What are the galacto-oligosaccharides in milk?

December 10, 2025 •

4 min read

Galacto-oligosaccharides, or GOS, are a type of non-digestible carbohydrate or prebiotic fiber that serves as a food source for beneficial bacteria in the large intestine. Found naturally in small amounts in mammalian milk, they are more prominently known as supplemental ingredients in infant formula and various functional foods for promoting gut health.

Quick Summary

Galacto-oligosaccharides (GOS) are dairy-derived prebiotics synthesized from lactose. They support digestive wellness and immune function by selectively feeding good bacteria in the gut, aiding infants and adults.

Key Points

What are galacto-oligosaccharides (GOS): GOS are a type of prebiotic fiber, derived from milk lactose, that are resistant to digestion and selectively promote the growth of beneficial gut bacteria.
Prebiotic Function: By acting as food for gut flora, GOS contribute to a balanced microbiome, which is essential for digestive health and a well-functioning immune system.
Origin in Milk: While present naturally in human milk, commercially produced GOS are synthesized from lactose, often sourced from dairy whey, using enzymes.
Benefits for Infants: Supplementing formula with GOS aids in establishing a healthy gut microbiota in infants, similar to that of breastfed babies, and supports immune development.
Digestive Benefits for Adults: For adults, GOS can alleviate symptoms of lactose intolerance and improve bowel regularity, especially for those experiencing constipation.
Mild Side Effects: High consumption of GOS may lead to minor gastrointestinal discomforts such as gas, bloating, and cramps, though they are generally well-tolerated.

The role of galacto-oligosaccharides (GOS)

Galacto-oligosaccharides (GOS) are important functional oligosaccharides, a type of carbohydrate with unique health benefits beyond basic nutrition. They are a group of sugar chains that are resistant to digestion in the upper gastrointestinal tract. This indigestible nature allows them to reach the colon intact, where they are fermented by the gut microbiota, primarily by beneficial bacteria such as Bifidobacterium and Lactobacillus.

Unlike simple sugars, GOS do not provide calories and contribute to gut health by promoting the growth of these helpful microorganisms. This process is the foundation of their prebiotic effect.

How GOS are synthesized from milk lactose

While present in natural milk, commercially available GOS are produced by enzymatic synthesis using lactose, the main sugar in milk, as a starting material. This process involves a transgalactosylation reaction catalyzed by the enzyme $\beta$-galactosidase. In this reaction, the enzyme transfers galactose units from lactose molecules to other lactose molecules, forming chains of galacto-oligosaccharides of varying lengths. This method offers a cost-effective way to produce GOS for use as a functional ingredient in various food products.

Comparison: GOS in human vs. cow's milk

Human milk contains a complex and diverse array of oligosaccharides (Human Milk Oligosaccharides or HMOs), which are structurally different and far more abundant than the oligosaccharides found in cow's milk. Cow's milk-based infant formulas, which naturally contain very low levels of oligosaccharides, are often supplemented with a specific blend of GOS and other prebiotics (like fructo-oligosaccharides or FOS) to mimic the prebiotic effects of human breast milk.

Human Milk Oligosaccharides (HMOs): Highly complex and diverse, acting as key prebiotics and immune modulators in infant nutrition.
Cow's Milk Oligosaccharides (BMOs): Much lower in concentration and simpler in structure than HMOs, but some dairy streams like whey can be processed to yield more complex oligosaccharides.

Health benefits of GOS from milk

The consumption of GOS derived from milk has been linked to numerous health benefits, particularly related to gut health and immunity.

For infants

Microbiota Development: GOS promote the growth of beneficial gut bacteria, which helps establish a healthy and balanced gut microbiota in infants, especially those fed formula.
Immune System Support: A healthy gut microbiome is crucial for immune system development. GOS can support immune function and may reduce the incidence of infections, like respiratory and gastrointestinal issues, in formula-fed infants.
Improved Digestion: GOS intake can lead to softer, more regular stools in infants, making consistency closer to that of breastfed babies.
Enhanced Mineral Absorption: Some studies suggest that GOS can increase the absorption of minerals like calcium and iron.

For adults

Digestive Regulation: GOS consumption can relieve symptoms of constipation by increasing stool frequency and promoting softer stools.
Lactose Intolerance Relief: In individuals with lactose intolerance, GOS may help mitigate symptoms like pain and bloating by improving lactose digestion through the beneficial bacteria in the gut.
Gut Flora Modulation: GOS support a balanced gut microbiota in adults by fostering the growth of Bifidobacterium and other beneficial bacteria.

GOS vs. Other Common Prebiotics

Understanding how different prebiotics compare can help in choosing the right functional foods or supplements. The primary differences lie in their molecular structure, origin, and the specific bacteria they nourish.


Feature	Galacto-Oligosaccharides (GOS)	Fructo-Oligosaccharides (FOS) & Inulin	Dietary Fiber
Origin	Derived from lactose in milk (dairy).	Found in plants like onions, bananas, and chicory root.	Broad category from various plant sources.
Structure	Chains of galactose units.	Chains of fructose units.	Varies widely in composition and solubility.
Beneficial Bacteria	Selectively promotes Bifidobacteria and Lactobacillus.	Primarily stimulates Bifidobacteria and Lactobacilli.	Varies depending on fermentability.
Effect on Gut	Fermented in the colon, producing beneficial short-chain fatty acids (SCFAs).	Fermented in the colon, also producing SCFAs.	Promotes bowel regularity and gut health.
Digestion	Resists digestion in the upper GI tract.	Resists digestion until reaching the colon.	Varies, with some types being fully indigestible.
Taste	Mildly sweet.	Mildly sweet to tasteless.	Tasteless.

Potential side effects and considerations

While generally recognized as safe, GOS may cause mild gastrointestinal side effects in some individuals, particularly at higher doses. This is a normal physiological response to the fermentation process in the colon. Common side effects can include gas, bloating, and stomach cramps. It is advisable to start with a lower dose and gradually increase it to allow the gut to adjust.

Some research also suggests GOS can increase immune system activity, which could be a concern for individuals with autoimmune diseases. Anyone with a pre-existing condition should consult a healthcare provider before starting GOS supplementation. Additionally, individuals allergic to certain types of dust mites may have a higher risk of allergic reactions to GOS, as some research has indicated.

Conclusion

Galacto-oligosaccharides are valuable prebiotics derived from milk lactose that play a significant role in promoting a healthy gut microbiome. By feeding beneficial bacteria like Bifidobacterium, they contribute to improved digestion, enhanced immunity, and nutrient absorption in both infants and adults. While naturally present in human milk, they are added to infant formulas and functional foods to replicate these important prebiotic effects. Despite their benefits, it's wise to start with small doses to minimize potential mild side effects like bloating and gas. With growing interest in gut health, GOS continue to be a key ingredient in functional foods and nutritional supplements aiming to support overall digestive wellness. For more on how GOS interact with the gut, you can review research on the NIH website.

Frequently Asked Questions

GOS are prebiotics, meaning they are non-living, non-digestible fibers that serve as food for beneficial bacteria already present in the gut. Probiotics, on the other hand, are live, beneficial bacteria themselves that are introduced into the body.

No, human milk contains significantly higher concentrations and a more diverse range of oligosaccharides (HMOs) than cow's milk. Commercial GOS are often added to cow's milk-based infant formulas to replicate the prebiotic benefits of breast milk.

GOS are commonly added to infant formulas as a prebiotic ingredient to foster the growth of beneficial bacteria, such as Bifidobacteria and Lactobacilli, in the infant's developing gut.

Yes, some studies show that GOS can alleviate symptoms of lactose intolerance by promoting the growth of beneficial bacteria that can assist with lactose digestion in the colon.

At higher doses, some individuals may experience mild side effects such as bloating, gas, stomach cramps, and diarrhea. It is generally recommended to introduce GOS gradually to minimize these effects.

The main function of GOS is to act as a fermentable food source for beneficial gut bacteria, which in turn produce short-chain fatty acids (SCFAs). These SCFAs nourish the cells lining the colon and contribute to overall gut health.

Yes, GOS can benefit adults by improving gut microbiota balance, relieving constipation, and potentially aiding those with lactose intolerance. They are found in many functional foods and supplements.

Commercially produced GOS are typically manufactured from lactose derived from dairy by-products, such as whey. An enzyme, $\beta$-galactosidase, is used to convert the lactose into GOS chains.