Transport and metabolism of water-soluble vitamins in intestine.

An overview of intestinal absorption of water-soluble vitamins is presented. Evidence relating to absorption of ascorbic acid by the "sodium-gradient" hypothesis and folic acid by the "acidic microclimate" hypothesis is discussed. Over the last decade, vitamin absorption by the intestine has been attributed in large part to simple diffusion. Recently, however, this view has been changing as information continues to be gathered which suggests the participation of membrane-bound "carriers" in transport and the involvement of cellular enzymes in metabolism of the vitamins during absorption. The circumstances that make vitamin absorption difficult to investigate are described. A brief look toward the future stresses the types of information that will be helpful to acquire, identifies some technological advances that will promote progress, and discusses potential application of the results to health care. Some reasons are given for confusion that has prevailed in descriptions of vitamin absorption during the last decade; suggestions are made that might help investigators to correct this situation.