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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 digest more diverse foods.
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).
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 biologic process.
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.
Comparision to other milks
Other mammals also produce milk, but the composition of milk for each species varies widely and other kinds of milk often very different from human breast milk. In particular, cow's milk and goat's milk are not acceptable substitutes for breast milk.
Whole cow's milk contains insufficient Vitamin E, iron, and essential fatty acids, which can make infants fed on cow's milk anaemic. 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 protein and fat in whole cow's milk are more difficult for an infant to digest and absorb than breast milk.  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 intolerant of lactose or allergic to one or more of the consitituents of cow's milk. These problems can also affect formula milk derived from cow's milk.
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 can cause intestinal irritation and anaemia.
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