Milk oligosaccharides seem to be more abundant in humans than in other animals and to be more complex and varied. Oligosaccharides in primate milk are generally more complex and diverse than in non-primates.
Human milk oligosaccharides (HMOs) form the third most abundant solid component (dissolved or emulsified or suspended in water) of human milk, after lactose and fat. HMOs are present in a concentration of 11.3 – 17.7 g/L (1.5 oz/gal – 2.36 oz/gal) in human milk, depending on lactation stages. Approximately 200 structurally different human milk oligosaccharides are known, and they can be categorized into fucosylated, sialylated and neutral core HMOs. The composition of human milk oligosaccharides in breast milk is individual to each mother and varies over the period of lactation. The dominant oligosaccharide in 80% of all women is 2′-fucosyllactose, which is present in human breast milk at a concentration of approximately 2.5 g/L; other abundant oligosacchadies include lacto-N-tetraose, lacto-N-neotetraose, and lacto-N-fucopentaose. It has been found by numerous studies that the concentration of each individual human milk oligosaccharide changes throughout the different periods of lactation (colostrum, transitional, mature and late milk) and depend on various factors such as the mother's genetic secretor status and length of gestation.