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 carbohydrates in the brush border of the mature enterocytes. The glucose, galactose, and fructose produced are absorbed across the enterocytes of the upper half of the villus. Glucose and galactose (and other glucalogues) are actively transported into the enterocyte by the Na(+)-glucose cotransporter SGLT1 (gene on chromosome 22) via the transmembrane electrochemical Na+ gradient, and exit across the basolateral membrane by the glucose transporter GLUT2 (gene on chromosome 3). The critical importance of the correct expression of SGLT1 for human sugar absorption is shown by the rare genetic disease of glucose-galactose malabsorption. People with this disease cannot absorb hexoses and have severe watery diarrhea, which, if untreated, is terminal. Fructose absorption is by an Na(+)-independent transport system that has not been fully characterized (possibly GLUT5). Despite many kinetic and other studies in animals, and some in humans, that suggest multiple Na(+)-glucose transporters, only SGLT1 is expressed in enterocytes. Absorption of monosaccharides from disaccharides appears to have a kinetic advantage (disaccharide-related transport system). Hexose absorption is enhanced by dietary intake of hexoses by increased activity of SGLT1 and GLUT2 and by increased enterocyte numbers.