captured from the mother’s blood and taken into the al-veolar cells. These proteins are then secreted into the milk ducts.
The Letdown Reflex
The letdown reflex stimulates milk release from the breast. The stimuli from the infant suckling are passed through nerves to the hypothalamus, which responds by promoting oxytocin release from the posterior pituitary gland (Illustration 6.4). The oxytocin causes contraction of the myoepithelial cells surrounding the secretory cells.
As a result, milk is released through the ducts, making it available to the infant. Other stimuli, such as hear-ing a baby cry, sexual arousal, and thinkhear-ing about nurs-ing, can also cause letdown, and milk will leak from the breasts.
Table 6.5 Human and cow’s milk composition
source: Adapted from USDA National Nutrient Database for Standard Reference, Release 22, 2009.17
Nutrient Units
Human Milk (1 fl oz)
Whole Cow’s Milk (1 fl oz)
Water g 26.95 26.94
Energy kcal 22 19
Energy kj 90 78
Protein g 0.32 0.96
Total lipids (fat) g 1.35 0.99
Carbohydrate g 2.12 1. 46
Fiber, total dietary g 0.0 0.0
Sugars, total g 2.12 1.60
Minerals
Calcium, Ca mg 10 34
Iron, Fe mg 0.01 0.01
Magnesium, Mg mg 1 3
Phosphorus, P mg 4 26
Potassium, K mg 16 40
Sodium, Na mg 5 13
Zinc, Zn mg 0.05 0.11
Copper, Cu mg 0.016 0.008
Manganese, Mn mg 0.008 0.001
Selenium, Se mcg 0.6 1.1
Vitamins Vitamin C, total ascorbic acid
mg 1.5 0.0
Thiamin mg 0.004 0.014
Riboflavin mg 0.011 0.052
Niacin mg 0.055 0.027
Pantothenic acid mg 0.069 0.114
Vitamin B6 mg 0.003 0.011
Folate, DFE mcg_DFE 2 2
Vitamin B12 mcg 0.02 0.14
Vitamin A, RAE mcg_RAE 19 14 Vitamin E (alpha
tocopherol)
mg 0.02 0.02
Vitamin D IU 1 16
Vitamin K
(phylloquinone)
mcg 0.1 0.1
Nutrient Units
Human Milk (1 fl oz)
Whole Cow’s Milk (1 fl oz) Lipids
Saturated Fatty acids, total
g 0.619 0.569
4:0 g 0.000 0.023
6:0 g 0.000 0.023
8:0 g 0.000 0.023
10:0 g 0.019 0.023
12:0 g 0.079 0.023
14:0 g 0.099 0.091
16:0 g 0.283 0.000
18:0 g 0.090 0.253
Monounsaturated Fatty acids, total
g 0.511 0.000
Polyunsaturated Fatty acids, total
g 0.153 0.000
Cholesterol mg 4 0.000
Amino acids 0.000
Tryptophan g 0.005 0.022
Threonine g 0.014 0.043
Isoleucine g 0.017 0.049
Leucine g 0.029 0.079
Lysine g 0.021 0.042
Methionine g 0.006 0.022
Cystine g 0.006 0.005
Phenylalanine g 0.014 0.044
Tyrosine g 0.016 0.045
Valine g 0.019 0.057
Arginine g 0.013 0.022
Histidine g 0.007 0.022
Alanine g 0.011 0.031
Aspartic acid g 0.025 0.071
Glutamic acid g 0.052 0.193
Glycine g 0.008 0.022
Proline g 0.025 0.102
Serine g 0.013 0.032
in colostrum, but other proteins present in mature milk are not present. The concentration of mononuclear cells (a specific type of white blood cell from the mother that provides immune protection) is highest in colostrum. Co-lostrum has higher concentrations of sodium, potassium, and chloride than more mature milk.
Water
Breast milk is isotonic with plasma. This biological de-sign of milk means that babies do not need water or
other fluids to maintain hydration, even in hot climates.20 As a major component of human milk, water allows suspension of the milk sugars, proteins, immunoglobulin A, sodium, potassium, citrate, magnesium, calcium, chlo-ride, and water-soluble vitamins.
Energy
Human milk provides approximately 0.65 kcal/mL, although the energy content varies with its fat (and, to a lesser degree, protein and carbohydrate) composition.
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Trans Fatty Acids Trans fatty acids stemming from the mother’s diet are present in human milk.31 Trans fat con-centrations are similar in American and Canadian women, but lower in the milk of women from European and African countries. Removal of trans fatty acids from many food products in Canada led to lower levels of trans fat in human milk.32 Similar trends are expected in the United States.
Cholesterol Cholesterol, an essential component of all cell membranes, is needed for growth and replication of cells. Cholesterol concentration ranges from 10–20 mg/d and varies depending on the time of day.17 Breastfed in-fants have higher intakes of cholesterol and higher levels of serum cholesterol than infants fed HMS.33 Early con-sumption of cholesterol through breast milk appears to be related to lower blood cholesterol levels later in life.34
Protein
The protein content of mature human milk is relatively low (0.8–1.0%) compared to other mammalian milks, such as cow’s milk (Table 6.5). The concentration of pro-teins synthesized in the breast are more affected by the age of the infant (time since delivery) than maternal in-take and maternal serum proteins. Proteins synthesized by the breast are more variable because hormones that regulate gene expression and guide protein synthesis change with time.35 Despite the relatively low concentra-tion, human milk proteins have important nutritive and non-nutritive value. Proteins and their digestive products, such as peptides, exhibit a variety of antiviral and antimi-crobial effects.36 Enzymes present in human milk might also provide protection by facilitating actions that prevent inflammation.
Casein Casein is the major class of protein in mature milk from women who deliver either at term or preterm.37 Casein, calcium phosphate, and other ions such as magne-sium and citrate appear as an aggregate and are the source of milk’s white appearance.38 Casein’s digestive products, casein phosphopeptides, keep calcium in soluble form and facilitate its absorption.
Whey Proteins Whey proteins are the proteins that remain soluble in water after casein is precipitated from milk by acid or enzymes. Whey proteins include milk and serum proteins, enzymes, and immunoglobulins, among others. Several mineral-, hormone-, or vitamin-binding proteins are also identified as components of whey pro-teins. These include lactoferrin, which carries iron in a form that is easy to absorb and has bacteriostatic activity.
The enzymes present in whey proteins aid in digestion and protection against bacteria.
Nonprotein Nitrogen Nonprotein nitrogen provides 20–25% of the nitrogen in milk.38 Urea accounts for 30–50% of nonprotein nitrogen, and nucleotides for 20%, Breastfed infants consume fewer calories than infants
fed HMS.21,22 It is not known whether this difference in energy intake of breastfed infants has to do with the composition of human milk, the inability to see the volume of feedings when providing human milk, the differences in the suckling at the breast compared to an artificial nipple, or other factors. Infants who are breastfed are thinner for their weight at 8–11 months than infants fed HMS, but these differences disappear by 12–23 months of age and few differences are notable by 5 years of age.23
Lipids
Lipids are the second largest component of breast milk by concentration (3–5% in mature milk). Lipids provide half of the energy of human milk.17 Human milk fat is low at the beginning of a feeding in foremilk, and higher at the end in the hindmilk that follows.
Effect of Maternal Diet on Fat Composition The fatty acid profile of human milk varies with the diet of the mother.24 When diets rich in polyunsaturated fats are con-sumed, more polyunsaturated fatty acids are present in the milk.17 When a mother is losing weight, the fatty acid profile of her fat stores is reflected in the milk.25 When very low-fat diets with adequate calories from carbohy-drate and protein are fed, more medium-chain fatty acids are synthesized in the breast.
Clinicians use the value of 20 kcal/oz of human milk to calculate energy provided to an infant. However, fat content may vary considerably from mother to mother and may vary diurnally as well as within a feeding. Fat values in foremilk may range from 39.7–46.7% energy (17.9–23.6 kcal/oz) and hindmilk from 60.7–80.1% en-ergy (23.5–33.2 kcal/oz).26 Overall, the energy ranged from 20.9 to 26.2 calories per ounce.
DHA Milk DHA levels are increased by maternal sup-plementation.27 Recent interest in lipids in human milk stems from studies showing developmental advantages provided by docosahexaenioic acid (DHA).28 DHA is es-sential for retinal development and accumulates during the last months of pregnancy. The advantages of human milk seem particularly important to premature infants born before 37 weeks, perhaps because the concentrations of DHA are higher in the milk of mothers delivering pre-term infants as compared to full-pre-term infants.29 Advan-tages for term infants have been demonstrated as well.
For example, a Norwegian study suggests that cod liver oil supplementation during pregnancy was associated with higher IQ scores at 4 years of age in breastfed versus HMS-fed infants.30 Cod liver oil contains high levels of DHA as well as high levels of vitamin A and vitamin D, so it should be used with caution.
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40 μg of vitamin E per gram of lipid in the milk.46 Levels of alpha-tocopherol decrease from colostrum to transitional milk and to mature milk, whereas beta
and gamma tocopherols remain stable throughout each stage of lactation. The level of vitamin E present in human milk is adequate to meet the needs of full-term infants for muscle integrity and resistance of red blood cells to hemo-lysis (breaking of red blood cells). The levels of vitamin E in preterm milk have been reported to be the same47 and higher48 than in term milk. However, in both reports, the levels present were not considered adequate to meet the needs of preterm infants.
Vitamin K Vitamin K is present in human milk at levels of 2.3 μg/dL.46 Approximately 5% of breastfed infants are at risk for vitamin K deficiency based on vitamin K- dependent clotting factors. There are cases of vitamin K deficiency among exclusively breastfed infants who did not receive vitamin K at birth.
Water-Soluble Vitamins
Water-soluble vitamins in human milk are generally responsive to the content of the maternal diet or sup-plements (vitamin C, riboflavin, niacin, B6, and biotin).
Clinical problems relating to water-soluble vitamins are rare in infants nursed by mothers with inadequate diets.13 Vitamin B6 is considered most likely to be deficient in hu-man milk; levels of B6 in human milkdirectly reflect ma-ternal intake. 49
Vitamin B12 and Folic Acid Vitamin B12 and folic acid are bound to whey proteins in human milk; there-fore, their content in milk is less influenced by maternal intake of these vitamins than are the other water-soluble vitamins. Factors that influence protein secretion (hor-mones and the age of the infant, or time since delivery) are more likely to alter the human milk levels of B12 and folate than is dietary intake.1,50 Infant illness associated with low folate levels in milk has not been reported.
Folate levels increase with the duration of lactation de-spite a decrease in maternal serum and red blood cell folate levels.1 B12 deficiency, or low levels of B12, in milk has been reported for women who have had gastric bypass surgery, have hypothyroidism, consume vegan diets, have latent pernicious anemia, or are generally malnourished.51
Minerals in Human Milk
The minerals in human milk contribute substantially to the osmolality of human milk. Monovalent ion secretion is man-aged closely by the alveolar cells, in balance with lactose, to maintain the isosmotic composition of human milk.
depending on the stage of lactation and the diet of the mother. Some of this nitrogen is available for the infant to use for producing nonessential amino acids. Some of the nonprotein nitrogen is used to produce other proteins with biological roles such as hormones, growth factors, free amino acids, nucleic acids, nucleotides, and carnitine.
The role of individual nucleotides in human milk is under investigation; however, nucleotides appear to play impor-tant roles in growth and disease resistance.
Milk Carbohydrates
Lactose is the dominant carbohydrate in human milk.
Other carbohydrates—including monosaccharides (such as glucose), polysaccharides, oligosaccharides, and protein-bound carbohydrates—are also present.39 Lactose enhances calcium absorption. As the second largest carbohydrate component, oligosaccharides contribute calories at low osmolality, stimulate the growth of bifidus bacteria in the gut, and inhibit the growth of E. coli and other potentially harmful bacteria.
Oligosaccharides Oligosaccharides are medium-length carbohydrates containing lactose on one end. Oligosac-charides can be free, or bound to proteins as glycopro-teins, or bound to lipids as glycolipids, or they can bind to other structures. The conjugated and unconjugated oligosaccharides are classified as glycans. Over 130 differ-ent oligosaccharides are presdiffer-ent as functional ingredidiffer-ents of human milk.40,41 Oligosaccharides in human milk pre-vent the binding of pathogenic microorganisms to the gut, thereby preventing infection and diarrhea.
Fat-Soluble Vitamins
Vitamin A Colostrum has approximately twice the con-centration of vitamin A as mature milk does. Some of the vitamin A in human milk is in the form of beta-carotene.
Its presence is responsible for the characteristic yellow color of colostrum. In mature milk, vitamin A is present at 75 μg/dl or 280 IU/dl.42 These levels are adequate to meet infant needs.
Vitamin D Vitamin D is present in both lipid and aque-ous (water) compartments of human milk. Most vitamin D is in the form of 25-OH2 vitamin D and vitamin D3. Vitamin D levels of human milk vary with maternal diet and exposure to sunshine.43 Maternal exposure to sunlight has been reported to increase the vitamin D3 level in milk tenfold.44 It is yet unknown how much maternal vitamin D supplement is needed to ensure adequate maternal and in-fant vitamin D status when sunlight exposure is insufficient, though researchers are actively pursuing the answer.45 Vitamin E The level of total tocopherols in human milk is related to the milk’s fat content. Human milk contains
Osmolality A measure of the concentration of particles in solution.
Monovalent Ion An atom with an electrical charge of 11 or –1.
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Taste of Human Milk
“. . . too full o’ th’ milk of human kindness to catch the nearest way.”
Shakespeare’s Macbeth, Act I, Scene V
This line from Shakespeare reflects the centuries-old belief that a breastfeeding woman’s diet influences the composi-tion of her milk and has a long-lasting influence on the child. The flavor of human milk is an important taste ex-perience for newborn infants, but flavor of human milk is often ignored when the benefits of human milk or its composition is considered. Human milk is slightly sweet61 and it carries the flavors of compounds ingested, such as mint, garlic, vanilla, and alcohol.62
The transfer of flavor compounds appears to occur se-lectively and in relatively low amounts.63 Infant responses to flavors in milk seem to depend on the length of time since the mother consumed the food, and the amount and frequency of the flavor that the mother consumed (new versus repeated exposure). Infants seem more interested in their mother’s milk when flavors are new to them. Re-searchers found that infants nursed at the breast longer if a flavor (garlic) was new to them than if the mother had taken garlic tablets for several days.64 Infants who were exposed to carrot juice flavor in their mother’s milk ate less of a carrot-flavored cereal and spent less time feed-ing at the breast than infants who had not been exposed to the carrot flavor. Thus, exposing infants to a variety of flavors in human milk may contribute to their interest in and consumption of human milk as well as their accept-ance of new flavors in solid foods.65