After lactose and fat, HMOs are the third largest component of breast milk. They are a complex mixture of indigestible carbohydrates and have a high structural diversity. Over 150 different HMOs have already been identified.

The types and concentrations of HMOs vary greatly between breastfeeding women and also during the course of breastfeeding. Nevertheless, the two HMOs 2'-fucosyllactose (2'-FL; fucolysed) and lacto-N-neotetraose (LNnT; neutral) are found in human milk in the largest amounts.
Technical progress means it is now possible to produce HMOs industrially and add them to infant milks.

The HMOs can be classed as dietary fibres, meaning that the infant cannot digest the HMOs directly. Instead, they are metabolised via the intestinal bacteria. This promotes the growth of healthy bacteria, such as bifidobacteria and lactic acid bacteria, and thus suppresses the growth of pathogens. During the metabolism of HMO, the bacteria form short-chain fatty acids, which change the intestinal environment in a way that reduces the growth of pathogenic germs.