Lactose Intolerance

Lactose intolerance is a common condition characterized by abdominal pain, bloating, flatulence, and diarrhea after ingesting lactose, a disaccharide found in dairy products.  It usually occurs from lactose malabsorption by the small intestine due to a deficiency of lactase, an enzyme necessary for its breakdown.

Lactose is a disaccharide present in milk and other dairy products.  Because only monosaccharides can be absorbed by the small intestine, lactose must be hydrolyzed into its monosaccharide components.  Lactase, a brush border enzyme concentrated in epithelial cells of the small intestine, hydrolyzes lactose into glucose and galactose.  Following breakdown, the monosaccharides are fully absorbed within the small intestine.

Without lactase, undigested lactose can enter the large intestine, where colonic bacteria ferments it into short-chain fatty acids (eg, acetate, butyrate, propionate) that acidify the stool.  Hydrogen gas is also produced, leading to bloating and flatulence.  Undigested lactose also acts as an osmotic agent, drawing water into the lumen and causing osmotic diarrhea.