Category: sugar
Aliases: Milk sugar, Lactose monohydrate, Lactobiose, β-D-galactopyranosyl-(1→4)-D-glucose, 4-O-β-D-galactopyranosyl-D-glucose
Reducing disaccharide of galactose β-(1→4)-linked to glucose; the principal carbohydrate of mammalian milk. Not absorbed intact — brush-border lactase-phlorizin hydrolase (LCT) on small-intestinal enterocytes cleaves it to free [[glucose]] and [[galactose]], which are taken up by SGLT1 (Na⁺-coupled) and GLUT2. Glucose feeds glycolysis and elicits a glucose-driven [[insulin]] response; galactose is routed through the Leloir pathway (galactokinase → GALT → epimerase) to UDP-glucose/glycogen in the liver. Constitutional or acquired lactase deficiency leaves unhydrolyzed lactose in the colon, where bacterial fermentation to short-chain fatty acids, H₂, CH₄ and CO₂ plus its osmotic load produces the bloating and osmotic diarrhea of lactose intolerance. About one-fifth as sweet as sucrose; widely used as a pharmaceutical tablet/capsule diluent and dry-powder-inhaler carrier.