Human milk proteins: key components for the biological activity of human milk.

Human milk contains a wide array of proteins that provide biologic activities ranging from antimicrobial effects to immunostimulatory functions. Proteins like lactoferrin, secretory IgA, kappa-casein, lactoperoxidase, haptocorrin, lactadherin and peptides formed from human milk proteins during digestion can inhibit the growth of pathogenic bacteria and viruses and therefore protect against infection. At the same time, proteins like lactoferrin, bile-salt stimulated lipase, haptocorrin, kappa-casein, and folate-binding protein can facilitate the absorption of nutrients in the neonatal gut. However, the proteins in human milk themselves also provide adequate amounts of essential amino acids to the growing infant. This suggests a highly adapted digestive system, which allows the survival of some proteins and peptides in the upper gastrointestinal tract, while still allowing amino acid utilization from these proteins further down in the gut. It is now possible to produce recombinant human milk proteins in transgenic plants and animals, which makes it possible to further study the bioactivity of these proteins. Provided these proteins can be produced in large scale at low cost, that they show biologic activity and pose no safety concerns, it may be possible to add some human milk proteins to infant diets, such as formula and complementary foods. Human milk proteins produced in rice or potatoes, for example, could be added without much purification, because these staples commonly are used in weaning foods. Thus, some qualities provided by human milk may be included into other diets, although it is highly unlikely that all unique components of human milk can be copied this way.