The overall process of carbohydrate digestion and absorption is so efficient in healthy individuals that ordinarily all digestible dietary car- bohydrate is absorbed by the time the ingested material reaches the lower jejunum. However, because it is mono saccharides that are absorbed, any defect in a specific disaccharidase activity of the intestinal mucosa causes the passage of undigested carbohydrate into the large intestine. As a consequence of the presence of this osmotically active material, water is drawn from the mucosa into the large intestine, causing osmotic diarrhea. This is reinforced by the bacterial fermentation of the remaining carbohydrate to two- and three-carbon compounds (which are also osmotically active) plus large volumes of CO 2 and H 2 gas, causing abdominal cramps, diar- rhea, and flatulence.