Breastfeeding: Unraveling the Mysteries of Mother's Milk

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.^[13-15]

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.^[17-20]

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.

Breast milk provides not only essential nutrients but also a great number of other specific functions in the newborn. For example, major nutrients, protein, carbohydrate, and fat, in addition to serving as building blocks for the infant's tissue, carry out anti-infective as well as nutrient-enhancing functions, such as transporting essential elements and aiding digestion. Furthermore, even when concentrations in human milk are markedly lower than in bovine milk or formula, nutrients from human milk might have much greater bioavailability for the infant because of specific biologic factors, such as the infant's receptor-mediated uptake of iron from human milk. Thus, in spite of a relatively low concentration of some nutrients, human milk might be superior to other nutrient sources in providing these nutrients to the infant. The apparently lower concentration of some nutrients in human milk such as vitamin D, pantothenic acid, and folate, might be due to the fact that they are bound to other components or, lower concentrations may be due to shifts from the aqueous phase to the fat phase of milk upon standing after the milk has been expressed from the breast (vitamin D).

All proteins in human milk have bioactive functions in addition to providing amino acids for protein synthesis by the newborn. Whey proteins, for example, have been reported to provide immune and nonimmune protection.^[41,42] Recently, casein has been shown to prevent the attachment of Helicobacter pylori to human gastric mucosa.^[43]

Most proteins in human milk are heavily glycosylated^[44] and are therefore resistant to proteolysis both after ingestion by the infant^[42,45] and after short-term storage (4-24 hours) at low to moderate ambient temperatures (15deg.-25deg.C).^[46,47]

Early in studies of human milk, researchers became aware that certain substances--most notably, IgA, lysozyme, and lactoferrin--that are abundant in human milk (compared with bovine milk)^[41] might protect the infant from infection.^[47] This observation has progressed within the last 2 decades to a fuller appreciation of several characteristics of breast milk's protective features:

• Immunoprotective substances act at mucosal sites.
• Because of their resistance to digestive enzymes, protective factors are well adapted to persist in the hostile environment of the gastrointestinal tract.
• They kill certain bacterial pathogens synergistically.
• Protection is achieved without triggering inflammatory reactions.
• The daily production of many immunoprotective factors changes as lactation proceeds.
• The secretion of many soluble defense agents by the mammary gland is inversely related to the capacity of the recipient infant to produce them at mucosal sites.^{[41, 49-51]}

The presence in milk of immunomodulators that fine-tune the interrelationships among the various protective agents has recently been reported and is currently being investigated (Table II).^[52] Secretory immunoglobulin A (sIgA), dimeric IgA coupled to the secretory component, is the main immunoglobulin in human milk. IgG and IgM are also present in milk, but at much lower concentrations. The changing concentration of these immunoglobulins in milk provides an example of the interaction between milk components and the functional development of the infant: while IgG and IgM rise rapidly after birth, the newborn maintains low levels of endogenous IgA during the first year of life. IgA is produced in the mammary gland in B cells, which originate at maternal sites of high environmental pathogen exposure (eg, the small intestine or respiratory tract), and therefore protects the infant against pathogens present in the immediate environment.

Table III summarizes the enteric and respiratory pathogens against which the infant is protected by specific IgA antibodies in human milk. IgA is resistant to proteolysis, acts at mucosal surfaces, and protects by noninflammatory mechanisms; all of these properties enable efficient action in the infant.

Human milk lacks inflammatory mediators, and contains anti-inflammatory agents such as antiproteases, antioxidants, and enzymes that degrade inflammatory mediators and modulators of leukocyte activation (Table IV).^[49] Furthermore, IgE (the principal immunoglobulin responsible for immediate hypersensitivity reactions), basophils, mast cells, eosinophils (the principal effector cells in these reactions), and the mediators from these cells are absent in human milk. Immune and nonimmune protecting agents are present in milk throughout lactation and some, such as lysozyme, are present at higher concentrations during prolonged lactation than during the early stages. Therefore, although it is strongly advocated that breast-fed infants receive food supplements after 4 to 6 months of exclusive breast-feeding, it is advisable to breast-feed for longer periods in geographic areas where the environment may be contaminated with pathogenic microorganisms, in order to provide the infant and toddler the benefits of milk-borne protective agents.

Studies also indicate that a glycoconjugate present in human milk, but absent in either human serum or bovine milk, inhibits the binding of HIV envelope glycoprotein (gp120) to the CD4 receptor of T lymphocytes.^[53,54]

In addition to soluble antigens and anti-infective agents, human milk contains leukocytes; the majority (90%) are neutrophils and macrophages. Lymphocytes account for approximately 10%. The number and type of leukocytes change with duration of lactation. Most of the lymphocytes in milk are T cells. The proportions of CD4 (helper) to CD8 (suppressor/cytotoxic) cells in human milk are similar to those in blood. Cytokines in human milk (eg, TNF-alpha and IL-1-beta) have been shown to enhance the anti-infective function of milk leukocytes. Milk macrophages might participate in the process of immunogenesis in the infant.

The immune and nonimmune protection provided by milk results in a lower incidence of necrotizing enterocolitis^[55] and other gastrointestinal and respiratory infections in breast-fed infants than in formula-fed infants^[56]. The incidence of otitis media is also lower than in formula-fed infants. In addition to protection against some infectious diseases, breast-fed infants might also be protected at later ages from diseases that are sequelae of infectious insults (eg, insulin-dependent diabetes mellitus, lymphoma, and Crohn's disease). Immune factors provided by human milk that compensate for their delayed production by the infant are summarized in Table V.

Oligosaccharides (which amount to 1.0-1.5g/100mL of human milk),^[53] glycoconjugates, mucins, and glycolipids act as receptor analogs and thereby inhibit the binding of enteric and respiratory microorganisms and their toxins.^[57] In addition, the hydrolysis of milk triglycerides (the major component of milk fat) during digestion in the stomach and intestine^[59] produces free fatty acids and monoglycerides that have been shown to have antiviral, antiprotozoan, and possibly also antibacterial activity.^[60]

The presence of growth factors and hormones in milk and their function has been known for some time (Table VI, VII).^[61-64] Interestingly, the concentration of many growth factors and hormones is higher in a woman's milk than in her plasma. The milk hormones, however, often differ in structure from their maternal serum counterparts, suggesting modification (often post-translational processing such as glycosylation) within the mammary gland. These glycosylated forms often are difficult to detect by standard RIA techniques and have to be quantitated by specific bioassays.^[62] The stronger glycosylation protects these bioactive components during passage through the gastrointestinal tract and probably enables the newborn to absorb growth factors and hormones from mother's milk.

It appears that variants of prolactin are present in the circulation of the newborn and that the prolactin acquired from breast milk, and not endogenous prolactin secreted by the newborn's pituitary gland, is essential for the normal development of the neuroendocrine regulation of prolactin in the newborn.^[62,65]

Many hormones act in the newborn. While the exact mechanisms of uptake from milk and their mode and site of action in the newborn are known for some, further study is needed to identify these mechanisms for most hormones. Agents in milk seem to stabilize hormones in the gastrointestinal tract of the newborn.

In addition to prolactin, other hormones such as progesterone are present in different form in breast milk than in maternal serum. Transfer of these hormones from milk to infant was documented in some studies directly; in other studies, this transfer is inferred from the documentation of higher serum level of the hormone--for example, thyrotropin releasing hormone (TRH) and somatostatin--in breast-fed than in formula-fed newborns^[61]. The milk hormones may also be modified as they pass through the gastrointestinal tract and prior to release into the newborn's blood.

Human milk contains a great number of enzymes, many of which have specific transport functions (Table VIII). For instance, xanthine oxidase acts as a carrier of iron^[65] and glutathione peroxidase carries selenium.^[66] Although proteases are present in human milk, it is not known how much of that activity is expressed because of the antiprotease activity of human milk itself.^[66] One can postulate that antiproteases might protect the mammary gland from local proteolysis (caused by leukocytic or lysosomal proteases) and might prevent the proteolytic breakdown of milk proteins, many of which have to reach the infant intact (eg, immunoglobulins, digestive enzymes). The antitryptic and antichymotryptic activity of human milk might prevent the absorption of endogenous and bacterial proteases in infants and thereby contribute to the passive protection of extraintestinal organs such as the liver.^[67] The high activity of antiproteases in colostrum coincides with the period of greatest transfer of nonimmunoglobulin protein from the intestine to the systemic circulation of the newborn.

The digestive enzymes in milk (amylase and digestive lipase) act in the newborn to compensate for immature pancreatic function. These enzymes are remarkably stable for years in milk stored at low temperature (-20deg.C or -70deg.C). Moreover, activity is unchanged after storage for 24 hours at 38deg.C. The stability of enzymes and of other proteins in milk might be due to the antiprotease activity of milk. Furthermore, many enzymes are stable in the gastrointestinal tract of the newborn.

Amylase,^[68] an enzyme identified in milk more than a century ago,^[69] may be more important to the infant after initiation of starch supplements^[70] or when formula that contains oligosaccharides hydrolyzed by amylase is fed to partially breast-fed infants. Amylase activity in the duodenum of the newborn is only 0.2% to 0.5% of the adult level. At the time of supplementation (after 4 to 6 months of exclusive breast-feeding), the infant is still deficient in endogenously produced amylase.^[71] The latter secreted from salivary glands and pancreas does not reach adequate levels until 2 years after birth. Other infants and toddlers who might benefit from milk amylase are those with pancreatic insufficiency caused by diseases such as cystic fibrosis^[72] or malnutrition.^[73-75] Because of the potential of bile salt-dependent lipase in milk^[76] to compensate for the low pancreatic lipase in the newborn,^[77,78] this enzyme has received great attention in the past decade.^[44,66] The characteristics of the digestive enzymes of human milk are summarized in Table IX.

Several milk components are essential because they have to be provided to the newborn, while older children and adults have the ability to synthesize these components. Among these are carnitine,^[79] taurine,^[80] and LC-PUFAs^[26] that are produced by elongation and desaturation of the precursor fatty acids, linoleic (C 18:2, n-6), and linolenic (C18:3, n-3) acids, and nucleotides^[81] that have to be provided to the intestine and lymphatic tissues because they cannot be synthesized either from the diet or de novo in other organs.^[82] The need for these essential components might be even greater in premature infants who are born before fetal intrauterine reserves have been laid down.

The breakdown of milk casein produces beta-casomorphins; these short peptides have been shown to affect a variety of physiologic systems.^[83] Because they are opioid agonists, these peptides also have behavioral effects, such as lowering response to pain and elevating mood, that can affect the nursing mother or the newborn. Most of the effects of the beta-casomorphins have been studied in such animals as rats, pigs, and chickens^[83].

The milk produced by women who deliver prematurely differs from that produced after a full-term pregnancy. Specifically, during the first month after parturition, preterm milk maintains a composition similar to that of colostrum. Colostrum, secreted during the first few days after parturition, contains higher concentrations of protein (including higher levels of protective proteins such as secretory IgA, lactoferrin, and lysozyme), sodium, and chloride, and contains lower amounts of potassium, carbohydrate, fat, and certain vitamins. While the transition from colostrum to mature milk is rapid after full-term pregnancy, it proceeds much more slowly after premature delivery.^[84]

Some of the nutritional needs of preterm infants, therefore, cannot be met by feeding the preemie breast milk only. While the mother's own preterm milk is preferable to donor-banked full-term milk, either diet has to be supplemented with protein, calcium, and phosphorus in the preterm infant.^[85] However, given the many benefits to the preterm infant that accrue from the mother's own milk, efforts should be made to encourage mothers of preterm infants to breast-feed, even if during the early stages this might necessitate milk pumping while the infant is hospitalized or is too immature to nurse.

Human milk not only is beneficial during infancy,^[1,2,7,8] but it also may protect the child from chronic diseases that develop at later ages, such as Crohn's,^[86] diabetes mellitus,^[87] and lymphomas.^[88] Also, cognitive development, assessed at 7.5-8.0 years of age, seems to be affected by early diet in the preterm infant. A significantly higher score on the Wechsler Intelligence Scales for Children-Revised (WISC-R) was found in children fed expressed human milk than in those fed formula in early infancy.^[89,90] Similar findings have been reported for full-term infants.^[91]

Given the short-term and long-term benefits of breast-feeding, many working women continue to breast-feed after returning to work. Collection and proper storage of milk in the workplace might not always be easy, because it may be difficult to find a quiet, isolated place where the mother can pump milk, or a refrigerator for milk storage. However, one study showed that milk can be safely stored for up to 24 hours at 60deg.F,^[47] a temperature that can be maintained in a styrofoam box with a frozen ice pack. Efforts should be made to make the workplace an easier environment in which women who choose to breast-feed can do so.

We have just begun to assess the many components in human milk and their interaction with the infant. Much work lies ahead to understand in depth the immediate and long-term effects of feeding mother's milk to newborns. As researchers continue to discover the unique features of breast milk, clinicians need to encourage the practice for the sake of the benefits breast-feeding can bring to both mothers and infants.

Major nutrients	g/liter
Carbohydrate	72.0±2.5
Protein	10.5±2.0
Fat	39.0±4.0
Macronutrients
Minerals	mg/liter
Calcium	280±26
Chloride	420±60
Magnesium	35±2
Phosphorus	140±22
Potassium	525±35
Trace Elements	ug/liter
Chromium	50±5
Copper	250±30
Fluoride	16±5
Iodine	110±40
Iron	300±100
Manganese	6±2
Molybdenum	NR
Selenium	20±5
Zinc	1200±200
Vitamins
Fat-soluble	mg/liter
Vitamin A, RE*	670±200 (2230 IU)
Vitamin D	0.55±0.10
Vitamin E	2300±1000
Vitamin K	2.1±0.1
Water-soluble	mg/liter
Vitamin B6	93,000±8,000
Vitamin B12	0.97
Biotin	4±1
Vitamin C	40,000±10,000
Folate	85±37
Niacin	1500±200
Pantothenic acid	1800±200
Riboflavin	350±25
Thiamin	210±35

Reprinted from Hamosh et al: Nutrition During Lactation, (1991, p 116), Copyright (c) 1991, National Academy Press.Data (means ± SD); IU = international units; NR = not reported; RE = retinol equivalents.

Cytokines	Possible Functions	Concentrations
IL-1b	Activates T cells	~ 1130pg/mL
IL-6	Enhances IgA production	~ 151pg/mL
IL-8	Chemotaxin for neutrophils/T cells	~ 3500pg/mL
IL-10	Decreased inflammatory cytokine synthesis	~ 3500pg/mL
TNF-a	Increased secretory component production	~ 620pg/mL
TGF-b	Enhances Ig isotype switching to IgA+ B cells	~ 130pg/mL
M-CSF	Induce proliferation and differentiation of macrophages	~ 2000-9000 U/mL

*Milk collected during the first several days of lactation. Data are mean values.
From Goldman AS, et al.^[42]

Enteric Pathogens	Respiratory Pathogens
* Bacteria, Toxins, Virulence Factors Clostridium difficile Escherichia coli Salmonella spp Shigella spp Vibrio cholerae * Parasites Giardia lamblia * Viruses Polioviruses Rotaviruses	* Bacteria Haemophilus influenzae Streptococcus pneumoniae Klebsiella pneumoniae * Viruses Influenza viruses Respiratory syncytial virus * Fungi Candida albicans * Food Proteins Cow's milk Soy

From Goldman AS, Goldblum RM. Immunologic systems in human milk: Characteristics and effects, in Lebenthal E (ed): Textbook of Gastroenterology and Nutrition in Infancy, ed 2. New York, Raven Press, 1989, pp 135-142.

Component Enzymes	Function
Catalase	Degrades hydrogen peroxide
Histaminase	Degrades histamine
Arysulfatase	Degrades leucotrienes
Antioxidants a-Tocopherol Cysteine Ascorbic acid	Scavengers of oxygen radicals
Antiproteases a -1-antitrypsin a -1-antichymotrypsin	Neutralize enzymes that act in inflammation
Prostaglandins PG-E2 PG-F2	Cytoprotective

Reprinted from Acta Paediatr Scand (1986; 689), Copyright (c) 1986, Scandinavian University Press.

Immune Factors in Human Milk	When Immune Factors Mature in the Infant
Secretory IgA (sIgA)	~ 4-12 months
Full antibody repertoire	~ 24 months
Lysozyme	~ 1-2 years
Lactoferrin	?
Interleukin-6	?
PAF-acetylhydrolase	?
Memory T cells	2 years

Reprinted from Pediatr Infect Dis J (1993; 12:664-672), Copyright (c) 1993, Williams and Wilkins.

Growth Factor	Colostrum	Milk
EGF*	6-73 nM	3-19 nM
NGF	Not quantified
Insulin*	21.5±5mg/L	2.6±0.3mg/L
IGF-I	10.9±5.3mg/L	7.1-19.1mg/L
IGF-II	NR	2.7±0.7mg/L
Relaxin	327±110mg/L	509±5.3ng/L
TGF-a	2.2-7.2mg/L	0-8.4mg/L

* EGF concentration higher in preterm colostrum and milk, insulin concentration lower in preterm colostrum and milk than in term milk. From Donovan et al. ^[64]

Growth Factor/Hormone	Maternal Mammary Gland	Newborn
PRL	Maintenance of lactation	Neuroendocrine and immune system
Corticosterone	Synthetic capacity (enzymes, specific proteins, etc.)	Response to stress in the adult
Insulin	Growth via IGF-II or IGF-I	Neonatal glycemia
IGFs	Growth and (?) differentiation of gland	GI growth, affect IGF receptors in intestine (?) systemic growth effects
Relaxin	Growth and differentiation
EGF, TGF-a	Growth	GI growth, gut closure, eye opening
TGF-b	Inhibits growth	Inhibits enterocyte growth in ovarian GnRH receptors
GnRH		(?) GH secretion
GRH		(?) GH secretion
TRH		(?) TSH secretion
PTHrP	(?) Ca/P/Mg in milk
Salmon calcitonin-like peptide	PRL inhibiting factor
Erythropoietin		Stimulates erythropoiesis
Prostaglandins	Cytoprotection for intestine

EGF: epidermal growth factor; IGF: insulin like growth factor; PRL: prolactin.
From Grosvenor et al.^[62]

Function	Enzyme(s)	Process(s)
Biosynthesis of milk components in the mammary gland	Phosphoglucomutase	Synthesis of lactose
	Lactose synthetase	Synthesis of lactose
	Fatty acid synthetase	Synthesis of medium-chain fatty acids
	Lipoprotein lipase	Uptake of circulating triglyceride fatty acids
Digestive function in the infant	Amylase	Hydrolysis of polysaccharides
	Lipase (bile salt-dependent)	Hydrolysis of triglycerides
	Proteases*	Proteolysis (not verified)
Transport in the infant	Xanthine oxidase	Carrier of iron, molybdenum
	Glutathione peroxidase	Carrier of selenium
	Alkaline phosphatase	Carrier of zinc, magnesium
Preservation of milk components	Antiproteases	Protection of bioactive proteins (ie, enzymes and immunoglobulins)
	Sulfhydryl oxidase	Maintenance of structure and function of proteins containingS-S bonds
Anti-infective agents	Lysozyme	Bactericidal
	Peroxidase	Bactericidal
	Lipases (lipoprotein lipase, bile salt-dependent lipase)	Release of free fatty acids that have antibacterial, antiviral,and antiprotozoan actions
Protection against enterocolitis	PAF-AH	Hydrolysis of platelet necrotizing activity factor

*It is not known whether the proteolytic enzymes of milk are active because of possible interaction with milk antiproteases. PAF-AH = Platelet activity factor acetyl hydrolase.
From Hamosh.^[66]

Characteristic Maternal factors	Amylase	Bile salt-dependent lipase
High parity (>10)	Low activity	?
Malnutrition	?	Decrease in activity
Diurnal and within feed activity	Constant	Constant
Pattern of secretion
Prepartum	?	Present
Colostrum	Colostrum greater than milk	Colostrum lower than milk
Milk after preterm (PT) and term (T) delivery	Equal activity PT and T	Equal activity PT and T
Weaning	?	Activity constant independent of milk volume
Distribution in milk	Aqueous phase	Aqueous phase
Effect of milk storage Temperature
Cold: -20deg.C to -70deg.C	Stable	Stable
Warm: +15deg.C to +38deg.C	Stable (at least 24 hrs)
Stable (at least 24 hrs)
Effect of pH
Low pH (pH>3.0) (passage through stomach)	Stable	Stable
pH optimum	6.5-7.5	7.4-8.5
Enzyme character	Identical to salivary amylase isozyme	Identical to pancreatic carboxyl ester lipase
Evidence of activity in infant's intestine	Yes	Yes
Presence in milk of other species	?	Yes, in primates and carnivores

From Hamosh.^[66]

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