Figure 1.

Digestion and absorption of starch and sugar. Starch is a polymer of several thousand molecules of glucose, which is digested by the pancreatic enzyme alpha-amylase into maltose, isomaltose, maltotriose (not represented in the figure) and maltodextrins. At the level of the brush border of the intestinal mucosa, specific enzymes generate glucose from maltose (sucrase, maltase), isomaltose (isomaltase) and maltodextrins (glucoamylase). Glucose is then absorbed into the enterocyte by an apical co-transport with NaCl (Sodium-glucose-transporter-1, SGLT1) and transferred to the blood at the basolateral membrane through a facilitated transport mediated by GLUT2. Sucrose is cleaved into glucose and fructose by sucrase at the brush border. Fructose is transported into the enterocyte independently of Na by GLUT5, and due to the presence of fructose metabolizing, gluconeogenic and lipogenic enzymes, part of the absorbed fructose may be metabolized to lactate, glucose, and fatty acids within the enterocytes. Unmetabolized fructose is transferred to the blood at the basolateral membrane by GLUT2.