Galacto-oligosaccharides (GOS) are naturally functioning oligosaccharides formed by transgalactosylation of β-galactosidase. It is an important active substance in milk and a commonly used prebiotic. It has the ability to prevent diarrhoea or intestinal obstruction, regulate intestinal flora, promote the proliferation of bifidobacteria, prevent cancer, have a positive effect on lipid metabolism, stimulate the absorption of minerals, and have immunomodulatory properties. It can be used to treat eczema (atopic dermatitis), infantile colic, lactose intolerance, constipation and intestinal disorders caused by ulcerative colitis.