After delivery, the breasts begin to secrete colostrum, which is a deep lemon-yellow liquid. It usually can be expressed from the nipples by the second postpartum day. Compared with mature milk, colostrum is rich in immunological components and contains more minerals and amino acids (Ballard, 2013). It also has more protein, much of which is globulin, but less sugar and fat. Secretion persists for 5 days to 2 weeks, with gradual conversion from "transitional" to mature milk by 4 to 6 weeks. The colostrum content of immunoglobulin A (IgA) ofers the newborn protection against enteric pathogens. Other host resistance factors found in colostrum and milk include complement, macrophages, lymphocytes, lactoferrin, lactoperoxidase, and lysozymes.
Mature milk is a complex and dynamic biological luid that includes fat, proteins, carbohydrates, bioactive factors, minerals, vitamins, hormones, and many cellular products (Table 36-2). The concentrations and contents of human milk change even during a single feed and are inluenced by maternal diet and by newborn age, health, and needs. A nursing mother easily produces 600 mL of milk daily, and maternal gestational
FIGURE 36-5 Schematic of the alveolar and ductal system during lactation. Note the myoepithelial fibers (M) that surround the outside of the uppermost alveolus. The secretions from the glandular elements are extruded into the lumen of the alveoli (A) and ejected by the myoepithelial cells into the ductal system (0), which empties through the nipple. Arterial blood supply to the alveolus is identified by the upper right arrow and venous drainage by the arrow beneath.
PUFA = Polyunsaturated fatty acids. The Puerperium 657 weight gain has little impact on its quantity or quality. Milk is isotonic with plasma, and lactose accounts for half of the osmotic pressure. Essential amino acids are derived from blood, and nonessential amino acids are derived in part from blood or synthesized in the mammary gland. Most milk proteins are unique and include alpha-lactalbumin, beta-lactoglobulin, and casein. Fatty acids are synthesized in the alveoli from glucose and are secreted by an apocrine-like process. Most vitamins are found in human milk, but in variable amounts. Vitamin K is virtually absent, and thus, an intramuscular dose is given to the newborn (Chap. 33, p. 626). Vitamin D content is low 22 IU/mL, and newborn supplementation is also recommended by the American Academy of Pediatrics (Wagner, 2008).
Whey is milk serum and has been shown to contain large amounts of interleukin-6 (Saito, 1991). Human milk has a whey-to-casein ratio of 60:40, considered ideal for absorption.
Prolactin appears to be actively secreted into breast milk.
Epidermal growth octor (EGF) has been identiied, and because it is not destroyed by gastric proteolytic enzymes, it may be absorbed to promote growth and maturation of newborn intes tinal mucosa (McCleary, 1991). Other critical components in human milk include lactoferrin, melatonin, oligosaccharides, and essential fatty acids.
• Endocrinology of Lactation