An understanding of osmosis and the intestinal absorption of salt and glucose forms the basis for a simple therapy that saves millions of lives each year, particularly in developing countries. In these countries, cholera and other intestinal pathogens are major causes of death for young children. A toxin released by these bacteria activates chloride secretion from the apical surfaces of intestinal epithelial cells into the lumen; water follows osmotically, and the resultant massive loss of water causes diarrhea, dehydration, and ultimately death. A cure demands not only killing the bacteria with antibiotics but also rehydration: replacement of the water that is lost from the blood and other tissues.
Simply drinking water does not help because it is excreted from the gastrointestinal tract almost as soon as it enters. However, as we have just learned, the coordinated transport of glucose and Na+ across the intestinal epithelium creates a transepithelial osmotic gradient, forcing water to move from the intestinal lumen across the epithelial cell layer and ultimately into the blood. Thus giving affected children a solution of sugar and salt to drink (but not sugar or salt alone) causes increased sodium and sugar transepithelial transport and, consequently, increased osmotic flow of water into the blood from the intestinal lumen, leading to rehydration. Similar sugar-