Lactose is a disaccharide sugar that is naturally present in the milk of many mammals. Structurally, it is a polymer composed of one molecule of glucose and one molecule of galactose, linked together by a β-1,4 glycosidic bond. The metabolism and digestion of lactose rely on the enzyme lactase, which is secreted by the small intestine. In individuals who do not produce sufficient levels of this enzyme, the sugar remains undigested in the digestive tract. This undigested lactose can draw water into the colon, leading to symptoms of gastrointestinal distress, characteristic of lactose intolerance.