Human milk is, of course, the default nutritional source for human infants. Besides the milk sugar, lactose, human milk is composed of hundreds or thousands of bioactive compounds with anti-microbial and anti-inflammatory activity, besides their key role in organ development, immune system maturation, and the evolution of a protective microbiome.
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Breast milk is a dynamic fluid, with obvious changes in its composition between the first watery colostrum to the carbohydrate-rich milk of late lactation. Even more, it varies over a single feed, with the time of the day, as well as distinct inter-individual and inter-population variations.
Milk is made up of many compartments, including solutions, colloids or micelles of casein, membranes and membrane-bound fat globules, and live cells. It may be said to contain aqueous and lipid fractions.
Colostrum to mature milk
Colostrum is predominantly meant to strengthen the infant’s immunity and stimulate the growth of the body tissues. Produced in the first few days after childbirth, it contains abundant secretory immunoglobulin A (sIgA) antibodies, as well as lactoferrin and white blood cells. Growth factors are also present.
It is distinctly yellowish due to high levels of carotenoids, such as α-arotene, β-carotene, β-crytoxanthin, lutein, and xeaxanthin. The concentrations of these constituents fall ten-fold by the time mature milk is secreted, at 0.1 to 0.3 mg/liter in colostrum vs 0.34 to 7.57 mg/liter in mature milk. Lactose concentrations are low, as are potassium and calcium.
Over a few days, the mammary gland epithelial cells close up as tight junctions are formed, and the sodium levels fall while lactose levels increase. The secretory cells of the mammary gland are now activated, and transitional milk is produced, in stage II of milk production.
At this point, the amount of milk being produced increases as well. This period lasts for up to two weeks. After this period, mature milk is secreted and remains stable in composition.
In every 100 mL of mature milk produced after a term delivery, there are approximately 0.9 to 1.2 grams of protein, with 3.2 to 3.6 grams of fat. Lactose makes up 6.7 to 7.8 grams. This yields 65 to 70 kilocalories of energy when metabolized, though the amount of fat is the primary determinant.
Preterm milk contains more protein and fat, corresponding to the increased energy needs of a preterm baby.
However, at about four months after childbirth, milk composition depends on factors such as the maternal fat mass in relation to the lean mass, number of children, the presence of amenorrhea, and the number of breastfeedings per day.
The maternal diet is not associated with milk protein concentrations. The greater the amount of milk produced by the woman, the lower is the fat and protein concentrations, but the higher the carbohydrate content.
Lactose accounts for about 60-70% of total osmotic pressure in milk, with twice the energy value of glucose per unit of osmotic pressure.
Proteins and other nitrogenous compounds
Milk proteins include both whey and casein fractions, each being made up of several related proteins. Most of the protein comes from casein, α-lactalbumin, lactoferrin, sIgA, lysozyme, and albumin.
Casein forms micelles along with calcium, magnesium, and phosphate, allowing a large amount of these minerals to be incorporated into milk compared to the aqueous solution. Whey proteins like lactoferrin are made in the breast tissue, while others are transported into milk from the maternal blood.
Lactoferrin is an iron-binding protein with antibacterial, antifungal, and antiviral activity, especially preventing the proliferation of siderophilic (iron-absorbing) bacteria.
There are also nitrogenous compounds that are not proteins, such as urea, creatinine, uric acid, amino acids, and nucleotides, from which about a quarter of the nitrogen in human milk is derived.
The essential amino acid profile of human milk is fine-tuned to that which optimally promotes growth in human infants.
Fats in milk
About 90% of fats in milk are made up of triglycerides, produced in the alveolar cells of the breast. These fatty acids come either from the maternal blood or are newly synthesized within the breast tissue, with the latter predominating as lactation progresses.
Trans fats come from the maternal intake of hydrogenated fats, as well as during postpartum weight loss, from the fat tissue being broken down. Cholesterol and phospholipid levels are not diet-dependent. Phospholipids are present at about 75 mg/100 mL.
Lipoprotein lipase, an enzyme that breaks down triglycerides, is differentially increased in the mammary gland, but reduced in fatty tissue, upon prolactin secretion, before and during lactation.
Human milk has high palmitic and oleic acid content and represents the most variable component of milk. It is two to three times higher in hindmilk, or the last part of the feed, compared to the foremilk. It is also higher in the afternoon and evening feedings.
A high protein intake may improve fat levels in milk. Long-chain polyunsaturated fatty acids (LCPUFAs) in milk from Western women contain more omega-6 fatty acids, considered to be pro-inflammatory, with especially low levels of the omega-3 fatty acid docosahexaenoic acid (DHA), which may therefore be required as a supplement for women in North America who breastfeed.
Polyunsaturated fatty acids (PUFAs), linoleic acid, and total omega-6 fatty acids are higher in younger mothers. Overall, a very high-fat diet causes a dramatic increase in linoleic and linolenic acid, but a low-fat, low-calorie diet causes long-chain fatty acids to predominate, from body fat stores.
The mineral constituents of breast milk comprise 0.2%, including sodium, potassium, calcium, magnesium, phosphorus, and chlorine. Potassium, chloride, and calcium are the most abundant, though the former two are twice as high as the latter.
Sodium and phosphorus are also plentiful. Mineral concentration is not significantly affected by maternal diet. However, iodine supplementation may be helpful as it does improve the level of iodine.
Iron is required for many metabolic processes, and its deficiency especially impacts the rapidly growing infant. Copper is essential for cell respiration and iron metabolism, as well as connective tissue synthesis. Zinc is vital for growth and immune function.
Calcium is a critical second messenger in cell signaling pathways, while phosphorus is a key part of cell membranes and nucleic acids, in addition to the abundance found in bones (which also requires magnesium). It is also vital for cell signaling and energy production.
Magnesium is involved in more than 300 metabolic processes. Iodine is essential for physical and mental growth in infancy, and survival. Selenium is an important component of the cell’s antioxidant and deiodinase proteins.
Human milk also contains vitamins A, B1, B2, B6, B12, D, and iodine, though to a varying degree depending on the maternal diet. This makes multivitamin supplementation during this period a valuable recommendation.
Vitamin C is also plentiful but is rapidly degraded on heating the milk. It is involved in leukocyte activation, enhanced antibody production, and increased interferon synthesis.
Vitamin K is low in milk since it does not cross the placenta easily. Therefore a single vitamin K injection is given at birth to prevent hemorrhagic disease of the newborn. Vitamin D is also low, especially in situations of low sunlight exposure.
Vitamin E has antioxidant activity as well as immunostimulatory properties, and most of it is present as α-tocopherol.
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Bioactives in breastmilk
Bioactive constituents in breast milk are secreted by the mammary epithelium in some cases, such as milk fat globule (MFG). Others are carried across from the mother’s blood by receptor-mediated transport, as illustrated by polymeric immunoglobulin receptors (pIgR) carrying sIgA into the mammary duct lumen.
Still, others come from cells within the milk. Vascular endothelial growth factor (VEGF) comes from the gland itself.
It is important to recognize that the biological activity of such molecules may be impaired following freeze-thaw cycles, as when milk is stored in milk banks, or even by the processes of collection, storage, and pasteurization.
Growth factors in milk
Epidermal growth factor (EGF), which is highest in colostrum at 2000-fold the serum levels, promotes intestinal mucosal maturation and repair. In mature milk, too, it is a hundred times higher.
In the infant, it allows for mucosal integrity by preventing pro-inflammatory TNF-α-mediated loss of tight junctions in the intestine, and by inhibiting programmed cell death. It is also critical in the healing of hypoxic injuries, as well as those caused by ischemia-reperfusion, or by hemorrhagic shock, or necrotizing enterocolitis.
Brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) are vital neurotransmitters in the enteric nervous system, enhancing peristalsis and keeping neurons intact.
The insulin-like growth factor (IGF) family consisting of members like IGF-I and IGF-II, as well as IGF binding proteins and IGF-specific proteases, are again abundant in colostrum, declining over time, and is essential for tissue growth, though its functions are not clear.
Others include VEGF, Erythropoietin (Epo) for red blood cell production, calcitonin and somatostatin to regulate infant growth, adiponectin which modulates metabolism and inflammation, as well as other metabolism-regulating hormones like leptin and ghrelin.
Antibodies and other immune factors
Immunological factors include cells like antibodies, macrophages, stem cells, lymphocytes and T cells, cytokines, chemokines, and immune factors. The presence of sIgA against many pathogens protects the infant until its own secretory immune system matures, which may not be for several months after birth.
The sIgA-producing B cells in the breast tissue come from the small intestine or the respiratory tract, which encounters most pathogens. They travel to the breast via the blood, under the influence of lactogenic hormones. In the breast, they become plasma cells producing dimeric IgA.
This sIgA is effective at mucosal surfaces, resists breakdown by proteases, and does not induce inflammation during its antimicrobial action. It is also synergistic with other agents in human milk that resist infection.
Lysozymes break down some bacterial cell walls, thus killing them in association with other host agents. It is high in human milk unlike bovine milk and resists proteolysis or denaturation by stomach acid.
Leukocytes, cytokines, and growth factors
Up to 1010 leukocytes are swallowed each day by the infant, via breast milk, about 80% being macrophages migrating from the blood into the milk. As they engulf human milk constituents, they transform into a phenotype that can differentiate into dendritic cells that stimulate T cell activity in the infant, protecting the baby against pathogens while promoting maturation of the immune system.
Cytokines are signaling peptides with multiple functions, some enhancing inflammation or preventing infection, and others with immunomodulatory functions. Among the most plentiful in human milk is the TGF-β family, dominated by TGF-β2. These prevent allergic responses and modulate the inflammatory and repair processes. It is secreted as a prohormone and converted to its active form by stomach acid.
Granulocyte-colony stimulating factor (G-CSF) promotes intestinal development, while the inflammatory mediators TNF-α, IL-6, IL-8, and IFNγ are present at lower and declining levels after delivery. These may recruit neutrophils and enhance an inflammatory rather than allergic response to antigens.
Lactadherin is found in the milk fat globulin (MFG) and protects against rotavirus infection. It also triggers the phagocytosis of cells undergoing apoptosis and modulates inflammation.
This important molecule also improves intestinal healing and promotes tolerance in intestinal macrophages and dendritic cells. The MFG also contains mucins from the maternal cell membrane, which protect against infection.
Bile salt stimulating lipase (BSSL) is another glycoprotein that makes milk fat energy available to the infant, also preventing viral infections.
Human milk oligosaccharides (HMOs) contain 3-32 sugar molecules and are unique to our species among mammals. They are present at up to 2.5 grams/100 mL in colostrum, decreasing slightly to 1 gram/100 mL in mature milk.
They cannot be used for energy or growth, but are enzymes called glycosyltransferases that have a prebiotic function, enhancing the establishment of probiotic organisms.
They are also decoy soluble receptors that bind pathogens and thus inhibit infection. Though HMOs composition varies between mothers, this only broadens the range of pathogen binding overall. These molecules may also feed the microbes in milk, helping them colonize the infant's intestine.
While breast milk composition remains largely intact irrespective of maternal nutrition, it is important to remember that when the mother is ill-nourished, the nutrients in milk come from the mother’s tissues, if her stored reserves have been used up. This is especially true of vitamins B6, B12, A, and D.
Conversely, occasional shortages in the diet do not alter the nutrient profile of breast milk. Excessive vitamin D or iodine is to be avoided to prevent toxicity to the infant.
Some constituents of breast milk are being explored as novel treatment approaches, such as lactoferrin, lactoadherin, HMOs, or EPO. These could be of great use in preventing and treating infection in preterm infants.