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Breast milk usually refers to the milk produced by a human female which is usually fed to infants by breastfeeding. It provides the primary source of nutrition for newborns before they are able to eat solid food and digest a wider variety of food.

Under the influence of the hormones prolactin and oxytocin, women produce milk after pregnancy to feed their baby. The initial milk produced is often 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 along with creating a mild laxative effect, expelling meconium and helping to prevent the build up of bilirubin (a contributory factor in jaundice).

There are many reasons a mother may not produce enough breast milk. Some of the most common are: improper latch, not nursing or pumping enough to meet supply, certain medications, birth control, illness, dehydration, or (not commonly^{[citation needed]}) a physical inability to produce. Lack of supply can be addressed by nursing or/and pumping more frequently. The more the mother nurses her baby, or pumps, the more milk is produced. It is very helpful to nurse on demand - to nurse when the baby wants to nurse rather than on a schedule. If pumping; it is helpful to have an electric high grade pump so that all of the milk ducts are stimulated. Some mothers try to increase their milk supply in other ways - by taking the herb fenugreek, used for hundreds of years to increase supply ("Mother's Milk" teas contain fenugreek as well as other supply-increasing herbs); there are also prescription medications that can be used, such as Domperidone (off-label use) and Reglan.

The exact integrated properties of breast milk are not entirely understood, but the nutrient content after this period is relatively consistent and draws its ingredients from the mother's food supply. If that supply is found lacking, content is obtained from the mother's bodily stores. The exact composition of breast milk varies from day to day, depending on food consumption and environment, meaning that the ratio of water to fat fluctuates. Foremilk, the milk released at the beginning of a feed, is watery, low in fat and high in carbohydrates relative to the creamier hindmilk which is released as the feed progresses. The breast can never be truly "emptied" since milk production is a continuous biological process.

Human milk contains 0.8% to 0.9% protein, 3% to 5% fat, 6.9% to 7.2% carbohydrates and 0.2% ash (minerals). Carbohydrates are mainly lactose; several lactose-based oligosaccharides have been identified as minor components. The principal proteins are casein homologous to bovine beta-casein, alpha-lactalbumin, lactoferrin, IgA, lysozyme and serum albumin. Non-protein nitrogen-containing compounds, making up 25% of the milk's nitrogen, include urea, uric acid, creatine, creatinine, amino acids and nucleotides.^[1][2]

Mother's milk has been shown to supply a type of endocannabinoid (the natural neurotransmitters which marijuana simulates), 2-Arachidonoyl glycerol.^[3]

Though now it is almost universally prescribed, 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, today it is now 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 also provides vitamins, minerals, digestive enzymes and hormones - all of the things that a growing infant will require. Breast milk also contains antibodies from the mother that may help the baby to resist infections.

Women who are breastfeeding should consult with their physician regarding things that can be unwittingly passed to the infant via breast milk, such as alcohol, viruses (HIV or HTLV-1) or medications.

Most women who do not breastfeed use infant formula, but breast milk donated by volunteers to human milk banks can be obtained by prescription. [1] Cow's milk is recommended as a substitute, but only for children over one year old.

All mammal 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.

Whole cow's milk does not contain sufficient vitamin E, iron, or essential fatty acids, which can make infants fed on cow's milk anemic. Whole cow's milk also contains excessive amounts of protein, sodium, and potassium which may put a strain on an infant's immature kidneys. In addition, the proteins and fats in whole cow's milk are more difficult for an infant to digest and absorb than the ones in breast milk.^[4] Evaporated milk may be easier to digest due to the processing of the protein but is still nutritionally inadequate. A significant minority of infants are allergic to one or more of the constituents of cow's milk. These problems can also affect formula milk derived from cow's milk.^[vague]

Goat's milk does not contain agglutinin, which means that the fat globules in goat's milk do not cluster together like they do in cow's milk, which makes goat's milk easier for an infant to digest. Goat's milk also does not contain many of the allergens found in cow's milk. However, like cow's milk, goat's milk is also unsuitable for infants as it also does not have appropriate concentrations of electrolytes and can cause intestinal irritation and anemia.

Human milk is similar in flavor and texture to cow milk, but noticeably thinner and sweeter. Left in a cup, the cream will rise and form a thin layer.

• Erotic lactation
• Male lactation
• Breast pump
• Human milk banking in North America
• Infant formula

1. ^ Jenness R (Jul 1979). "The composition of human milk". Seminars in Perinatology 3 (3): 225-239. PMID 392766. 
2. ^ Thorell L; Sjöberg LB, Hernell O (Dec 1996). "Nucleotides in human milk: sources and metabolism by the newborn infant.". Pediatric Research 40 (6): 845-852. PMID 8947961. 
3. ^ Fride E, Bregman T, Kirkham TC. (April 2005). "PEndocannabinoids and food intake: newborn suckling and appetite regulation in adulthood.". Experimental Biology and Medicine 230 (4): 225-234. doi:10.1371/journal.pbio.0020286. PMID 15792943. 
4. ^ MedlinePlus Medical Encyclopedia: Cow's milk for infants and children

• Comparison of Human Milk and Formula
• How to pump and store breast milk
• La Leche League Human Milk Storage Information
• Children's Health Topics: Breastfeeding
• Infant nutrition information from Seattle's Children's Hospital and Regional Medical Center