An understanding of osmosis and the intestinal absorption of glucose forms the basis for a simple therapy that has saved millions of lives, particularly in less-developed countries. In these countries, diarrhea caused by cholera and other intestinal pathogens is a major cause of death of young children. 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. To understand the simple therapy that is used, recall that absorption of glucose by the small intestine involves the coordinated movement of Na+; one cannot be transported without the other (see figure above). The movement of NaCl and glucose from the intestinal lumen, across the epithelial cells, and into the blood creates a transepithelial osmotic gradient, forcing movement of water from the intestinal lumen into the blood. Thus, giving affected children a solution of sugar and salt to drink (but not sugar or salt alone) causes the bulk flow of water into the blood from the intestinal lumen and leads to rehydration.