Intestinal water absorption--implications for the formulation of rehydration solutions

Absorption of ingested water and most solutes occurs in the proximal small intestine, therefore the rate at which beverages are emptied from the stomach is an important factor in determining the rate of water absorption. In the small intestine, water absorption is brought about by the creation of suitable osmotic gradients that promote net uptake of water from the intestinal lumen. The absorption of solute, especially that brought about by active carriers, are highly effective in creating the osmotic gradients that promote net water uptake. The activation of these transporters also increases the permeability of the mucosa which helps absorption. Moderate hypotonicity of the luminal contents potentiates solute-induced water absorption while hypertonicity slows fluid absorption. Dilute hypotonic glucose-sodium solutions are highly effective oral rehydration solutions, and the type of carbohydrate used does not appear to be important. The addition of other actively absorbed solutes gives little benefit in potentiating water uptake. The inclusion of sodium in rehydration solutions may not be required to stimulate water absorption but probably assists the overall rehydration process. The amount of glucose required to stimulate water absorption is relatively small and for rehydration purposes ingestion of an adequate amount of a dilute solution is more beneficial than drinking a smaller volume of a more concentrated beverage.