Breast Milk

Breast milk is a unique bioactive substance that changes composition, within and between feedings and over time, to suit the needs of the growing infant. More than 200 different constituents of breast milk have been identified, many of which have dual roles, and more continue to be discovered as analytic techniques improve. Breast milk includes true solutions, colloids, membranes, membrane-bound globules, and living cells. Its three distinct stages occur when colostrum, transitional, and mature milk are secreted; each aids newborns in their physiologic adaptation to extrauterine life.

Compared to mature milk, colostrum, produced during the first week of life, has a higher protein and lower fat content and is rich in immunoglobu-lins, antibodies, and antioxidants. Compared to colostrum, transitional milk, produced from 7 days to between 10 days and 2 weeks postpartum, has lower immunoglobulins and total protein content and higher lactose, fat, and total caloric content.

Women experience the shift to transitional milk as a feeling of fullness in the breasts, which occurs between 40 and 72 h after birth. This occurs sooner for multiparas than primiparas. Milk volume increases dramatically after birth from less than 100 ml/day to approximately 500 ml/day by day 5 and approximately 650 ml/day by month 1 and 750ml/day by month 3 (Figure 2). Exclusively breast fed infants consume 714, 784, and 776 ml/day between 0 and 2, 3 and 5, and 6 and 8 months, respectively. Mature milk has an energy density of approximately 75 kcal/ml, which translates into a

800 700 600 500 ml 400 300 200 100 0

Day 1 Day 5 1 month 3 months

Time post-partum Figure 2 Average volume of breast milk production.

daily caloric intake of 450-550 kcal/day. This is entirely sufficient to satisfy current energy recommendations for infants for the first 6 months of life.

Once breast milk supply has been established, the volume produced depends on infant demand; frequent exclusive breast feeding is critical for stimulating optimal milk production. This is why it is so important that infants be breast fed on demand, day and night: The more often milk is removed from the breasts, the more milk a woman produces. The process of lactation requires both milk synthesis and its release into the alveoli and the lactiferous sinuses for removal by the suckling infant. Milk production is mediated by prolactin and its release by oxytocin. The decrease in plasma progesterone after birth initiates the process of lactogenesis. For several days following birth, lactogenesis does not depend on suckling; however, by day 3 or 4, milk secretion declines if milk is not removed from the breast. Therefore, any practices, such as the use of prelac-teal feeds, supplemental bottles, or feeding on a schedule, that interfere with the infant's desire or ability to nurse effectively are likely to undermine the successful establishment of lactation. Also, because infant demand is the primary determinant of milk production, the early introduction of other liquids or complementary foods will displace the energy and nutrients provided by breast milk rather than provide an additional source of nutrition.

Lipids comprise the primary source of energy and are the most variable constituent of breast milk and vary in concentration during a feeding, between breasts, during the day, and over time. They also vary in concentration among women by as much as 50%. Research on the lipid component of breast milk has focused on its association with improved cognitive development and possible role in the prevention of obesity and other chronic degenerative diseases in breast fed children compared to formula-fed children. Particular attention has been paid to the dietary omega-3 polyunsaturated fatty acid, doc-osahexaenoic acid (DHA), which has been shown to enhance retinal development and visual acuity and may provide a physiologic explanation for the superior cognitive development that is documented in breast fed children. Following the lead of Japan and numerous European countries, the US Food and Drug Administration has permitted the addition of DHA to infant formula.

Proteins in breast milk include casein, serum, albumin, a-lactabumin, ^-lactoglobulins, immuno-globulins, and other glycoproteins. Lactoferrin, an iron binding protein, inhibits the growth of certain iron-dependent bacteria in the gastrointestinal track and may protect against certain gastrointestinal infections. Lactose is the predominant carbohydrate of breast milk. It has been shown to enhance calcium absorption as well as provide a readily available source of galactose, which is essential to central nervous system development. Although concerns have been raised about the adequacy of breast milk to satisfy protein requirements for 6 months, a number of well-controlled studies have shown that protein needs can met through exclusive breast feeding.

The nutrients in breast milk most affected by maternal nutritional status are the water-soluble vitamins and the fat-soluble vitamins. With few exceptions, maternal stores and intake do not affect the mineral content of breast milk. Micronutrients affected by maternal intake and nutritional status include thiamin, riboflavin, vitamin B6, vitamin B12, vitamin A, iodine, and selenium. Those not affected include folate, vitamin D, calcium, iron, copper, and zinc. Particular attention has been paid to iron content of breast milk because of the relatively low breast milk content compared to theoretical needs. Breast milk iron, however, is highly bioavailable and exclusively breast fed term infants of normal birth weight are not at risk for iron deficiency anemia or depletion of iron stores. Iron supplements, beginning at 2 months, are recommended for preterm or low-birth-weight infants. Zinc, another mineral essential to human development, is also highly bioavailable in breast milk. Breast milk volume does not appear to be influenced by maternal nutritional status, except in situations of extreme food deprivation and famine.

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