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Milk Composition & Synthesis
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Milk Composition

Lactose :
  • is the major carbohydrate in milk.
  • is present in milk of most mammals.
  • is a readily digestible source of glucose (energy) for the neonate.
  • is digested by lactase enzyme in the neonate.
  • is unique to the mammary gland.
  • is the major osmole of milk (the primary component that draws water into the gland).

Lactose is the major carbohydrate in the milk of most species. Lactose is a disaccharide composed of the monosaccharides D-glucose and D-galactose, joined in a ß-1,4-glycosidic linkage. The chemical name for lactose is 4-0-ß-D-galactopyranosyl-D-glucopyranose. It is essentially unique to milk, although it has been identified in the fruit of certain plants. Of the mammalian species where information is available, only some marsupials have an alternative sugar other than lactose, and those sugars are generally trisaccharides of glucose and galactose. Lactose plays a major role in milk synthesis. It is the major osmole in milk and the process of synthesis of lactose is responsible for drawing water into the milk as it is being formed in the mammary epithelial cells. Because of the close relationship between lactose synthesis and the amount of water drawn into milk, lactose content is the least variable component of milk.

Lactose is a disaccharide composed of D-galactose and D-glucose.

Lactose is not as sweet as other disaccharides such as sucrose (a glucose-glucose sugar), or the monosaccharides fructose or glucose. Lactose is cleaved to glucose and galactose in the intestine of the neonate by an enzyme activity called lactase (or ß-galactosidase). The galactose is then converted to another glucose by a different enzyme. Lactose is a major, readily digestible source of glucose which provides energy for the neonate. Lactose intolerance can occur in adult animals or animals who do not have lactase activity in their intestines. Aspects of lactose in milk will be discussed further in the Lactose Synthesis Lesson.

Study Question

Lactase is an enzyme that potentially would be very useful to people who are lactose intolerant. Suppose someone produced a transgenic sow that expressed lactase only in its mammary gland and only during lactation, with the intent of isolating that lactase from the milk and using it for lactose intolerant people. What would you expect to be the consequences of expression of such a transgene to the sow's production of milk and milk composition?

Other carbohydrates are found in milk, but at low concentrations. Low concentrations of free glucose (about 0.1 mM) and free galactose (about 0.2 mM) are found in cow milk and milk of other species. Other carbohydrates found free in milk include amino sugars, sugar phosphates, neutral and acid oligosaccharides, and nucleotide sugars. Some of the complex oligosaccharides are thought to be important in helping establish the microflora of the neonate intestine (such as the bifidis factor identified in human milk; see Human Lactation Lesson). Some milk proteins are glycosylated and some milk lipids contain carbohydrate moieties.

Milk Composition Module