BIOCHEMISTRY OF HUMAN MILK

BIOCHEMISTRY OF HUMAN MILK
Human milk is a unique, species-specific, complex nutritive fluid with immunologic- and growth-promoting properties. This unique fluid actually evolves to meet the changing needs of the baby during growth and maturation. The synthesis and secretion of milk by the mammary gland involve numerous cellular pathways and processes. The processing and packaging of nutrients within human milk changes over time as the recipient infant matures. For example, early milk or colostrum has lower concentrations of fat than mature milk but higher concentrations of protein and minerals. This relationship reverses as the infant matures.

Fore and hind milk - important differences
In addition to the changes from colostrum to mature milk that mirror the needs of the developing neonate, variation exists within a given breastfeeding session. The milk first ingested by the infant (fore milk) has a lower fat content. As the infant continues to breastfeed over the next several minutes, the fat content increases (hind milk), which is thought to facilitate satiety in the infant. Finally, diurnal variations in breast milk exist, which reflect maternal diet and daily hormonal fluctuations.

Specific enzymes to aid neonatal digestion
Various enzymes are components of human milk, some specific for the biosynthesis of milk components in the mammary gland (eg, lactose synthetase, fatty acid synthetase, thioesterase) and others specific for the digestion of proteins, fats, and carbohydrates that facilitate food breakdown and absorption of human milk by the infant. Certain enzymes also serve as transport moieties for other substances such as zinc, selenium, and magnesium.

Three-dimensional structure of human milk
Under a microscope, the appearance of human milk is truly amazing. While a fluid, human milk has substantial structure in the form of compartmentation. Within the various compartments of human milk, nutrients and bioactive substances are sequestered. The most elegant example of this structure involves lipids. Lipids are enveloped at the time of secretion from the apical mammary epithelial cell within its plasma membrane, becoming the milk-fat globule. Certain proteins, growth factors, and vitamins also become sequestered within this milk-fat globule and are embedded within the membrane itself.

The membrane acts as a stabilizing interface between the aqueous milk components and compartmentalized fat. This interface allows controlled release of the products of lipolysis and transfer of polar materials into milk serum (aqueous phase). The bipolar characteristics of the membrane are also necessary for the emulsion stability of the globules themselves; thus, the structure of human milk provides readily available fatty acids and cholesterol for micellar absorption in the small intestine.

Proteins, carbohydrates, and designer fats for optimal brain development
Human milk provides appropriate amounts of proteins (the major components of which are alpha-lactalbumin and whey), carbohydrates (lactose), minerals, vitamins, and fats for the growing term infant. The fats are comprised of cholesterol, triglycerides, short-chain fatty acids, and long-chain polyunsaturated (LCP) fatty acids. The LCP fatty acids (18- to 22-carbon length) are needed for brain and retinal development. Large amounts of omega-6 and omega-3 LCP fatty acids, predominately the 20-carbon arachidonic acid (AA) and the 22-carbon docosahexaenoic acids (DHAs), are deposited in the developing brain and retina during prenatal and early postnatal growth.

An infant, particularly a preterm infant, may have a limited ability to synthesize optimal levels of AA and DHA from linoleic and linolenic acid. These 2 fatty acids may be essential. Recently, some infant formulas in the United States have added AA or DHA. Increasing evidence suggests that breastfed infants have better visual acuity at 4 months and slightly enhanced cognitive development than formula-fed infants, even when socioeconomic factors are taken into account. These differences are more pronounced in premature infants. Rather than causing better vision or greater intelligence, breast milk may somehow protect the developing neonatal brain from injury or less optimal development by providing necessary building materials and growth factors.