Human Milk Composition and Function in the Infant

Introduction

Lactation is the defining characteristic of “mammal” and has enabled the wide distribution of more than the 4000 species. The evolution of a large brain that consumes approximately 23% of resting energy has given humankind a significant competitive intellectual advantage over all other mammals. Unlike other mammals, most brain growth in humans occurs after birth and is facilitated by the nutrients present in human milk (HM). The synthesis of HM occurs in the lactating mammary gland, which, similar to the brain, is a very active organ, with the energy in HM representing approximately 30% of resting energy. Its evolution as a secretory organ is closely associated with the innate and acquired immune systems. However, because there is only a small body of basic research on human lactation, evidence-based medical assessment and treatment of lactation difficulties are very limited. Unlike other metabolically equivalent organs in the body, no clinical tests are currently available to assess the normal function of the lactating breast. This is of particular importance when considering the composition of HM and its function in the infant.

After birth, the transfer of nutritional and bioactive components from mother to infant continues through colostrum and milk. The substitution of infant formula for HM deprives the infant not only of nutrients ( Table 23.1 ) that are more accessible from HM than formula (e.g., essential amino acids, casein), but also of many bioactive and immune-protective factors (e.g., oligosaccharides, lactoferrin, lysozyme, leukocytes) directed specifically against pathogens in the infant’s environment, thus placing the formula-fed infant at a distinct disadvantage ( Table 23.2 ). HM components also compensate for immature function in the newborn, in whom endogenous production of digestive enzymes, immunoglobulin A (IgA), taurine, choline, nucleotides, and long-chain polyunsaturated fatty acids (LCPUFAs) is insufficient. Furthermore, recent research has revealed the presence of stem cells in HM, some of which are capable of surviving the infant’s gastrointestinal tract and integrating into its organs, potentially providing developmental benefits. These nutritional and bioactive components are the reason for the superiority of HM to infant formula.

The composition of HM is indeed spectacular. In addition to viable maternal cells, it contains more than 900 proteins, many bioactive peptides, 200 oligosaccharides, thousands of triacylglycerols (TAGs), approximately 100 known metabolites, hormones, cytokines, and a full complement of minerals and vitamins. Some of these components vary from the beginning to the end of a breast-feed, over the day, with diet, and over the lactation period. Unfortunately, with the notable exception of the growth of breast-fed infants (World Health Organization [WHO] growth charts), there are no reference ranges for normality (predicted values that cover 95% of individuals) for milk production and milk composition. Thus the values given here for the concentration of the components in HM are only a guide to reference ranges for normality.

Milk Volume

Daily milk production in women is relatively constant regardless of maternal nutritional status. Exclusively breast-fed healthy infants have a mean daily milk intake of 750 to 800 mL/24 hours from 1 to 6 months of lactation. However, a wide range of intake volumes from 450 to 1200 mL/24 hours has been reported. Despite these differences in milk intake, there is a significant relationship between milk intake and infant growth rate. Milk intake remains consistent from 1 to 6 months. This is not surprising, considering younger infants (1 to 3 months of age) grow more rapidly than older infants (4 to 6 months of age). Smaller infants also have a larger surface area to volume ratio and therefore have a higher metabolic rate per kilogram of body weight and use more of their nutrient intake for maintenance of body temperature than do older, heavier infants.

Similar levels of milk production have been reported for mothers globally, including in developing countries, although maternal nutritional status may be subject to seasonal variation and may be less than adequate based on industrial country standards. Increasing the intake of fluids does not seem to affect milk production; therefore lactating women should maintain adequate fluid intake, but they should be aware that excess fluids have no impact on milk volume.

The measurement of daily milk production provides an objective measure of mammary gland function and has been shown to be useful to both the clinician and the mother without undermining maternal confidence. In contrast, measurement of milk intake at a single incidence of breast-feeding is of little value because milk intake is controlled by the infant’s appetite and can vary greatly from one breast feeding to the next.

Milk Composition

HM is a unique dynamic fluid that is clearly species specific. It contains nutritional components such as fat, protein, lactose, and micronutrients at levels suitable for optimal growth, as well as bioactive and cellular components that provide protection and promote infant development. These components enter the milk via two different routes; they are either synthesized by the lactocytes that line the mammary alveoli, or they are transferred directly from the maternal circulation and/or modified within the lactocyte after uptake from the blood.

Protein

The nitrogen content of HM (1.71 ± 0.31 g/L) consists of protein and nonprotein (approximately 25%; 0.42 ± 0.10 g/L) components.

Nonprotein nitrogen is derived from a number of components such as free amino acids, peptides, creatine and creatinine, nucleic acids and nucleotides, urea, uric acid, ammonia, amino sugars, polyamines, carnitine, and other compounds. A number of these components have functions for the infant; for example, nucleotides and nucleosides become semiessential during periods of high physiologic growth such as in the preterm infant, with evidence suggesting their involvement in lymphocyte production and regulation of gut development. Carnitine is also essential for fatty acid metabolism and is involved in lipolysis, ketogenesis, and thermogenesis. Similarly, taurine is involved in fat absorption, bile acid secretion, hepatic function, and retinal function.

Protein levels in HM are relatively low, constituting 0.8% to 0.9% by weight; nevertheless, they are highly bioavailable to the infant. Protein content fluctuates during lactation with high levels during early lactation (15.8 ± 4.2 g/L) that gradually decline to a relatively stable level in mature milk (6.9 ± 1.2 g/L).

Three distinct groups of protein exist in HM: caseins as micellar structures, whey water-soluble proteins, and mucins, which are associated with the milk fat globule membrane.

Caseins are a major protein in mammalian milk. Casein micelles consist of several casein subunits, calcium phosphate, and other ionic constituents and give bovine milk its characteristic white appearance. Caseins are less abundant in HM, which accounts for its distinctive pale blue appearance. The functions of caseins are largely nutritive, providing essential amino acids and minerals to the infant. For example, the casein micelle is the main source of calcium and phosphorous for the infant and is essential for bone mineralization. Intrinsic protease activity in the breast and in the infant’s stomach creates casein-derived peptides that have a myriad of effects, including antimicrobial (e.g., casecidin), immunomodulatory, antithrombic (e.g., casopiastrin), antihypertensive, opioid (β-casomorphins), and gastrointestinal functions. HM has low casein content: less than 10% total protein content of colostrum, 40% in transitional milk, and 50% in mature milk. Low levels of casein result in soft curds formed in the infant’s stomach, which are easily digested, facilitating gastric emptying, and are therefore more compatible with frequent feeding. In comparison, the concentration of casein in bovine milk is more than 10 times that of HM, resulting in the formation of harder curds in the infant’s stomach, the effects of which may be offset to some degree by addition of whey protein to bovine-based formula. Lower casein content is also commensurate with the slow growth rate of human infants compared with the offspring of other mammals.

Whey proteins account for the major proportion of protein content in HM (90% of the total protein content in colostrum, 60% in mature milk, and 50% in late lactation). Whey is a complex protein fraction comprising a large number of proteins. Several proteins are abundant in the whey portion; however, there are many lower abundant proteins that have yet to be well characterized. The major immunologic proteins present in the whey fraction are lactoferrin, lysozyme, and secretory IgA (sIgA), α-lactalbumin and BSSL.

sIgA is the most abundant immunoglobulin in HM, accounting for more than 90% of immunoglobulins and up to 25% of total protein content. The sIgA concentration is higher in early lactation (1 to 2 g/L) and lower in later lactation (0.5 to 10 g/L). Adaptive immunity is achieved by IgA via the enteromammary pathway, in which IgA-producing lymphocytes in the maternal intestine are transferred to the mammary gland during lactation. sIgA is resistant to digestion, thereby boosting the infant’s immature immune system. Protection is afforded mainly against pathogens from the intestine.

Lactoferrin, another major protein in HM, is present at 305 g/L in colostrum, falling to 1.9 g/L as milk production is established. Each molecule of lactoferrin has the capability to bind two ferric ions, and binds most of the iron in HM is bound by lactoferrin. Sequestering of iron contributes to lactoferrin’s bacteriostatic properties. Proteolysis of lactoferrin in the infant stomach produces lactoferricins that possess diverse functions. In vitro studies in cultured human intestinal cells also show that lactoferrin facilitates the uptake of iron due to a lactoferrin receptor specific to enterocytes.

α-Lactalbumin comprises 10% to 20% of the total protein in HM and has 40% gene similarity to lysozyme, implying that it was intimately entwined in the evolution of the mammary gland from the innate immune system. α-Lactalbumin binds Ca ²⁺ and Zn ²⁺ and is involved in lactose synthesis. Furthermore, the amino acid composition of HM α-lactalbumin is similar to the infant’s requirements for amino acids. Although supplementation of infant formula with bovine α-lactalbumin increases absorption of zinc and iron in infant rhesus monkeys, the effect on mineral absorption in breast-fed infants has not been investigated.

Lysozyme, one of the major three proteins in HM, is found in particularly high concentrations in HM compared with the milk of other species. It is thought to be synthesized by the mammary epithelial cells. The HM form of lysozyme is considered identical in structure to lysozyme in other bodily fluids such as pancreatic juice and saliva. It possesses bacteriostatic properties by lysis of gram-positive and some gram-negative bacteria.

BSSL levels are relatively low in HM (1% to 2% of total milk proteins), but BSSL has an important role in efficient digestion of dietary fats. It is present in bovine milk but is lost from infant formula during the manufacturing process. In the intestinal lumen, activation by bile salts enables BSSL to hydrolyze lipid substrates, such as short- and long-chain triacylglycerides, diacylglycerides, monoacylglycerides, cholesteryl esters, retinol esters, and p -nitrophenyl esters. BSSL is unfortunately inactivated by heat pasteurization, resulting in reduced fat absorption in preterm infants. A novel method of pasteurization with ultraviolet (UV)-C radiation has been shown to retain almost all of the BSSL activity when compared with raw milk, but this has yet to be tested clinically to determine whether fat absorption is improved in infants fed with UV-C–pasteurized milk.