Bioactive peptides in milk and their biological and health implications

Milk contains a large number of bioactive peptides with various biological activities. These peptides either exist naturally in milk or derive from milk protein hydrolyses. The natural bioactive peptides found in milk include epidermal growth factor (EGF), transforming growth factor (TGF), nerve growth factor (NGF), insulin, and insulinlike growth factors I and II (IGF-I and IGF-II). The concentrations of these peptides are generally high in colostrum, and the concentrations are often higher than those found in blood circulation. Due to a limited protease activity in the gastrointestinal tract of neonatal animals and the existence of protease inhibitors in the milk, these milk-borne bioactive peptides are likely to survive the gastrointestinal digestion in suckling animals. Oral administration of EGF, insulin, or IGF-I can stimulate gastrointestinal maturation in newborn animals. It is speculated that colostrum-borne bioactive peptides may play a role in regulating postnatal gut development in suckling neonates. The second group of bioactive peptides found in milk include casomorphins, immunostimulating peptides, and ACE-inhibitory peptides. They are built into the primary structures of milk proteins and can be released by enzymatic hydrolyses. These peptides may act as potential physiological modulators during the gastrointestinal digestion of milk. In addition, they may also be a potential source of bioactive ingredients for designing "functional" foods. This review gives a brief summary of various bioactive peptides found in milk and a brief discussion on biological and health implications of such peptides on consumers.