Human Milk Proteins: Composition and Physiological Significance

Human milk (HM) contains hundreds of proteins with very diverse functions that likely contribute to the short- and long-term beneficial effects of breastfeeding. These functions include serving as a source of amino acids, improving the bioavailability of micronutrients, including vitamins, minerals, and trace elements, providing immunologic defense, stimulating intestinal growth and maturation, shaping the microbiome, and enhancing learning and memory. Human milk proteins can be broadly classified into 3 categories: caseins, whey proteins, and mucins, which are present in the milk fat globule membrane. HM is whey predominant; however, the whey/casein ratio of HM changes from 90/10 in colostrum to 60/40 in mature HM. The whey proteins present in significant quantities in the whey fraction are α-lactalbumin, lactoferrin, IgA, osteopontin, and lysozyme. Additionally, bioactive peptides are formed during digestion of casein and whey, and glycans from glycoproteins are bifidogenic, adding further complexity to the functional properties of HM proteins. Recent advances in dairy technology have enabled isolation of bioactive milk proteins from bovine milk in sufficient quantities for clinical studies and, in some cases, addition to commercially available infant formula. Herein, the current evidence on HM protein composition and bioactivity of HM proteins is reviewed.