IngredientsGalactooligosaccharides (GOS) are oligosaccharides composed of different galactosyl residues — from 2 to 9 units — and a terminal glucose linked by β-glycosidic bonds, such as β-(1–2), β-(1–3), β-(1–4), and β-(1–6). GOS are galactose-containing oligosaccharides whose chemical structures vary by chain length, branching, and glycosyl linkages; depending on the source of β-galactosidase, the galactose released during enzymatic hydrolysis of lactose can be transferred through β(1→6), β(1→4), or β(1→3) glycosidic linkages.
GOS consists mainly of compounds with a degree of polymerization (DP) varying from 2–8 with diverse glycosidic linkages. More than 100 diverse oligosaccharide molecules have been identified in GOS mixtures, and some of these structures are also present in human milk, such as 3′-galactosyllactose (3′-GL), 4′-galactosyllactose (4′-GL), and 6′-galactosyllactose (6′-GL).
There are two main types of GOS: α-GOS and β-GOS. α-GOS typically contain between one to four galactose moieties connected with alpha bonds and function as storage reserves and cryoprotectants in frost-resistant plant organs. Another type comprises natural isolates from soybeans: β-galactooligosaccharides including raffinose, stachyose, and verbascose, which consist of galactose residues linked β-1,6 to the glucose moiety of sucrose.
The commercially authorized novel food form is mainly composed of GOS consisting of different galactosyl residues (two to nine) linked to a terminal glucose by a β-glycosidic bond, but also contains lactose and its monomers (galactose and glucose).
GOS naturally occur at low concentrations in the milk of many animals, including humans and cows, but they can also be produced by chemical glycosylation or biocatalysis. GOS are carbohydrates naturally found in legumes such as lentils and chickpeas, as well as dairy products and certain root vegetables.
GOS are of special interest to human nutrition because of the presence of structurally related oligosaccharides together with different complex structures in human breast milk. They have remarkable prebiotic effects and structural similarity to human milk oligosaccharides.
These oligosaccharides are synthesised from lactose by a β-galactosidase transfer reaction, resulting in the formation of a family of di- to hexasaccharides, with the end products depending on the source of the enzyme. They can also be produced enzymatically through the transgalactosylation of galactosidases, and the chemical composition of GOS can vary significantly depending on the enzyme source and reaction conditions used in their production.
β-galactosidases from Kluyveromyces lactis and Aspergillus oryzae mainly produce β-1,6-linked GOS. The commercial novel food form is produced as a syrup containing ≥ 55% GOS (w/w dry matter) and is produced enzymatically by two β-galactosidases.
The production methods of GOS include natural extraction (primarily from the seeds of leguminous plants), hydrolysis of polysaccharides, chemical synthesis, and enzymatic synthesis via the transglycosidase activity of β-galactosidase. The enzymatic reaction has the advantages of safety and high efficiency and does not cause environmental pollution, and is currently the main method used.
GOS is available commercially in several forms, including syrups, powders, and capsule/tablet dosage forms. GOS has been authorized for use as an ingredient in milk-based, non-exempt infant formula for term infants, and in milk and milk products, soups, bakery products, cereals, fruit and vegetable juices, jellies and jams, and nonalcoholic beverages. A new generation of lactose-free prebiotic GOS substances has also emerged with the development of plant-based galactooligosaccharides extracted from legumes.
At the beginning of the 20th century, it was believed that the high acidity of the feces of breastfed infants was due to the action of lactobacilli present in the colonic microbiota, and afterward it was found that the growth-stimulating factor of colonic Lactobacillus bifidus growth consisted of a variety of oligosaccharides occurring in human milk.
Human milk oligosaccharides (HMO) are prominent among the functional components of human breast milk. While HMO have potential applications in both infants and adults, this potential is limited by the difficulties in manufacturing these complex structures. Consequently, functional alternatives such as galacto-oligosaccharides are under investigation, and nowadays, infant formulae are supplemented with galacto-oligosaccharides to mimic the biological effects of HMO.
Non-digestible oligosaccharides (NDO) are a major constituent of human milk, and human NDO promote the growth of a beneficial microbiota dominated by bifidobacteria. Some human NDO are also known to prevent directly the adhesion of pathogens and toxins. The presence of a microbiota rich in bifidobacteria is associated with a reduced risk of atopic diseases such as atopic dermatitis, food allergy, and asthma and with a reduced risk of infections with pathogens. Non-digestible oligosaccharides are therefore thought responsible for the lower incidence of infections and of atopic diseases observed in human milk–fed infants compared with formula-fed infants.
Although GOS as an isolated or enriched ingredient is a relatively recent development in food science, the compositional basis for its use — the selective bifidogenic action of milk oligosaccharides — has been recognized since at least the early 20th century in infant nutrition research. Currently, only three dietary ingredients have achieved prebiotic status in the European Union: fructooligosaccharides (and inulin), galactooligosaccharides, and lactulose.
GOS generally comprise two to eight sugar units, including galactose and glucose, which are synthesized from substrate lactose by microbial β-galactosidase. The bioactivity of GOS is structurally determined: the type and position of glycosidic linkages, degree of polymerization, and branching pattern all influence which colonic bacterial populations preferentially ferment them.
GOS is an indigestible food component that can pass through the upper gastrointestinal tract relatively intact and ferment in the colon to produce short-chain fatty acids (SCFAs) that further regulate the body's intestinal flora. Most HMO and related GOS structures are resistant to hydrolysis by intestinal lactase (β-galactosidase) or other brush border enzymes, and there is evidence that the majority survive passage through the small intestine and enter the colon where they act as prebiotics that stimulate the growth of beneficial colonic bacteria.
GOS are bifidogenic and lactogenic prebiotics; however, GOS utilization is strain-dependent. There is convincing in vivo evidence that prebiotics can promote growth of bifidobacteria in the intestinal tract of infants and adults, and for GOS in particular, 2 to 3 log increases in the number of bifidobacteria in fecal samples obtained from individual adults have been reported.
SCFAs are the end products of fermentation by microorganisms such as Bifidobacterium and Lactobacillus. In an in vitro test, fecal microbiota obtained from breastfed infants produced the same SCFAs as those obtained from supplemental GOS feeding. SCFAs produced by dietary fiber fermentation increase intestinal production of GLP-1 and PYY, which improves insulin secretion.
Pro- and prebiotic components, including GOS, by themselves improved markers of colonic permeability, providing a rationale for their use in pathologies with an underlying leakiness of the gut.
Several mechanisms have been proposed for GOS-enhanced mineral absorption: (1) bacterial fermentation of acidic metabolites in the colon reduces the local pH of the intestine, thereby increasing the luminal concentration of calcium ions and increasing passive calcium absorption; (2) SCFAs modify the charge of calcium, promote calcium channels, and increase calcium absorption.
GOS have been found to modulate immune function, improve nutrient absorption (e.g., calcium and iron), reduce stress-related symptoms and neuroinflammation, and support gut barrier function.
The gut and the brain are intimately connected via the gut–brain axis, which involves bidirectional communication via neural, endocrine, and immune pathways. GOS are prebiotics that modulate gut microbiota and are implicated in the gut–brain axis (GBA), with preclinical models reporting effects on neurochemistry, brain function, and cognition.
Evidence strength: Strong; multiple human RCTs
GOS are among the most established prebiotics, and many studies have shown that GOS impact gut microbiota in various target groups, exerting a beneficial role on gut microbiota composition and activity, and health outcomes. An increase of Bifidobacterium abundance is robustly reported in all these studies.
A study by Tamai et al. (1992) described the bifidogenic effect of 20 days' consumption of 2 g GOS per day in healthy males aged 25–60 years. Additionally, 1 g GOS per day for 21 days increased levels of bifidobacteria in a study with healthy men aged 26–57 years.
Results obtained in formula-fed infants showed that GOS was able to induce the development of gut microbiota similar to breast-fed infants. A clinical study (Moro et al., 2002) showed that supplementation of GOS at a ratio of 90–10% produced bifidobacterial effects similar to breastfeeding.
A double-blind, placebo-controlled, crossover study in healthy women investigated the effects of low-dose GOS supplementation. As supplements in the form of capsules or tablets can accommodate only a limited amount of ingredients, there is interest in substantiating low dosages of GOS; so far, only a limited number of studies have addressed the effectiveness of low daily dosages (≤2 g) on gut microbiota composition in a healthy population.
Evidence strength: Moderate to strong; multiple RCTs in infants
Galactooligosaccharides are mainly used in infant nutrition, follow-on formulas, and growing-up milks. Their nutritional benefits are the most important driver for their use in infant nutrition, and GOS are used in infant food products as human milk oligosaccharide–mimicking fractions. Infant formulas typically contain 6 to 8 g of GOS per liter of formula.
Different levels of GOS in infant formula increase stool frequency, and compared with infants receiving standard formula, infants receiving prebiotic supplements had softer stools and stool consistency similar to that of breastfed infants.
GOS preparations have been studied under in vitro and in vivo conditions for their bifidogenic and health-promoting properties, and they have a generally recognized status as an effective prebiotic. Their applications in infant nutrition have been well established, yet the precise and long-term impact of GOS inclusion on the microbiota development and its developmental consequences for the host require significant additional investigations.
Evidence strength: Moderate; small RCTs with positive but not entirely consistent findings
A clinical investigation examined the efficacy of a novel prebiotic trans-galactooligosaccharide in IBS sufferers. In all, 44 patients with Rome II positive IBS completed a 12-week single-centre parallel crossover controlled clinical trial. Patients were randomized to receive either 3.5 g/d prebiotic, 7 g/d prebiotic, or 7 g/d placebo. IBS symptoms were monitored weekly and scored according to a 7-point Likert scale. Changes in faecal microflora, stool frequency and form, subjective global assessment (SGA), anxiety, depression, and quality-of-life scores were also monitored.
The prebiotic significantly enhanced faecal bifidobacteria (3.5 g/d, P < 0.005; 7 g/d, P < 0.001). The prebiotic at 7 g/d significantly improved SGA (P < 0.05) and anxiety scores (P < 0.05). The galactooligosaccharide acted as a prebiotic in specifically stimulating gut bifidobacteria in IBS patients and was effective in alleviating symptoms.
A subsequent study explored combined GOS and low-FODMAP dietary approaches. A randomized, placebo-controlled, 3-arm trial enrolled 69 Rome III adult IBS patients from secondary care in the United Kingdom, who were randomized to a sham diet with placebo supplement (control), or low-FODMAP diet (LFD) supplemented with either placebo or 1.4 g/d B-GOS for 4 weeks.
Evidence strength: Moderate; RCTs in adults, including the elderly
GOS plays an important role in alleviating lactose intolerance and preventing constipation. A clinical trial demonstrated that women with constipation who took GOS for 3 weeks significantly relieved constipation symptoms (Teuri and Korpela, 1998). Prebiotics increase the water-binding capacity of the gut; these movements increase stool weight and frequency, soften stools, and result in reduced transit time.
Evidence strength: Moderate; placebo-controlled RCTs in elderly populations
Aging is associated with reduced numbers of beneficial colonic bifidobacteria and impaired immunity. A double-blind, placebo-controlled, crossover study assessed the effect of a prebiotic GOS mixture (B-GOS) on immune function and fecal microflora composition in healthy elderly subjects. In this study, 44 elderly subjects were randomly assigned to receive either a placebo or B-GOS treatment (5.5 g/d), consuming treatments for 10 weeks followed by a 4-week washout period before switching to the other treatment for the final 10 weeks.
A subsequent study examined the influence of a galacto-oligosaccharide mixture (B-GOS) on gut microbiota, immune parameters, and metabonomics in elderly persons (Vulevic et al., 2015).
Primary outcomes across adult GOS trials have varied from immune read-outs, iron absorption, mental wellbeing, and gut health.
Evidence strength: Preliminary to moderate; several RCTs but results need replication
One study (2009) showed that GOS supplementation in high-risk infants reduced total immunoglobulin responses, modulated allergic responses, and reduced IgE. The presence of a microbiota rich in bifidobacteria is associated with a reduced risk of atopic diseases such as atopic dermatitis, food allergy, and asthma.
A randomized, double-blind, placebo-controlled trial by Kukkonen et al. (2007) examined probiotics and prebiotic galacto-oligosaccharides in the prevention of allergic diseases. More recently, searching PubMed retrieved clinical trials published in 2025 covering topics including the effects of GOS-containing synbiotics on infant allergy outcomes following maternal supplementation.
Evidence strength: Moderate; double-blind crossover RCTs in humans
The beneficial effects of GOS on calcium absorption and bone mineralization have been demonstrated in postmenopausal women (Abrams et al., 2005). A double-blind crossover trial (Whisner et al., 2013) found that galacto-oligosaccharides increase calcium absorption and gut bifidobacteria in young girls.
Among the mechanisms proposed: bacterial fermentation of acidic metabolites in the colon reduces intestinal pH, increasing luminal calcium ion concentrations and passive calcium absorption. One study (Scholz-Ahrens et al., 2002) demonstrated in ovariectomized rats and pigs that administration of GOS decreased intestinal pH, increased bone mineralization, inhibited estrogen-deficiency–induced bone degradation, and preserved bone structure. The animal data cannot be directly extrapolated to humans and clinical evidence in this area, while promising, remains limited.
Evidence strength: Moderate; one adequately powered double-blind RCT
Prebiotics have attracted interest for their ability to positively affect colonic microbiota composition, thus increasing resistance to infection and diarrheal disease. One study assessed the effectiveness of a prebiotic galacto-oligosaccharide mixture (B-GOS) on the severity and/or incidence of traveler's diarrhea in healthy subjects. The study was a placebo-controlled, randomized, double-blind parallel-design study in 159 healthy volunteers who traveled for a minimum of 2 weeks to a country of low or high risk for traveler's diarrhea, with the investigational product B-GOS compared against a maltodextrin placebo. Results showed significant differences between the B-GOS and the placebo group in the incidence (P < 0.05) and duration (P < 0.05) of traveler's diarrhea.
Evidence strength: Preliminary; small RCTs with promising but mixed results requiring replication
A Scientific Reports study (Johnstone et al., 2021) reported effects of the GOS intervention on self-reported high trait anxiety, attentional bias, and bacterial abundance, suggesting that dietary supplementation with a GOS prebiotic may improve indices of pre-clinical anxiety. The results showed that GOS supplementation improved self-reported anxiety levels, reduced attentional bias to negative stimuli, and promoted the growth of Bifidobacterium, thereby altering the gut microbiome composition in participants.
A subsequent, larger, double-blind, placebo-controlled trial attempted to replicate and extend these findings. This trial enrolled 83 healthy females (17–25 years) who received GOS or placebo for 28 days, with assessments at baseline, endline, and 28 days post-supplementation. The primary outcome was trait anxiety; secondary outcomes included brain-based levels of GABA and glutamate in the dorsolateral prefrontal cortex, anterior cingulate cortex, and inferior occipital gyrus (measured with 1H-MRS), and gut microbiome composition.
Trait anxiety did not differ significantly between groups at endline, though trends favoured lower anxiety in the GOS group at follow-up (p = 0.069). GOS reduced GABA at trend significance in the inferior occipital gyrus and dorsolateral prefrontal cortex in high-anxious participants, with effects persisting at follow-up. GOS increased Bifidobacterium abundance (p = 0.001) but did not affect overall microbiome diversity.
Reviews and meta-analyses on the efficacy of prebiotics for reducing anxiety symptomology are mixed, calling for further well-controlled trials in human participants.
An independent crossover study by Dahl et al. (2023) assessed FOS and GOS effects on biological markers of stress. That study aimed to determine fructooligosaccharide (FOS) and galactooligosaccharide (GOS) effects on biological markers of stress and inflammation and mental health symptoms in adults. This reflects the mixed state of evidence in this emerging area.
Evidence strength: Preliminary; small RCTs, mixed metabolic findings
GOS, as a non-digestible, soluble fibre with prebiotic properties, promotes the growth of beneficial bacteria such as bifidobacteria and lactobacilli, which produce SCFAs that improve insulin sensitivity and glucose metabolism. However, a key gastroenterology study referenced in the literature — "Supplementation of Diet With Galacto-oligosaccharides Increases Bifidobacteria, but Not Insulin Sensitivity, in Obese Prediabetic Individuals" (Gastroenterology, 2017) — found that while the bifidogenic effect of GOS was confirmed, no improvement in insulin sensitivity was demonstrated in an obese, prediabetic population. This underscores the importance of distinguishing microbiota effects from downstream metabolic outcomes.
A randomized, double-blind, placebo-controlled, parallel-arm clinical trial in obese humans compared the ecological and physiological impact of GOS and probiotic strains when used on their own or as synbiotic combinations. Although the synbiotic combinations tested in this study did not demonstrate functional synergism, findings clearly showed that the pro- and prebiotic components by themselves improved markers of colonic permeability.
Currently ongoing clinical trials are investigating GOS in applications such as effects on blood glucose levels in healthy adults and graft-versus-host disease following allogeneic stem cell transplants.
GOS is commercially available as a liquid syrup, powder, and in capsule or tablet form. Dosages reported in clinical studies vary considerably by population and indication:
Following a request from the European Commission, the EFSA Panel on Nutrition, Novel Foods and Food Allergens was asked to deliver an opinion on the extension of use of galacto-oligosaccharides as a novel food pursuant to Regulation (EU) 2015/2283. The novel food is a syrup containing ≥ 57% w/w GOS (w/w dry matter) consisting of different galactosyl residues linked to a terminal glucose by a β-glycosidic bond, also containing lactose and its constituent monomers. The novel food is already authorized and included in the Union list of novel foods.
The EFSA Panel concluded that the novel food, composed of ≥ 57% GOS dry matter, lactose, and related saccharides, is safe under the proposed conditions of use.
The US FDA has reviewed multiple GRAS notices for GOS, authorizing its use in milk-based, non-exempt infant formula for term infants at a level up to 7.2 g GOS/L of reconstituted or ready-to-feed formula, and in milk and milk products, soups, bakery products, cereals, fruit and vegetable juices, jellies and jams, and nonalcoholic beverages.
Clinical trials have generally found GOS to be very well tolerated in infants, adolescents, adults, the elderly, and during pregnancy, with gastrointestinal side effects (e.g., bloating) only noted with higher doses in most research. Supplemental GOS is generally very well tolerated, even in individuals who react to other prebiotics (e.g., FOS and lactulose) and even in those patients who react to food sources of GOS (i.e., legumes).
It is noted that the total intake at the highest mean (8.7–22.0 g/day) is below the adequate intake (AI) of 25 g/day for dietary fibre set to ensure normal laxation in adults, while the highest 95th percentile (27.2–41.6 g/day) is higher than the AI. When maximum use as a food supplement is added to the highest 95th-percentile combined intake from all proposed and authorized food categories, a total intake up to 58 g GOS/day is estimated. This highest intake level would exceed the AI for dietary fibre; however, no tolerable upper intake level for dietary fibre has been set, and only transient gastrointestinal symptoms may be related to high intake of fibre.
Three published subchronic oral repeat-dose studies using GOS in rats reported no toxicologically relevant adverse effects even at the highest doses tested. GOS does not produce adverse test-article–related effects in juvenile rats at doses up to 3 g GOS/kg bw/day except for increased weights of cecum. The cecal enlargement was not correlated with histopathological changes and was completely reversed following a 14-day recovery period. Cecal enlargement is a common finding for non-digestible substances administered in rodent toxicology tests.
Because the predominant commercial form of GOS is produced from lactose and contains residual lactose, galactose, and glucose as co-constituents, it is relevant for individuals with lactose intolerance or cow's milk allergy. The prevalence of lactose intolerance and cow's milk allergy is rising in infants and young children, and most prebiotics for infants are produced from dairy raw materials. A new generation of lactose-free prebiotic GOS substances has emerged with the development of plant-based galacto-oligosaccharides extracted from legumes.
Allergic reactions were first reported in Japanese oyster shuckers who developed anaphylaxis after consumption of GOS-supplemented lactic acid beverages. This was later shown to result from cross-reactivity of GOS with the Hoya antigen derived from sea squirts present on oyster shells. This is an uncommon and occupationally specific finding, but represents a documented adverse event in the scientific literature.
No pharmacokinetic drug interactions for GOS have been established in peer-reviewed clinical literature as of the current evidence base. Because GOS exerts its effects primarily in the colon via microbial fermentation rather than systemic absorption, direct pharmacological interactions are not anticipated. GOS is neither digested nor absorbed until it reaches the large intestine, where it is hydrolyzed to glucose and galactose and is subsequently metabolized to short-chain fatty acids, CO₂, and H₂ gas by the intestinal microbiota. Theoretically, because GOS alters colonic microbiota composition, it could indirectly influence the metabolism of co-administered substances whose biotransformation involves gut bacteria, though no specific interactions have been documented in controlled human trials.
Health conditions that Galactooligosaccharides may help support.
Galacto-oligosaccharides (GOS) are among the most evidence-supported prebiotics for infant and child gut health, widely used in infant formulas. Multiple clinical studies confirm GOS supplementation in infants sustains high levels of stool bifidobacteria, supports softer stools, and modulates gut microbiota favorably compared to unsupplemented formula. GOS are recommended in infant formula by international pediatric nutritional guidelines.
Galactooligosaccharides (GOS) are prebiotic fibers that promote Bifidobacterium growth in the infant and child gut and have been used in infant formula to support immune development. GOS combined with probiotic strains significantly reduced RTI incidence and severity in multiple large RCTs. GOS also enhances macrophage activity and intestinal IgA production relevant to mucosal immunity.
Galactooligosaccharides (GOS) are prebiotic fibers included in fiber mixture studies for constipation. A pediatric RCT of a fiber mixture containing transgalacto-oligosaccharides and inulin (8 weeks, n=135 children) showed results comparable to lactulose for childhood constipation. GOS selectively ferment by Bifidobacterium, increasing fecal bulk and improving gut transit.
Galactooligosaccharides (GOS) are well-established prebiotics synthesized from lactose that selectively promote Bifidobacterium and Lactobacillus. They are incorporated into infant formulas as HMO analogs and are among the prebiotics with strongest evidence from human RCTs for gut microbiota modulation.
Galacto-oligosaccharides (GOS) are prebiotics with clinical RCT evidence for gut-brain axis effects in humans. A controlled study showed GOS consumption reduced anxiety and increased beneficial gut microbial populations in healthy young females. GOS selectively feeds Bifidobacterium species, promotes SCFA and GABA production, and modulates neuroinflammatory pathways.
Galactooligosaccharides (GOS) have IBS-specific RCT evidence. A double-blind placebo-controlled RCT (Silk et al., Aliment Pharmacol Ther 2009, n=44 IBS patients) found 3.5 g/day GOS significantly improved stool consistency, bloating, and anxiety scores in IBS patients alongside beneficial microbiota changes. Unlike higher-FODMAP prebiotics, low-dose GOS appears tolerable and directly beneficial in IBS.
Galactooligosaccharides (GOS) are prebiotics that have been studied specifically for lactose intolerance in multiple clinical trials. A 377-subject randomized, double-blind, multisite placebo-controlled trial found that 30-day GOS supplementation significantly reduced LI symptoms and shifted the fecal microbiome toward lactose-fermenting bacteria. GOS exerts its benefit by selectively enriching Bifidobacterium and other colonic lactose-metabolizing species.
Body systems that Galactooligosaccharides may help support.
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