The milk produced by human mammary glands contains about the same percentage of fat and less protein than cow's milk, but it has more carbohydrate than that of other mammals. This carbohydrate is in the form of lactose, a disaccharide. Mammalian infants depend on their mother's milk for nourishrnent and immunological protection. As a fresh, temperature-controlled, uncontaminated food source, the milk from mother's breast is the best at meeting the needs of the infant. It also happens that in animals and most humans there is a decline in the level of production of lactase with aging.
Lactase is the enzyme that digests milk sugar.
Since the sugar lactose is known to occur only in milk, the advantages offered during infancy by this nutrient have been studied. A component of the disaccharide, called galactose, is not found free in nature, and in the synthesis of lactose by the body a molecule of glucose is converted to galactose then joined with another glucose molecule. During digestion lactose is digested and absorbed more slowly than other sugars and may improve calcium retention and absorption for increasing the mineral content of bone. In addition, growth factors, for Lactobacillus bifudus present in human milk may establish and maintain intestinal flora that are favorable. Galactose and glucose are combined in b -linkage to yield the sugar lactose.