Lactose (galactosyl beta-1,4-glucose or milk sugar) is a disaccharide present in milk.

synthesized in teh mammary gland by formation of a 1,4 glycosidic linkage between glucose and galactose.

Lactose is broken down into galactose and glucose by an intestinal brush-border disaccharidase called lactase (a type of beta-galactosidase more specifically known as lactase-phlorizin hydrolase)

Lactose intolerance

Primary - very common

particularly in people of African and Asian descent.

Secondary lactase deficiency

associated with intestinal mucosal diseases such as celiac sprue and viral gastroenteritis.

It is due to the fact that lactase is concentrated within epithelial cells in the microvilli of the small intestine (the brush border). When these cells are damaged in GI, they slough off and are replaced by immature cells that have low concentration of lactase.