Hormones and growth factors in milk

MILK and colostrum contain a variety of proteins, peptides, and steroids that possess biological activity. In studies in which the concentration of hormone/growth factor has been measured in both colostrum and milk, the highest concentrations occur in colostrum (1). This review will discuss pituitary, hypothalamic, pancreatic, thyroid, parathyroid, adrenal, gonadal, and gut hormones as well as growth factors, prostaglandins, and neuropeptides found in milk (see Table 1). Other biologically important proteins and peptides in milk such as the immunoglobulins, allergins, opiates, enzymes, casomorphins, and cyclic nucleotides will not be addressed. The reader is referred to excellent recently published reviews regarding hormones and other bioactive substances present in milk (2–4). Of the hormones and growth factors present in milk many exist in concentrations that exceed those found in maternal plasma (see Table 2). Some are rapidly transported into milk from the maternal circulation unchanged in structure and activity; others appear to be modified by glycosylation, phosphorylation, and proteolysis within the mammary gland and/ or milk. Several hormones [thyroid hormones, relaxin, PTHreleasing peptide (PTHrP), estrogen, GnRH. PRL], growth factors [insulin-like growth factor (IGF), epidermal growth factor (EGF)], and nonhormonal bioactive substances (lactoferrin, transferrin, casomorphins) appear to be synthesized within the mammary gland as well as transported from the maternal circulation (see Table 2).