Bioactive proteins in breast milk.

Human milk contains many proteins that have been shown to be bioactive, but it is still not known whether these activities are exerted in breast-fed infants. These bioactivities include enzyme activities, enhancement of nutrient absorption, growth stimulation, modulation of the immune system and defence against pathogens. The antimicrobial activities are very diverse, ranging from stimulation of beneficial microorganisms (i.e. prebiotic effects), killing or inhibition of growth of pathogens, to mechanisms preventing attachment or invasion of harmful microorganisms. Among the bioactive proteins are lactoferrin, lysozyme, secretory immunoglobulin A, haptocorrin, lactoperoxidase, α-lactalbumin, bile salt stimulated lipase, β- and κ-casein, and tumour growth factor β. Human milk proteins may be largely resistant against digestion in the gastrointestinal tract, be partially digested into bioactive peptides, or be more or less completely digested and utilised as a source of amino acids. These events can be studied using an in vitro digestion model, which is useful for predicting results in human infants. Some bovine milk proteins, for example, lactoferrin and tumour growth factor β, may also resist proteolysis and be capable of exerting bioactivities similar to those of human milk proteins.