Milk-borne prolactin and neonatal development.

Milk is primarily regarded as a food furnishing essential nutrients for infant growth and development, but milk can also serve as a vehicle for mother to neonate transfer of molecules that regulate development. A wide array of biologically active compounds such as hormones, cytokines and enzymes are present in milk, especially early milk. The premise that prolactin (PRL) in milk is an important and possibly essential developmental factor for the newborn is explored. Both PRL and structurally modified isoforms are abundant in early milk and gradually diminish with the progression of lactation. Milk PRL is absorbed and biologically active in the neonate. Assays of PRL variants, experimental paradigms to test them as developmental regulators and the body of evidence supporting the hypothesis that milk PRL regulates differentiation and maturation of neonatal neuroendocrine, reproductive, and immune systems is presented.