Question: How much protein is contained in human milk?

Breast milk


Breast milk is the milk produced by the breasts (or mammary glands) of a human female to feed a child. Milk is the primary source of nutrition for newborns before they are able to eat and digest other foods; older infants and toddlers may continue to be breastfed,  in combination with other foods from six months of age when solid foods should be introduced.
Methods
The baby nursing from its own mother is the most common way of obtaining breast milk, but the milk can be pumped and then fed by baby bottle, cup and/or spoon, supplementation drip system, or nasogastric tube. In preterm children who do not have the ability to suck during their early days of life, avoiding bottles and tubes, and use of cups to feed expressed milk and other supplements is reported to result in better breastfeeding extent and duration subsequently.[1] Breast milk can be supplied by a woman other than the baby's mother, either via donated pumped milk (generally from a milk bank or via informal milk donation), or when a woman nurses a child other than her own at her breast, a practice known as wetnursing.
The World Health Organization recommends exclusive breastfeeding for the first six months of life, with solids gradually being introduced around this age when signs of readiness are shown. Supplemented breastfeeding is recommended until at least age two and then for as long as the mother and child wish.[2]
Benefits
Breastfeeding offers health benefits to mother and child even after infancy.[3] These benefits include a 73% decreased risk of sudden infant death syndrome,[4] increased intelligence,[5] decreased likelihood of contracting middle ear infections,[6] cold and flu resistance,[7] a tiny decrease in the risk of childhood leukemia,[8] lower risk of childhood onset diabetes,[9] decreased risk of asthma and eczema,[10] decreased dental problems,[10] decreased risk of obesity later in life,[11] and a decreased risk of developing psychological disorders, including in adopted children.[12][13] In addition, feeding an infant breast milk is associated with lower insulin levels and higher leptin levels compared feeding an infant via powdered-formula.[14]
Breastfeeding also provides health benefits for the mother. It assists the uterus in returning to its pre-pregnancy size and reduces post-partum bleeding, as well as assisting the mother in returning to her pre-pregnancy weight. Breastfeeding also reduces the risk of breast cancer later in life.[15][16] Lactation protects both mother and infant from both types of diabetes.[17] Lactation may protect the infant from specifically developing Type 2 diabetes because studies have shown that bioactive ingredients in human breast milk could prevent excess weight gain during childhood via contributing to a feeling of energy and satiety. A lower risk of child-onset diabetes may more applicable to infants who were born from diabetic mothers.[14] The reason is because while breastfeeding for at least the first 6 months of life minimizes the risk of Type 1 diabetes from occurring in the infant, inadequate breastfeeding in an infant prenatally exposed to diabetes was associated with a higher risk of the child developing diabetes later on.[14] However, it can be argued that human breastfeeding may contribute to protective effects against the development of Type 1 diabetes due to the fact that the alternative of bottle-feeding may expose infants to unhygienic feeding conditions.[18]
Though it now is almost universally prescribed, in some countries in the 1950s the practice of breastfeeding went through a period where it was out of vogue and the use of infant formula was considered superior to breast milk. However, it is now universally recognized that there is no commercial formula that can equal breast milk. In addition to the appropriate amounts of carbohydrate, protein, and fat, breast milk provides vitamins, minerals, digestive enzymes,[19] and hormones.[19] Breast milk also contains antibodies and lymphocytes from the mother that help the baby resist infections.[20] The immune function of breast milk is individualized, as the mother, through her touching and taking care of the baby, comes into contact with pathogens that colonize the baby, and, as a consequence, her body makes the appropriate antibodies and immune cells.[21]
At around four months of age, the internal iron supplies of the infant, held in the hepatic cells of the liver, are exhausted. The American Academy of Pediatrics recommends that at this time that an iron supplement should be introduced,[22] however, other health organisations such as the NHS in the UK have no such recommendation.[23] Breast milk contains less iron than formula, because it is more bioavailable as lactoferrin, which carries more safety for mothers and children than ferrous sulphate.[24]
Both the AAP[25] and the NHS recommend vitamin D supplementation for breastfed infants.[23] Vitamin D can be synthesised by the infant via exposure to sunlight, however, many infants are deficient due being kept indoors or living in areas with insufficient sunlight. Formula is supplemented with vitamin D for this reason.[23][25]
Production

Under the influence of the hormones prolactin and oxytocin, women produce milk after childbirth to feed the baby. The initial milk produced is referred to as colostrum, which is high in the immunoglobulin IgA, which coats the gastrointestinal tract. This helps to protect the newborn until its own immune system is functioning properly. It also creates a mild laxative effect, expelling meconium and helping to prevent the build-up of bilirubin (a contributory factor in jaundice).
Actual inability to produce enough milk is rare, with studies showing that mothers from developing countries experiencing nutritional hardship still produce amounts of milk of similar quality to that of mothers in developed countries.[26] There are many reasons a mother may not produce enough breast milk. Some of the most common reasons are an improper latch (i.e., the baby does not connect efficiently with the nipple), not nursing or pumping enough to meet supply, certain medications (including estrogen-containing hormonal contraceptives), illness, and dehydration. A rarer reason is Sheehan's syndrome, also known as postpartum hypopituitarism, which is associated with prolactin deficiency and may require hormone replacement.
The amount of milk produced depends on how often the mother is nursing and/or pumping: the more the mother nurses her baby or pumps, the more milk is produced.[27][28][29] It is beneficial to nurse when the baby wants to nurse rather than on a schedule. A Cochrane review came to the conclusion that a greater volume of milk is expressed whilst listening to relaxing audio during breastfeeding, along with warming and massaging of the breast prior to and during feeding. A greater volume of milk expressed can also be attributed to instances where the mother starts pumping milk sooner, even if the infant is unable to breastfeed.[30]
Sodium concentration is higher in hand-expressed milk, when compared with the use of manual and electric pumps, and fat content is higher when the breast has been massaged, in conjunction with listening to relaxing audio. This may be important for low birthweight infants.[30] If pumping, it is helpful to have an electric, high-grade pump so that all of the milk ducts are stimulated. Galactagogues increase milk supply, although even herbal variants carry risks; therefore non-pharmaceutical methods should be tried first.[31][32]
Composition


Breast milk contains complex proteins, lipids, carbohydrates and other biologically active components. The composition changes over a single feed as well as over the period of lactation.[34]

During the first few days after delivery, the mother produces colostrum. This is a thin yellowish fluid that is the same fluid that sometimes leaks from the breasts during pregnancy. It is rich in protein and antibodies that provide passive immunity to the baby (the baby's immune system is not fully developed at birth). Colostrum also helps the newborn's digestive system to grow and function properly.
Colostrum will gradually change to become mature milk. In the first 3–4 days it will appear thin and watery and will taste very sweet; later, the milk will be thicker and creamier. Human milk quenches the baby's thirst and hunger and provides the proteins, sugar, minerals, and antibodies that the baby needs.
In the 1980s and 1990s, lactation professionals (De Cleats) used to make a differentiation between foremilk and hindmilk. But this differentiation causes confusion as there are not two types of milk. Instead, as a baby breastfeeds, the fat content very gradually increases, with the milk becoming fattier and fattier over time.[35]
The level of Immunoglobulin A (IgA) in breast milk remains high from day 10 until at least 7.5 months post-partum.[36]
Human milk contains 0.8% to 0.9% protein, 4.5% fat, 7.1% carbohydrates, and 0.2% ash (minerals).[37] Carbohydrates are mainly lactose; several lactose-based oligosaccharides have been identified as minor components. The fat fraction contains specific triglycerides of palmitic and oleic acid (O-P-O triglycerides), and also lipids with trans bonds (see: trans fat). The lipids are vaccenic acid, and Conjugated linoleic acid (CLA) accounting for up to 6% of the human milk fat.[38][39]
The principal proteins are alpha-lactalbumin, lactoferrin (apo-lactoferrin), IgA, lysozyme, and serum albumin. In an acidic environment such as the stomach, alpha-lactalbumin unfolds into a different form and binds oleic acid to form a complex called HAMLET that kills tumor cells. This is thought to contribute to the protection of breastfed babies against cancer.[40]
Non-protein nitrogen-containing compounds, making up 25% of the milk's nitrogen, include urea, uric acid, creatine, creatinine, amino acids, and nucleotides.[41][42] Breast milk has circadian variations; some of the nucleotides are more commonly produced during the night, others during the day.[43]
Mother's milk has been shown to supply endocannabinoids (the natural neurotransmitters that cannabis simulates) 2-Arachidonoyl glycerol,[44] anandamide,[45][46] oleoylethanolamide,[47] palmitoylethanolamide,[47] N-arachidonoyl glycine,[47] eicosapentaenoyl ethanolamide,[47] docosahexaenoyl ethanolamide,[47] N-palmitoleoyl-ethanolamine,[47] dihomo-γ-linolenoylethanolamine,[47] N-stearoylethanolamine,[47] prostaglandin F2alpha ethanolamides[47] and prostaglandin F2 ethanolamides,[47] Palmitic acid esters of hydroxy-stearic acids (PAHSAs).[48]  They may act as an appetite stimulant, but they also regulate appetite so infants don't eat too much. That may be why formula-fed babies have a higher caloric intake than breastfed babies.[49]
Breast milk isn't sterile, but contains as many as 600 different species of various bacteria, including beneficial Bifidobacterium breve, B. adolescentis, B. longum, B. bifidum, and B. dentium.[50]
Breast milk contains a unique type of sugars, human milk oligosaccharides (HMOs), which are not present in infant formula. HMOs are not digested by the infant but help to make up the intestinal flora. They act as decoy receptors that block the attachment of disease causing pathogens, which may help to prevent infectious diseases. They also alter immune cell responses, which may benefit the infant. To date (2015) more than a hundred different HMOs have been identified; both the number and composition vary between women and each HMO may have a distinct functionality.[51]
The breast milk of diabetic mothers has been shown to have a different composition from that of non-diabetic mothers. It may contain elevated levels of glucose and insulin and decreased polyunsaturated fatty acids. A dose-dependent effect of diabetic breast milk on increasing language delays in infants has also been noted, although doctors recommend that diabetic mothers breastfeed despite this potential risk.[52]
Women breastfeeding should consult with their physician regarding substances that can be unwittingly passed to the infant via breast milk, such as alcohol, viruses (HIV or HTLV-1) or medications. Even though most infants infected with HIV contract the disease from breastfeeding, most infants that are breastfed by their HIV positive mothers never contract the disease.[53] While this paradoxical phenomenon suggests that the risk of HIV transmission between an HIV positive mother and her child via breastfeeding is small, studies have also shown that feeding infants with breast milk of HIV-positive mothers can actually have a preventative effect against HIV transmission between the mother and child.[53] This inhibitory effect against the infant contracting HIV is likely due to unspecified factors exclusively present in breast milk of HIV-positive mothers.[53]
Most women that do not breastfeed use infant formula, but breast milk donated by volunteers to human milk banks can be obtained by prescription in some countries.[54] In addition, research has shown that women who rely on infant formula could minimize the gap between the level of immunity protection and cognitive abilities a breastfed child benefits from versus the degree to which a bottle-fed child benefits from them. This can be done by supplementing formula-fed infants with bovine milk fat globule membranes (MFGM) meant to mimic the positive effects of the MFGMs which are present in human breast milk.[55]
Storage of expressed breast milk


Expressed breast milk can be stored. Lipase may cause thawed milk to taste soapy or rancid due to milk fat breakdown. It is still safe to use, and most babies will drink it. Scalding it will prevent rancid taste at the expense of antibodies.[56] It should be stored with airtight seals.  Some plastic bags are designed for storage periods of less than 72 hours. Others can be used for up to 12 months if frozen. This table describes safe storage time limits.[57]

Comparison to other milks
All mammalian species produce milk, but the composition of milk for each species varies widely and other kinds of milk are often very different from human breast milk. As a rule, the milk of mammals that nurse frequently (including human babies) is less rich, or more watery, than the milk of mammals whose young nurse less often. Human milk is noticeably thinner and sweeter than cow's milk.
Whole cow's milk contains too little iron, retinol, vitamin E, vitamin C, vitamin D, unsaturated fats or essential fatty acids for human babies.[58][59][60][61] Whole cow's milk also contains too much protein, sodium, potassium, phosphorus and chloride which may put a strain on an infant's immature kidneys. In addition, the proteins, fats and calcium in whole cow's milk are more difficult for an infant to digest and absorb than the ones in breast milk.[59][62][63] Evaporated milk may be easier to digest due to the processing of the protein but is still nutritionally inadequate. Some infants are allergic to cow's milk protein, this problem may be associated with infant formulas derived from cow's milk.[64]

Effects of medications and other substances on milk content
Almost all medicines pass into breastmilk in small amounts. Some have no effect on the baby and can be used while breastfeeding.[67] Women with hypothyroidism may be unable to produce milk.[68][69] Alcohol use during pregnancy carries a significant risk of serious birth defects, but consuming alcohol after the birth of the infant is considered safe.[70]
Pesticides and other toxic substances bioaccumulate; i.e., creatures higher up the food chain will store more of them in their body fat. This is an issue in particular for the Inuit, whose traditional diet is predominantly meat. Studies are looking at the effects of polychlorinated biphenyls and persistent organic pollutants in the body; the breast milk of Inuit mothers is extraordinarily high in toxic compounds.[71]
Market

There is a market for human breast milk, both in the form of wet nurse service and milk product. As a product, breast milk is exchanged by human milk banks as well as directly between milk donors and customers mediated by websites on the Internet. Human milk banks generally have standardized measures for screening donors and storing the milk, while donors on websites vary in regard to these measures. A study in 2013 came to the conclusion that 74% of breast milk samples from providers found from websites were colonized with Gram-negative bacteria or had more than 10,000 colony-forming units/mL of aerobic bacteria.[72] Growth happens during transit.[72] According to the FDA, the bacteria in fresh milk doubles every 20 minutes.[73] Breast milk is considered to be healthier than cow's milk and infant formula when it comes to feeding an infant in the first 6 months of life, but only under extreme situations do international health organizations support feeding an infant breast milk from a healthy wet nurse rather than that of its biological mother.[74] One reason is because the unregulated breast milk market is fraught with risks such as drugs of abuse and prescription medications being present in donated breast milk.[74] The transmission of these substances through breast milk can do more harm than good when it comes to the health outcomes of the infant recipient.[74]
Alternative uses
A minority of people, including restaurateurs Hans Lochen of Switzerland and Daniel Angerer of Austria, who operates a restaurant in New York City, have used human breast milk, or at least advocated its use, as a substitute for cow's milk in dairy products and food recipes.[75][76] An Icecreamist in London's Covent Garden started selling an ice cream named Baby Gaga in February 2011. Each serving cost £14. All the milk was donated by a Mrs Hiley who earned £15 for every 10 ounces and called it a "great recession beater".[77] The ice cream sold out on its first day. Despite the success of the new flavour, the Westminster Council officers removed the product from the menu to make sure that it was, as they said, "fit for human consumption."[78]Tammy Frissell-Deppe, a family counsellor specialized in attachment parenting, published a book, titled A Breastfeeding Mother's Secret Recipes, providing a lengthy compilation of detailed food and beverage recipes containing human breast milk.[79] Human breast milk is not produced or distributed industrially or commercially, because the use of human breast milk as an adult food is considered unusual to the majority of cultures around the world, and most disapprove of such a practice.[80]
While there is no scientific evidence that shows that breast milk is more advantageous for adults than cow's milk, according to several 2015 news sources breast milk is being used by bodybuilders for its nutritional value. In a February 2015 ABC News article one former competitive body builder said, "It isn’t common, but I’ve known people who have done this. It’s certainly talked about quite a bit on the bodybuilding forums on the Internet." Calling bodybuilders "a strange breed of individuals,” he said, “Even if this type of thing is completely unsupported by research, they’re prone to gym lore and willing to give it a shot if there is any potential effect.” At the time the article was written, in the U.S. the price of breast milk procured from milk banks that pasteurize the milk, and have expensive quality and safety controls, was about $10 an ounce, and the price in the alternative market online, bought directly from mothers, ranges from $1 to $4 per US fluid ounce, compared to cow's milk at about $3.44 a gallon.[81][82]
A 2015 CBS article cites an editorial led by Dr. Sarah Steele in the Journal of the Royal Society of Medicine, in which they say that "the health claims do not stand up clinically and that raw human milk purchased online poses many health risks." CBS found a study from the Center for Biobehavioral Health at Nationwide Children's Hospital in Columbus that "found that 11 out of 102 breast milk samples purchased online were actually blended with cow's milk." The article also explains that milk purchased online may be improperly sanitized or stored, so it may contain food-borne illness and infectious diseases such as hepatitis and HIV.[83]
See also
Blocked milk duct
Breast milk jewelry
Breast pump
Human milk banking in North America
Infant formula
La Leche League International
Lactation
Lactation room
Lactivism
Mary Rose Tully
Baby Gaga




 by Carol L. Wagner (Overview article, eMedicine, December 14, 2010)
 - including comparison of human and cow's milk ones

 and 
Meigs, EB (August 30, 1913) , J.Biol.Chem 147-168