DIGESTION AND ABSORPTION OF CARBOHYDRATES - FROM MOLECULES AND MEMBRANES TO HUMANS

Hydrolysis in the luminal bulk fluid by secreted enzymes is the major pathway for the breakdown of polysaccharides to oligosaccharides, and further hydrolysis is accomplished by a battery of carbohydrases in th e brush border of the mature enterocytes. The glucose, galactose, and fructose produced are absorbed across the enterocytes of the upper hal f of the villus. Glucose and galactose (and other glucalogues) are act ively transported into the enterocyte by the Na+-glucose cotransporter SGLT1 (gene on chromosome 22) via the transmembrane electrochemical N a+ gradient, and exit across the basolateral membrane by the glucose t ransporter GLUT2 (gene on chromosome 3). The critical importance of th e correct expression of SGLT1 for human sugar absorption is shown by t he rare genetic disease of glucose-galactose malabsorption. People wit h this disease cannot absorb hexoses and have severe watery diarrhea, which, if untreated, is terminal. Fructose absorption is by an Na+-ind ependent transport system that has not been fully characterized (possi bly GLUT5). Despite many kinetic and other studies in animals, and som e in humans, that suggest multiple Na+-glucose transporters, only SGLT 1 is expressed in enterocytes. Absorption of monosaccharides from disa ccharides appears to have a kinetic advantage (disaccharide-related tr ansport system). Hexose absorption is enhanced by dietary intake of he xoses by increased activity of SGLT1 and GLUT2 and by increased entero cyte numbers.