Breastfeeding: Unraveling the Mysteries of Mother's Milk

, Georgetown University Medical Center

In This Article

Milk Volume and Composition

Volume. Milk volume is relatively constant irrespective of maternal nutritional status (Fig. 1). In general, healthy infants consume an average of 750-800mL milk daily for the first 4-5 months after birth (range, 450 to 1200mL/day).[1,10,11] Similar findings were reported from developing countries where maternal nutrition is sometimes subject to greater seasonal variation and may be less adequate compared with industrial countries.[1,11] Increasing the intake of fluid does not seem to affect milk volume.[10] Therefore, lactating women should maintain adequate fluid intake but should not attempt to boost milk volume by consuming excess fluids.[1]

Figure 1. In general, healthy infants consume 450 to 1200mL/day first 4-5 months after birth. Milk volume is relatively constant irrespective of maternal nutritional status. Photo courtesy of Susanrachel Condon.

Major nutrients. Lactose, 5.5-6.0g/dL, is the most constant nutrient in human milk (Table I). Its concentration in breast milk is not affected by maternal nutrition.

Proteins amount to about 0.9g/dL in mature milk.[12]Recent studies comparing the impact of nutrition on lactation in industrialized and developing countries suggest that neither maternal diet nor body composition affects milk protein level.[1] However, limited data from earlier studies seem to indicate that short-term, high-protein diets can increase the protein and nonprotein nitrogen content of human milk,[13] while limiting maternal food intake can lead to lower milk protein levels.[13,14,15]

The majority of milk proteins provide the newborn with immune and nonimmune protection from infection. These proteins--immunoglobulins A, G, and M; lactoferrin; and lysozyme--have various functions in the newborn.[16] Early studies suggested that the level of these protective proteins in milk is affected by maternal diet, but more recent research suggests that immunoglobulins might be stable for a wide range of diets.[17,18,19,20]

Fat. While the amount and composition of carbohydrate and protein remain relatively constant in mature human milk, the composition of fat is highly variable and is affected within hours and to a large extent by maternal nutrition intake.[21] Gestation, lactation, parity, milk volume, caloric and carbohydrate intake, and weight changes are among the maternal factors that can alter the fat content and composition of breast milk. Specifically, phospholipid and cholesterol content are higher in colostrum preterm than term breast milk. Also, long chain polyunsaturated fatty acids (LC-PUFA) are higher in preterm and transitional milk and remain high for the first 6 months in women who deliver preterm. In term milk, on the other hand, LC-PUFA declines throughout the first 6 to 12 months of lactation. The endogenous synthesis of fatty acids (FA) declines with parity, most notably after 10 births, but FAs (C6-C16) rise with a high-carbohydrate diet. Palmitic acid (C16) content of breast milk increases in a low-calorie diet. Weight gain during pregnancy is positively associated with higher milk fat content. During infant feedings, fore milk has less fat content than hind milk. Also, the higher the volume of breast milk, the lower the milk fat concentration.[92] The lengths of both gestation and lactation affect phospholipid and cholesterol, the lipids that constitute the milk fat globule membrane.[22] In the early stage of lactation, because the milk fat globules are much smaller than in mature milk,[23,24] the total "membrane" lipid level is higher in colostrum and transitional milk than in mature milk. The period of colostrum lasts less than 10 days, but during this short time the higher lipid levels are beneficial in such processes as neonatal cell membrane production needed for growth, brain development, and bile salt synthesis. LC-PUFAs--C20:4n6 and C22:6n3, arachidonic, and docosahexaenoic acids, respectively--are milk fats essential for neonatal growth, brain development, and retinal function.[25,26] These fatty acids are stored in the fetus only in the last trimester of pregnancy; therefore, preterm infants are born with low reserves of LC-PUFA, and their best source for these essential fatty acids is human milk. LC-PUFA levels normally decrease in breast milk during lactation, but in women who have delivered infants before term, the levels remain constant in preterm milk for at least 6 months[27]. Holman and colleagues[28] have reported that levels of LC-PUFA often decline in pregnant and lactating women, suggesting that there is a preferential transfer of these essential fatty acids from mother to fetus or to the newborn through milk, even at the cost of possible depletion of maternal reserves. Depletion of maternal reserves might suggest the need for supplementation of pregnant and lactating women with LC-PUFA.

Milk fat content changes dramatically during each feeding[29,30] and fat composition is markedly affected by the maternal diet.[31] Some studies have shown that the mechanism for endogenous synthesis of fatty acids (ie, mainly medium chain fatty acids) seems to become exhausted in women of very high parity[32]; that infants who receive milk with low fat content (ie, less than 3.0 g/dl when the norm is 3.5 to 4.5 g/dl) tend to nurse more frequently and for longer time periods, thereby causing an increase in milk volume[33]; and that there is a strong positive relationship between weight gain during pregnancy and milk fat content.[34]

Vitamins and minerals. The vitamin content of human milk depends on the mother's vitamin status; when maternal intake of specific vitamins is chronically low, these vitamins in turn are found in low levels in the milk. Vitamin supplementation raises vitamin concentrations in milk. Water-soluble vitamins in milk are generally more responsive to maternal dietary intake than fat-soluble ones.[1]

The relationship between maternal intake of vitamins and their concentration in milk varies according to the specific vitamin. For example, excess vitamin C intake does not further increase the level in milk (above that associated with adequate intake), whereas vitamin B6 concentrations in milk continue to rise with higher intakes. Folate levels in milk remain normal even at the expense of maternal folate stores and do not decrease until the latter are depleted.[1] Based on infant needs and the concentrations of fat-soluble vitamins in human milk, the Institute of Medicine (IOM) advises that in the US all newborns receive 0.5-1.0mg vitamin K by injection or 1.0-2.0mg orally immediately after birth.[1,10] Infants should receive 5.0-7.5ug vitamin D per day if exposure to sunlight seems inadequate.

The concentration of trace minerals (iron, copper, zinc, selenium) varies as a function of length of lactation. Concentrations of iron[35,36] and fluoride[37] in milk seem to be independent of maternal nutrition. Concentrations of manganese,[38] iodine,[39] and selenium[40] depend on maternal nutrition. Iodine is unique among trace elements in that it is avidly accumulated by the mammary gland[1].

Because of the high bioavailability of iron in human milk, exclusively breast-fed infants do not need iron supplements during the first 6 months of life. When supplementary foods are introduced (as recommended after 4-6 months of exclusive breast-feeding), iron supplements should be added to the infant's nutrition[35,36]. It is recommended that breast-fed infants receive supplemental fluoride if the water supply in the area has only low levels (<0.3ppm).

It is important to assess not only the concentration of milk components but also the amount delivered to the infant. Thus, while some milk components are present at a higher concentration in colostrum than in milk, one has to consider the marked differences in volume: colostrum amounts to about 100mL/day, whereas average milk volumes are 750-850mL/day.


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