Nutrition is a critical capstone for the proper growth and development of infants. Breastfeeding of term infants by healthy mothers is the optimal mechanism for providing the caloric and nutrient needs of infants. Preterm infants can also benefit from breast milk and breastfeeding although supplementation and fortification of preterm breast milk may be required. Barring some unique circumstances, human breast milk can provide nutritional, social, and motor developmental benefits for most infants.

Despite increased emphasis on breastfeeding education, according to the 2005 National Immunization Survey conducted by the Center for Disease Control, approximately 75% of women chose to initiate breastfeeding. Of these women, only 43.1% were still breastfeeding at 6 months, and 21.4% of infants were still receiving breast milk at 1 year. The Department of Health and Human Services Healthy People 2010 initiative proposes to increase these numbers to 50% and 25% for infants at 6 and 12 months, respectively. The rates of women who were exclusively breastfeeding without formula supplementation were even lower with only 31.5% and 11.9% of infants ages 3 and 6 months, respectively, doing so. There are still, however, disparities associated with these rates, specifically for African American women. Breastfeeding rates are about 50% lower among all ages regardless of income or educational level in comparison to Caucasian women. Education of practitioners as well as their patients is an integral component of this initiative.

Most women presently of childbearing age were not breast-fed and report having no maternal relatives who breast-fed their children. Because evidence clearly suggests familial influences in the development of infant feeding practices, practitioners may find it difficult to encourage breastfeeding behaviors among women with no direct familial breastfeeding experience.

Efforts to alter knowledge, attitudes, and behaviors regarding breastfeeding must effectively address the numerous psychosocial barriers. Health care providers are critical conduits for maternal and familial education. All members of the health care team, including physicians, midwives, and nurses, are valuable sources of important evidence-based information as well as psychological support for mothers in search of guidance regarding infant feeding practices. Unless health care practitioners are properly educated regarding breastfeeding practices and barriers, efforts to achieve the Healthy People 2010 objectives will remain suboptimal.

Numerous studies have shown the superiority of breast milk and the health advantages that breast-fed children have. The literature has shown that infants who are breast-fed have fewer episodes of diarrheal illness, ear infections, and allergies. Exclusive breastfeeding for at least 4 months in infants at risk for developing atopic disease decreases the cumulative incidence of atopic dermatitis. Lower rates of childhood obesity, type 2 diabetes, sudden infant death syndrome (SIDS) and leukemia have also been associated with breastfeeding. There are likewise financial advantages to breastfeeding. Other somewhat controversial investigations suggest higher intelligence among breast-fed infants.

There are also Maternal benefits to breastfeeding. Mothers who breast-feed are less likely to develop premenopausal breast cancer. An association with decreased rates of Type 2 diabetes and ovarian cancer also exists. Studies are also looking at the relationship between breastfeeding and rates of postpartum depression and cardiovascular disease. Most importantly, however, is the bonding relationship breastfeeding promotes between mother and infant.

All major maternal-child health professional organizations recommend exclusive breastfeeding for the first 6 months prior to the introduction of age-appropriate solid foods and advise continued breastfeeding for the first year of life.

The American Academy of Pediatrics (AAP) Committee on Nutrition recommends breastfeeding for the first year of life with supplemental vitamin D at birth and the addition of supplemental iron at age 4 months and possible addition of fluoride at age 6 months for infants living in regions in which water is low in fluoride. Vitamin D supplementation is particularly applicable in regions with limited sunlight and for infants of mothers with decreased daily intake of cow’s milk. Further recommendations include delaying introduction of cow’s milk until after 1 year and delaying addition of reduced-fat milk until 2 years of age. To this end, new mothers should be encouraged to continue prenatal vitamins containing supplemental iron, calcium, and vitamin D. Supplemental solid foods should be considered at or around 6 months of age once the infant demonstrates appropriate readiness.

Women are able to produce milk by the age they are able to bear children. There is no evidence that breast function, breast milk production, or composition is different among younger women. The principal external structures of the mature human female breast are the nipple, areola, and Montgomery tubercles. The areola is the darker part of the breast, with the nipple being the central-most structure through which milk ducts open and milk is expressed. Within the areola are Montgomery tubercles, through which sebaceous and sweat glands (Montgomery glands) open, producing lubricating substances for the nipple.

Underlying structures include adipose tissue, mammary gland cells, and contractile myoepithelial cells surrounding the gland cells (allowing for milk ejection). Milk produced within the alveoli is ejected into the milk ducts which open out directly to the nipple. It was thought previously that milk was stored in lactiferous sinuses, however recent research has concluded that these sinuses do not exist.

Infant breastfeeding draws the nipple and areola into the mouth, causing elongation of the nipple. The elongated nipple is compressed between the palate and the tongue and milk is expressed less than 0.05 seconds after the nipple has elongated. Stimulation of the areola is essential for the oxytocin-mediated hormonal cascade that controls milk ejection.

Two principal hormones are required for breast milk production—oxytocin and prolactin—controlled by the hypothalamic-pituitary axis. Oxytocin production and secretion are under the control of the posterior pituitary and are stimulated by suckling. Oxytocin production in response to suckling is intermittent and stimulates ejection (“let down”) of breast milk. Oxytocin does not appear to affect breast milk production, although numerous stressors can negatively impact breast milk let down. Evidence suggests that lactogenesis may be delayed and let down reduced following stressful vaginal delivery or cesarean section.

Milk production is controlled primarily by the release of prolactin. Prolactin secretion is through a feedback loop under dopaminergic control with the primary action on prolactin receptors on mammary epithelium. Suckling likewise stimulates prolactin release. Furthermore, prolactin acts as an inhibitor of ovulation through hormonal feedback control, although breastfeeding is considered a relatively unreliable contraceptive mechanism.

Several additional hormones are required for milk production: cortisol, human growth hormone, insulin, thyroid and parathyroid hormones, and feedback inhibitor of lactation (FIL). Not entirely understood, FIL appears to act at the level of breast tissue to inhibit continued breast milk production when the breast is not completely emptied.

Milk production begins during the postpartum period with prolactin production and concomitant decreased estrogen and progesterone production following placental delivery. Milk production will persist under this hormonal control for the first several days; however, continued milk production beyond the initial 48 hours postpartum requires suckling. Although mothers will continue to produce milk between feedings once suckling has initiated the feedback loop, milk production significantly rises during breastfeeding.

Stages of Production

Production of human breast milk among healthy mothers who deliver full-term infants occurs in three phases—colostrum, transitional milk, and mature milk. Colostrum is a thick, yellow substance produced during the first several days postpartum. Healthy mothers produce approximately 80-100 mL daily. Colostrum is rich in calcium, antibodies, minerals, proteins, potassium, and fat-soluble vitamins. This milk has immunologic qualities that are vital to the infant and it possesses gastrointestinal properties to aid in secretion of meconium. Production of colostrum is followed for the next 5-6 days with transitional milk, which provides essential components more closely resembling mature breast milk. Most women will notice a significant change evidenced by the fullness of their breasts and the change in the consistency of the milk. True milk is white and sometimes has a bluish tint. The consistency is similar to cow’s milk with a sweet taste. Mature breast milk, produced beginning at or near postpartum day 10, produces key components, discussed in the next section.

Numerous factors may affect the supply of breast milk, including anxiety, medications, maternal nutritional status, sleep, exercise, breastfeeding frequency, tactile stimulation, and fluid intake. Breastfeeding mothers should be encouraged to consume generous amounts of fluids and express breast milk every 2-3 hours. The hormonal feedback loop that controls the production and release of prolactin and oxytocin is initiated by suckling or other tactile stimulation of the breast. The greater the amount of suckling or other tactile breast stimulation, the greater the milk supply.

Components

Mature human breast milk contains protein, carbohydrate, and fat components and provides approximately 20 kcal/oz and 1 g of protein. The principal protein elements of both mature and premature breast milk are casein (40%) and whey (60%). Breast milk contains approximately 2.5 g/L of casein. Also called “curds,” this protein forms calcium complexes. Higher concentrations of this protein are found in cow’s milk. Whey (approximately 6.4 g/L) is a protein component composed of α-lactalbumin, lactoferrin, lysozyme, immunoglobulins, and albumin.

Free nitrogen, vital for amino acid synthesis, is also a significant component of mature breast milk and is integral for multiple biochemical pathways, including production of uric acid, urea, ammonia, and creatinine. It is also a key component of insulin and epidermal growth factor.

There are approximately 70 g/L of lactose, the primary carbohydrate in mature breast milk. Composed of galactose and glucose, the lactose concentration continues to increase throughout breastfeeding. Human milk fat likewise increases with continued breastfeeding. Mature breast milk provides approximately 40 g/L and includes triacylglycerides, phospholipids, and essential fatty acids.

The principal electrolytes in breast milk are sodium, potassium, magnesium, and calcium. Calcium appears to be mediated through the parathyroid hormone–related protein, which allows for mobilization of calcium stores from bone in otherwise healthy women. Bone calcium levels return to normal after termination of breastfeeding. Regulation of sodium and potassium concentrations in breast milk occurs through corticosteroids.

Iron absorption is particularly high in newborns and infants, although the relative concentration of iron in mature breast milk is low. For infants younger than 6 months of age, the concentration of iron in breast milk is sufficient and supplementation is not necessary; however, recommendations for infants older than 6 months include supplemental iron from green vegetables, meats, and iron-rich cereals. The recommended amount of supplemental elemental iron is 1 mg/kg/d. Iron is an essential component in the synthesis of hemoglobin.

Vitamin K, a lipid-soluble vitamin and important component in the clotting cascade, is routinely provided in the immediate postpartum period as a 1-mg intramuscular injection. There is evidence that oral vitamin K may produce similar benefit as well as maternal supplementation of 5 mg/d of oral vitamin K for 12 weeks following delivery.

Another lipid-soluble component, vitamin D, is essential for bone formation. Women who have limited exposure to sunlight or suboptimal vitamin D intake will produce little or no vitamin D in breast milk. The recommended daily intake of vitamin D is 400 IU/d. Practitioners must be cognizant of mothers with special diets (ie, vegetarian diets) whose low vitamin D intake might indicate a need for supplemental vitamin D.

Other elemental minerals in breast milk (eg, zinc, copper, selenium, manganese, nickel, molybdenum, and chromium) are found in trace amounts but nonetheless are essential for a multitude of biochemical processes.

Composition of Preterm Breast Milk

The composition of breast milk in mothers of preterm infants is different from that in mothers of term infants. This difference persists for approximately 4 weeks before the composition approaches that of term infant breast milk. The difference in preterm milk composition reflects the increased nutrient demands of preterm infants. Preterm breast milk contains higher concentrations of total and bound nitrogen, immunoglobulins, sodium, iron, chloride, and medium-chain fatty acids. However, it may not contain sufficient amounts of phosphorus, calcium, copper, and zinc. Preterm infants are more likely to require fortification with human milk fortifiers (HMF) to correct these deficiencies.

Preparation for breastfeeding should begin in the preconception period or at the first contact with the patient. Most women choose their method of feeding prior to conception. Psychosocial support and education may encourage breastfeeding among women who might not otherwise have considered it. Evidence for this strategy, however, is anecdotal and requires further investigation.