Galactooligosaccharides (GOS)

Also called GOS, galacto-oligosaccharides, trans-galactooligosaccharides, B-GOS, Bimuno, Biotis GOS.

Best grade C across 11 graded outcomes Other Last checked Oct 21, 2026

The short answer

From 13 sources

Galactooligosaccharides are a prebiotic fibre made from milk sugar. Many trials show they raise gut bifidobacteria at 2.5 to 15 g a day, and several found less bloating, gas or constipation. GOS taken with an iron supplement raised iron absorption by about half. Insulin sensitivity and immune markers did not change.

Every graded outcome on this page

11 rows · line = participants

Grade CGut symptoms in students during exam stressGut symptomsn 427
Grade CCold or flu days in students during exam stressColdsn 427
Grade CStool frequency in self-reported constipationConstipationn 132
Grade CBloating, flatulence and abdominal painBloatingn 83
Grade CTrait anxiety in young womenAnxietyn 83
Grade CBowel movements in functional constipationConstipationn 63
Grade CIBS symptomsIBSn 44
Grade CIron absorption from an iron supplementIron absorptionn 34
Grade CIron absorption from a 100 mg iron tabletIron absorptionn 30
No effectInsulin sensitivity in prediabetesBlood sugarn 44
No effectImmune function and oxidative stress in older adultsImmunityn 44
8 more graded outcomes, in the evidence chapter

Key facts

What it is: Prebiotic carbohydrate made from lactose; Health Canada accepts 2 to 15 g/day as a prebiotic¹
Gut bacteria: Raised faecal bifidobacteria in each trial here that measured them, dose-dependently in constipated adults^2,7,8
Bloating and gas: 2.75 g/day cut bloating, flatulence and pain vs placebo within a week in 83 adults (crossover)⁵
Iron absorption: 15 g taken with ferrous fumarate raised iron absorption by 45% to 61% in iron-depleted women^9,10
Anxiety (no effect): 28 days of GOS did not lower trait anxiety vs placebo in 83 young women¹²
Funding: Several trials were run by GOS makers: FrieslandCampina (Biotis) and Clasado (B-GOS)^2,5

What it is

Galactooligosaccharides (GOS) are short chains of galactose made from lactose, used as a prebiotic that feeds bifidobacteria.

How it works

Established

GOS is not digested in the small intestine and is fermented by colonic bacteria, selectively increasing bifidobacteria. In iron studies, GOS given with ferrous fumarate increased iron absorption and, in the lab, iron dialyzability.^1,10

Forms

Form

The form on this page
Form	Elemental %	Absorption (human data)	GI tolerance	Studied for
GOS powder or syrup (e.g. B-GOS, Biotis GOS)	Varies¹Not applicable (carbohydrate; Health Canada counts only the declared oligosaccharide content)	Not digested in the small intestine; fermented in the colon.¹	No significant adverse reactions in a 4-week constipation trial; Health Canada lists stopping use if hypersensitivity or allergy occurs.^1,3	Constipation, bloating, IBS, exam-stress symptoms and iron absorption (2.75 to 15 g/day)

What the human research says

Human trials only, one row per outcome. The form line shows what each trial actually tested. Grades follow our evidence-grading policy.

Trial map

11 graded outcomes, largest study first

11 graded outcomes: 6 improved, 2 mixed, 3 no effect. The largest, a 2011 randomized trial of 427 people on cold or flu days in students during exam stress, found mixed results.

By sizeTable

1Cold or flu days in students during exam stress

Mixed · Grade C · RCT, 427 people, 2011

Healthy university students around final exams (University of Florida)

Dose, result and source

Every graded outcome on this page, largest study first
Outcome	Found	People	Study	Who was studied
Cold or flu days in students during exam stress	Mixed	427	Grade C · RCT, 2011	Healthy university students around final exams (University of Florida)
Gut symptoms in students during exam stress	Improved	427	Grade C · RCT, 2011	Healthy university students around final exams (University of Florida)
Stool frequency in self-reported constipation	Mixed	132	Grade C · RCT, 2022	Adults aged 18 to 59 with self-reported constipation (94% women)
Trait anxiety in young women	No effect	83	Grade C · RCT, 2025	Healthy women aged 17 to 25
Bloating, flatulence and abdominal pain	Improved	83	Grade C · Crossover RCT, 2018	Adults from the general population with bloating, abdominal pain or flatulence (UK)
Bowel movements in functional constipation	Improved	63	Grade C · RCT, 2024	Adults meeting Rome IV criteria for functional constipation (Korea; 34 GOS, 29 placebo)
IBS symptoms	Improved	44	Grade C · Crossover RCT, 2009	Adults with Rome II IBS (single centre, London)
Immune function and oxidative stress in older adults	No effect	44	No-effect grade · Crossover RCT, 2021	20 prefrail older adults and 24 healthy adults (Maastricht)
Insulin sensitivity in prediabetes	No effect	44	No-effect grade · RCT, 2017	Overweight or obese prediabetic adults aged 45 to 70 (Maastricht)
Iron absorption from an iron supplement	Improved	34	Grade C · PK study, 2019	Iron-depleted young women (median ferritin 16.4 ug/L, ETH Zurich)
Iron absorption from a 100 mg iron tablet	Improved	30	Grade C · Crossover RCT, 2022	Iron-depleted women (median ferritin 12.9 ug/L, ETH Zurich)

Every graded row

11 rows

Form tested

Showing 11 of 11 outcomes. Human trials only.

Grade CGut symptoms in students during exam stressGut symptomsFewer stress-related gut symptomsRCT
n 427 · 2011

Improved Scores for diarrhoea (P = 0.0298), constipation (P = 0.0342), abdominal pain (P = 0.0058) and indigestion (P = 0.0003) were lower after GOS supplementation.

Design: RCT, n = 427, 2011
Population: Healthy university students around final exams (University of Florida)
Form: GOS
Dose: 2.5 g or 5 g/day
Duration: 8 weeks
Source: Reference 4 · PubMed 21525194 (opens PubMed)

Grade CCold or flu days in students during exam stressColdsFewer cold days only in normal-weight studentsRCT
n 427 · 2011

Mixed 2.5 g was linked to lower cold or flu symptom intensity across stress levels, but 5 g only at lower stress; a 40% reduction in days with cold or flu was seen with 5 g in normal-weight students only, not in overweight or obese students.

Design: RCT, n = 427, 2011
Population: Healthy university students around final exams (University of Florida)
Form: GOS
Dose: 2.5 g or 5 g/day
Duration: 8 weeks
Source: Reference 4 · PubMed 21525194 (opens PubMed)

Grade CStool frequency in self-reported constipationConstipationTrend overall; more frequent stools in subgroups at 11 gRCT
n 132 · 2022

Mixed Overall, 11 g showed only a trend toward more frequent stools vs control. In those with 3 or fewer bowel movements a week at baseline and in those aged 35 or over, 11 g increased stool frequency. Faecal Bifidobacterium rose dose-dependently.

Design: RCT, n = 132, 2022
Population: Adults aged 18 to 59 with self-reported constipation (94% women)
Form: Biotis GOS
Dose: 5.5 g or 11 g/day
Duration: 3 weeks
Source: Reference 2 · PubMed 35057489 (opens PubMed)

Grade CBloating, flatulence and abdominal painBloatingLess bloating, gas and pain vs placeboCrossover RCT
n 83 · 2018

Improved Bloating, flatulence and abdominal pain scores were significantly lower than baseline and placebo by the end of week 1 (P < 0.001), sustained at week 2. No effect on bowel movements, stool consistency, quality of life or mood.

Design: Crossover RCT, n = 83, 2018
Population: Adults from the general population with bloating, abdominal pain or flatulence (UK)
Form: B-GOS
Dose: 2.75 g/day
Duration: 2 weeks per arm, 2-week washout
Source: Reference 5 · PubMed 30109908 (opens PubMed)

Grade CTrait anxiety in young womenAnxietyNo change in anxietyRCT
n 83 · 2025

No effect Trait anxiety did not differ from placebo at the end of supplementation (p = 0.443), with only a trend toward lower anxiety at follow-up (p = 0.069). Bifidobacterium rose transiently; social anxiety and depression scores did not change.

Design: RCT, n = 83, 2025
Population: Healthy women aged 17 to 25
Form: GOS
Dose: Not stated in the abstract
Duration: 28 days, with follow-up 28 days later
Source: Reference 12 · PubMed 40532960 (opens PubMed)

Grade CBowel movements in functional constipationConstipationMore bowel movements and better stool form vs placeboRCT
n 63 · 2024

Improved Bowel movements per day and stool shape improved vs control; constipation-related quality of life satisfaction increased; Bifidobacterium and Lactobacillus rose from baseline. No significant adverse reactions apart from pulse rate.

Design: RCT, n = 63, 2024
Population: Adults meeting Rome IV criteria for functional constipation (Korea; 34 GOS, 29 placebo)
Form: GOS
Dose: Not stated in the abstract
Duration: 4 weeks
Source: Reference 3 · PubMed 38787732 (opens PubMed)

Grade CIBS symptomsIBSFewer IBS symptoms, mainly at 3.5 gCrossover RCT
n 44 · 2009

Improved Bifidobacteria rose at both doses. 3.5 g improved stool consistency, flatulence, bloating, composite symptom score and global assessment; 7 g improved global assessment and anxiety scores. Placebo had no effect on clinical measures.

Design: Crossover RCT, n = 44, 2009
Population: Adults with Rome II IBS (single centre, London)
Form: Trans-galactooligosaccharide (B-GOS)
Dose: 3.5 g or 7 g/day (placebo 7 g)
Duration: 12 weeks (parallel crossover)
Source: Reference 6 · PubMed 19053980 (opens PubMed)

Grade CIron absorption from an iron supplementIron absorption28% to 61% more iron absorbed when taken togetherPK study
n 34 · 2019

Improved GOS increased fractional iron absorption from ferrous fumarate by 61% in water (P < 0.001) and 28% with a meal (P = 0.002); 4 weeks of daily GOS did not add a further effect.

Design: PK study, n = 34, 2019
Population: Iron-depleted young women (median ferritin 16.4 ug/L, ETH Zurich)
Form: GOS with 14 mg iron as ferrous fumarate
Dose: 15 g GOS per test dose; about 15 g/day for 4 weeks
Duration: Single test meals before and after 4 weeks
Source: Reference 9 · PubMed 31004135 (opens PubMed)

Grade CIron absorption from a 100 mg iron tabletIron absorptionAbout 10 mg more iron absorbed per doseCrossover RCT
n 30 · 2022

Improved Fractional iron absorption was 45% higher with GOS and 51% higher with FOS than control (P < 0.001); acacia gum had no effect (P = 0.688). Median total iron absorbed: 34.6 mg with GOS vs 23.9 mg control.

Design: Crossover RCT, n = 30, 2022
Population: Iron-depleted women (median ferritin 12.9 ug/L, ETH Zurich)
Form: GOS, FOS or acacia gum with 100 mg iron as ferrous fumarate
Dose: 15 g GOS in a single dose
Duration: Single doses, crossover
Source: Reference 10 · PubMed 35015879 (opens PubMed)

No effectInsulin sensitivity in prediabetesBlood sugarNo change in insulin sensitivityRCT
n 44 · 2017

No effect Faecal Bifidobacterium rose 5-fold, but peripheral and adipose insulin sensitivity (clamp), body composition, energy metabolism, short-chain fatty acids and inflammatory markers did not change.

Design: RCT, n = 44, 2017
Population: Overweight or obese prediabetic adults aged 45 to 70 (Maastricht)
Form: GOS
Dose: 15 g/day (placebo: maltodextrin)
Duration: 12 weeks
Source: Reference 7 · PubMed 28396144 (opens PubMed)

No effectImmune function and oxidative stress in older adultsImmunityMore bifidobacteria, no immune changeCrossover RCT
n 44 · 2021

No effect Bifidobacteria rose and microbial diversity fell in both groups; cytokine production, CRP and oxidative stress markers were not affected (P >= 0.236).

Design: Crossover RCT, n = 44, 2021
Population: 20 prefrail older adults and 24 healthy adults (Maastricht)
Form: Biotis GOS
Dose: 21.6 g/day product (15 g GOS)
Duration: 4 weeks per arm
Source: Reference 8 · PubMed 33509667 (opens PubMed)

What the numbers add up to

Each point is worked out from the data and the sources numbered on this page, with the arithmetic shown under it. It adds no new data and is not medical advice.

Bifidobacteria rise reliably; health outcomes vary

Every cited trial that measured bifidobacteria saw them rise, yet insulin sensitivity, immune markers and oxidative stress did not change.^2,7,8

An iron absorption booster

Two isotope studies from ETH Zurich found GOS taken with ferrous fumarate raised iron absorption, while acacia gum did not.^9,10

Studied doses

Doses used in research, shown against the official limits. They describe the studies, not a recommendation for you.

Studied doses in research, not advice. These are amounts used in research, not personal advice. Your clinician or pharmacist can say what fits you.

Doses studied in human trials

Doses studied in human trials ranged from 2.5 g/day for 8 weeks in stressed students to 15 g/day for 12 weeks in prediabetes; bloating relief was seen at 2.75 g/day and iron studies used single 15 g doses.^4,5,7,10

When trials measured an effect

3 outcomes

How long trials ran before effects were measured
Outcome	Measured at	Basis
Bloating, flatulence and abdominal pain	End of week 1 (sustained at week 2)	2.75 g/day B-GOS vs placebo in a crossover trial of 83 adults⁵
Stool frequency in constipation	Week 3 (end of trial)	11 g/day raised stool frequency in subgroups with 3 or fewer bowel movements a week or aged 35 or over²
Iron absorption	Same dose (taken together with the iron)	Single 15 g doses with ferrous fumarate; 4 weeks of daily GOS added no further effect^9,10

Interactions and safety

Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.

What Galactooligosaccharides meets

2 cited rows 2 tested together

Medicines 1

Galactooligosaccharides1 medicine · 1 supplement

Supplements and foods 1

Drug label or FDA notice
Measured in a human study
Tested together in a graded trial

Also cited: 2 tested together

Other medicines and supplements (timing)
- Drug labelDrug label or FDA notice
  Other medicines and supplements (timing): Health Canada's prebiotics monograph directs that GOS products be used a few hours before or after other medications or health products.
  Source 1, Health Canada, Natural and Non-prescription Health Products Directorate (2026)
IronIron page
- PK studyMeasured in a human study
  Iron supplements (ferrous fumarate): Taken together, 15 g GOS increased iron absorption from ferrous fumarate by 45% to 61% in iron-depleted women.
  Source 9, J Nutr (2019); Source 10, J Nutr (2022)
- Tested together
  Iron absorption from an iron supplement. Graded on the Galactooligosaccharides page; tested together is not a claim that they work better together.
- Tested together
  Iron absorption from a 100 mg iron tablet. Graded on the Galactooligosaccharides page; tested together is not a claim that they work better together.

Interactions

2 agents

Published interactions, grouped by kind
Agent	What the source says	Evidence
Medicines1
Other medicines and supplements (timing)	Health Canada's prebiotics monograph directs that GOS products be used a few hours before or after other medications or health products.¹	Drug label
Other supplements1
Iron supplements (ferrous fumarate)	Taken together, 15 g GOS increased iron absorption from ferrous fumarate by 45% to 61% in iron-depleted women.^9,10	PK study

Check with what you take

Kept on this device only

Add what you take. We list every interaction our evidence profiles cite between Galactooligosaccharides and your picks, and between your picks.

Add a supplement or medicine

Most cited:

Add a supplement or medicine you take to check it against Galactooligosaccharides.

Starting from one medicine? The interaction checker rates how serious each pair is, on a curated list of 19 entries for 11 medicines. This cabinet has no severity rating: it lists every interaction row our supplement pages cite.

Who should be careful

4 groups

Milk allergy and lactose: GOS is made from lactose; Health Canada requires a milk allergen statement where applicable and allows a statement that GOS may contain small amounts of lactose.¹
Allergy: Health Canada lists a known adverse reaction for GOS: stop use if hypersensitivity or allergy occurs.¹
Pregnancy, breastfeeding and children: The Health Canada monograph covers adults 18 and older, and LactMed has no GOS entry. In a 52-woman pilot trial in early pregnancy, a supplement supplying about 3.6 g GOS a day (with sialic acid) caused only mild symptoms, and gestational diabetes and newborn measures did not differ from a fructooligosaccharide placebo. NCCIH notes many supplements have not been tested in pregnant or nursing women or children.^1,13,11
Expecting metabolic benefits: 15 g a day for 12 weeks did not improve insulin sensitivity in prediabetic adults despite a 5-fold rise in bifidobacteria.⁷

How to read the label

What to check on the Supplement Facts panel.

How to read the label

2 checks

Count the GOS, not the powder

Health Canada counts only the declared oligosaccharide content; in one trial 21.6 g of product supplied 15 g of GOS.^1,8

Small doses can work for gas

Bloating relief was seen at 2.75 g a day; constipation trials used 5.5 to 11 g.^2,5

Have the bottle? Photograph its Supplement Facts panel and see each line, galactooligosaccharides (gos) included, beside the doses and forms the trials used in Label X-Ray.

Frequently asked questions

Short answers to common questions; the numbers link to the sources below.

Common questions

8 answers

Does GOS help constipation?

Somewhat. A Korean trial found more bowel movements over 4 weeks, while a Dutch trial found only a trend overall, with benefits at 11 g a day in people with few bowel movements.^2,3

Can GOS reduce bloating?

In 83 adults with bloating, 2.75 g a day of B-GOS lowered bloating, flatulence and pain scores vs placebo within a week. The trial was funded by the GOS maker.⁵

Does GOS help blood sugar or immunity?

Not in the trials here: 15 g a day did not change insulin sensitivity in prediabetes or immune markers in older adults.^7,8

Is GOS safe with lactose intolerance or milk allergy?

GOS is made from lactose. Health Canada requires milk allergen labelling where relevant and notes GOS may contain small amounts of lactose.¹

What are the side effects of GOS?

In the trials here, side effects were few: a 4-week constipation trial found no significant adverse reactions, and a pregnancy pilot reported only mild symptoms. Health Canada lists allergy as a known reaction, and GOS can contain small amounts of lactose.^1,3,13

Is GOS safe to take long term?

Long-term safety has not been studied: the trials here lasted 2 to 12 weeks. Health Canada accepts 2 to 15 g a day as a source of prebiotics for adults.^1,4,7

Should GOS be taken with iron?

In iron-depleted women, 15 g GOS taken with ferrous fumarate increased iron absorption by 45% to 61%. Health Canada's monograph otherwise directs spacing prebiotics a few hours from other products.^1,9,10

What does GOS do?

It is a prebiotic: it reaches the colon undigested and feeds bifidobacteria. Health Canada accepts 2 to 15 g a day as a source of prebiotics.¹

Sources

13 sources: trial abstracts, NIH fact sheets and reference databases. Numbers in the text link here.

Sources cited on this page

13 sources

1Prebiotics (Natural Health Product monograph, April 24, 2026)Guideline Health Canada, Natural and Non-prescription Health Products Directorate, 2026 · webprod.hc-sc.gc.ca
2Prebiotic Galacto-Oligosaccharides Impact Stool Frequency and Fecal Microbiota in Self-Reported Constipated Adults: A Randomized Clinical Trial.RCT Nutrients, 2022 · PMID 35057489
3Efficacy and safety of galacto-oligosaccharide in the treatment of functional constipation: randomized clinical trial.RCT Food Funct, 2024 · PMID 38787732
4Galactooligosaccharide supplementation reduces stress-induced gastrointestinal dysfunction and days of cold or flu: a randomized, double-blind, controlled trial in healthy university students.RCT Am J Clin Nutr, 2011 · PMID 21525194
5Effect of a prebiotic galactooligosaccharide mixture (B-GOS®) on gastrointestinal symptoms in adults selected from a general population who suffer with bloating, abdominal pain, or flatulence.Crossover RCT Neurogastroenterol Motil, 2018 · PMID 30109908
6Clinical trial: the effects of a trans-galactooligosaccharide prebiotic on faecal microbiota and symptoms in irritable bowel syndrome.Crossover RCT Aliment Pharmacol Ther, 2009 · PMID 19053980
7Supplementation of Diet With Galacto-oligosaccharides Increases Bifidobacteria, but Not Insulin Sensitivity, in Obese Prediabetic Individuals.RCT Gastroenterology, 2017 · PMID 28396144
8Galacto-oligosaccharides supplementation in prefrail older and healthy adults increased faecal bifidobacteria, but did not impact immune function and oxidative stress.Crossover RCT Clin Nutr, 2021 · PMID 33509667
9Consumption of Galacto-Oligosaccharides Increases Iron Absorption from Ferrous Fumarate: A Stable Iron Isotope Study in Iron-Depleted Young Women.PK study J Nutr, 2019 · PMID 31004135
10Prebiotic Galacto-Oligosaccharides and Fructo-Oligosaccharides, but Not Acacia Gum, Increase Iron Absorption from a Single High-Dose Ferrous Fumarate Supplement in Iron-Depleted Women.Crossover RCT J Nutr, 2022 · PMID 35015879
11Using Dietary Supplements WiselyFact sheet National Center for Complementary and Integrative Health (NCCIH), 2019 · nccih.nih.gov
12A randomised controlled trial of the effects of Galacto-Oligosaccharides on the gut brain-axis of young females.RCT Brain Behav Immun, 2025 · PMID 40532960
13Effects of galactooligosaccharides on maternal gut microbiota, glucose metabolism, lipid metabolism and inflammation in pregnancy: A randomized controlled pilot study.Pilot RCT Front Endocrinol (Lausanne), 2023 · PMID 36777355

What changed

Every dated change to this page: what changed, who made it and why.

Change log

2 entries

Dated changes to this page
Date	Change	Who and why
Oct 21, 2026all	Added a 2025 anxiety trial in 83 young women (PMID 40532960, null primary outcome) and a 2023 pregnancy pilot trial (PMID 36777355; GOS dose from the PMC full text) to the pregnancy note, and noted that LactMed has no GOS entry. Added time-to-effect rows from the bloating, constipation and iron trials, two FAQs, page questions and a gut goal link. PubMed search 2022 to 2026 found no GOS-specific meta-analysis in adults; trials of GOS combined with FOS or probiotics were not added. Interactions re-checked: nothing beyond iron and the Health Canada spacing direction.	Writer: Kymata Health EditorialTrigger: A–Z audit against docs/supplements/AUDIT-STANDARD.md
Oct 15, 2026all	Initial editorial evidence review. Every PMID checked against NCBI E-utilities esummary (title, year, journal; none retracted) and each cited abstract read; numbers taken from the abstracts. Funding: FrieslandCampina employees ran the Biotis constipation trial and Clasado funded the B-GOS bloating trial; others name no single funder. Safety from Health Canada's Prebiotics monograph (2026-04-24) and NCCIH's Using Dietary Supplements Wisely. Where to buy: we didn’t list a product; none of the stores we checked sells a single-ingredient GOS product.	Writer: Kymata Health EditorialTrigger: new page

Supplement & Nutrient Encyclopedia. Educational information only; it is not medical advice and does not replace a clinician. Dietary supplements are not intended to diagnose, treat, cure or prevent any disease. Statements about supplements have not been evaluated by the FDA.

Galactooligosaccharides (GOS)

Medicines 1

Supplements and foods 1

Other medicines and supplements (timing)

IronIron page