One of the major solid components of breastmilk are human milk oligosaccharides (HMOs).3They are complex sugars composed of five building blocks (D-glucose, D-galactose, N-acetylglucosamine, L-fucose and sialic acid). The arrangement of these building blocks determines their structure and function. More than 200 HMOs have been identified and can be grouped into three HMO categories.