Lactose intolerance results from insufficient lactase (\(\beta\)-galactosidase) in the small intestinal brush border. Undigested lactose reaches the colon and is fermented by colonic bacteria producing short-chain fatty acids, \(\text{H}_2\), \(\text{CO}_2\), and \(\text{CH}_4\), causing bloating, flatulence, cramping, and diarrhea.