant or breastfeeding. [9] Other groups are not described in the sources we read for GOS and FOS; ask a clinician, and see supplements before surgery.
- Product quality. See quality testing and certifications.
References
- Adv Nutr 2022 (PubMed abstract and open-access full text, PMC9776669) Gastrointestinal Effects and Tolerance of Nondigestible Carbohydrate Consumption. PubMed abstract Read 2026-10-23.
- Nutrients 2022 Effect of Fructooligosaccharides Supplementation on the Gut Microbiota in Human: A Systematic Review and Meta-Analysis. PubMed abstract Read 2026-10-23.
- Gastroenterology 2026 Fiber Supplementation in Irritable Bowel Syndrome: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. PubMed abstract Read 2026-10-23.
- Perit Dial Int 2016 Effects of Fructo-Oligosaccharide Supplementation on Constipation in Elderly Continuous Ambulatory Peritoneal Dialysis Patients. PubMed abstract Read 2026-10-23.
- Nutrients 2024 The Efficacy of Probiotics, Prebiotics, Synbiotics, and Fecal Microbiota Transplantation in Irritable Bowel Syndrome: A Systematic Review and Network Meta-Analysis. PubMed abstract Read 2026-10-23.
- Gastroenterology 2024 Efficacy and Findings of a Blinded Randomized Reintroduction Phase for the Low FODMAP Diet in Irritable Bowel Syndrome. PubMed abstract Read 2026-10-23.
- Br J Nutr 2018 Infant formulae supplemented with prebiotics: Are they better than unsupplemented formulae? An updated systematic review. PubMed abstract Read 2026-10-23.
- Nutr Res 2025 Effect of prebiotics on growth metrics in infants: A GRADE approach systematic review and meta-analysis of randomized clinical trials. PubMed abstract Read 2026-10-23.
- NHS (UK). Laxatives. Page last reviewed 30 April 2026. Read 2026-10-23.
- International Scientific Association for Probiotics and Prebiotics (ISAPP). Prebiotics. Read 2026-10-23.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.