Lactose is a disaccharide that consists of ß-D-galactose and ß-D-glucose molecules bonded through a ß1-4 glycosidic linkage. Lactose makes up around 2-8% of the solids in milk. The name comes from the Latin word for milk, plus the -ose ending used to name sugars. Its empirical formula is and its C12H22O11 molecular weight is 342.3 g/mol. 

Digestion of lactose

Infant mammals are fed on milk by their mothers. To digest it an enzyme called lactase (ß1-4 disaccharidase) is secreted by the intestinal villi, and this enzyme cleaves the molecule into its two subunits for absorption. Since lactose occurs mostly in milk, in most species the production of lactase gradually ceases with maturity, and they are then unable to metabolise lactose. This loss of lactase on maturation is also the default pattern in most adult humans. However, many people with ancestry in Europe, the Middle East, India, and the Maasai of East Africa, have a version of the gene for lactase that is not disabled after infancy, and in many of these cultures other mammals such as cattle, goats, and sheep are milked for food. This fact may cast doubt on some arguments by proponents of the Paleolithic diet, who argue that human metabolic needs have not changed since the last ice age. The process of retaining infant characteristics into adulthood is one of the simplest routes of adaptation, and is known as neoteny. 

What is lactose intolerance?

Lactose intolerance is the inability to digest significant amounts of lactose, the major sugar found in milk. Lactose intolerance is caused by a shortage of the enzyme lactase, which is produced by the cells that line the small intestine. Lactase breaks down milk sugar into two simpler forms of sugar called glucose and galactose, which are then absorbed into the bloodstream. Not all people deficient in lactase have the symptoms commonly associated with lactose intolerance, but those who do are said to have lactose intolerance.

People sometimes confuse lactose intolerance with cow’s milk intolerance because the symptoms are often the same. However, lactose intolerance and cow’s milk intolerance are not related. Being intolerant to cow’s milk is an allergic reaction triggered by the immune system. Lactose intolerance is a problem caused by the digestive system.

For more on lactose intolerance: See Lactose Intolerance