Lactose

sugar · digestive · Oral

Lactose, a sugar. Reducing disaccharide of galactose β-(1→4)-linked to glucose; the principal carbohydrate of mammalian milk. Not absorbed intact — brush-border…

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.

No E-number (a food ingredient, not an additive); roughly 0.2× as sweet as sucrose. GRAS with no numerical ADI. Key flag: poorly tolerated in lactase-deficient (lactose-intolerant) individuals and contraindicated in classic galactosemia; a near-ubiquitous tablet/capsule excipient, so trace exposure is common even off-diet.