Human Milk Oligosaccharide (HMO) is a natural nutrient found in human milk. Despite having "saccharides" in the name, they are not carbohydrates in the traditional sense that taste sweet, but rather a large group of structurally diverse oligosaccharides in human milk. Human Milk Oligosaccharides are the third most abundant solid component in human milk, after lactose and fats. In colostrum, the human milk oligosaccharide content is 20-25g/L, while in mature milk, it ranges from 7.5-20g/L.

Human Milk Oligosaccharides are composed of five monosaccharides (D-glucose, D-galactose, N-acetylglucosamine, L-fucose, and N-acetylneuraminic acid, also known as sialic acid) in varying proportions. Based on the type of monosaccharide at the molecular terminal, human milk oligosaccharides can be categorized into three major types: fucosylated human milk oligosaccharides (neutral), non-fucosylated human milk oligosaccharides (neutral), and sialylated human milk oligosaccharides (acidic). To date, scientists have identified more than 200 different oligosaccharides in human milk. Although most of these oligosaccharides are not digested or absorbed in the small intestine, they serve as prebiotics in the colon, stimulating the growth of beneficial bacteria and acting as receptor analogs to inhibit the adhesion of pathogenic microorganisms.

The functions of human milk oligosaccharides in breast milk include immune regulation, improving the gut microbiota, and promoting brain development.

- Immune regulation: Human Milk Oligosaccharides inhibit viral infections, resist bacterial invasion, and help strengthen the baby's own defense barriers.

- Improving gut microbiota: By stimulating the growth of beneficial bacteria such as bifidobacteria and lactobacilli in the gut, human milk oligosaccharides indirectly suppress the growth of harmful bacteria, maintaining a healthy gut microbiome balance in infants.

- Promoting brain development: Human Milk Oligosaccharides are rich in sialic acid, which helps promote brain development.

Clinical studies have shown that human milk oligosaccharides can increase the proportion of bifidobacteria in infants' intestines, reduce the incidence of lower respiratory tract infections and otitis media, and decrease the use of antipyretics and antibiotics.