Milk is a white secreted fluid containing nutrients produced by the mammary glands of female mammals. It contains nearly all the nutrients necessary to sustain life. It is the main source of nutrition for mammals from the early days of their life until they are able to digest other types of food. One of the most important ingredients in milk is calcium, responsible for the growth of strong bones and teeth. 
The milk also has a role in immunity. Substances present in human milk are able to kill intestinal protozoa such as Giardia lambia and, in addition, has the function of protecting infants against this and other infections. Breast milk contains immunological substances that confer passive immunity for the child, such as IgA, lactoferrin, lysozyme, macrophages and other antibodies.
The cow milk in its natural state contains approximately 87% water, 3.5% of fat, 3.6% protein, 4.9% carbohydrates (lactose) and minerals like calcium and phosphorus. The amount of water is reduced to approximately 4% when in the form of milk powder. Milk is also a source of vitamins C (human milk 3.8 mg/100ml and cow milk 1.5 mg/100ml) and D (human milk 0.8 μg/100ml and cow milk 0.15 μg/100ml). Lactose is a disaccharide found naturally only in milk and its derivatives and it is composed of galactose linked by a β 1-4 glycosidic bond to glucose.
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