Digestion and Absorption of Carbohydrates. Chemistry The principal components of dietary carbohydrates are: a. monosaccharides – single sugars (glucose, fructose, galactose) b. oligosaccharides – several sugar molecules (lactose and sucrose, which are disaccharides, are the important dietary ones) c. polysaccharides – many sugar containing molecules (starch, glycogen) The word saccharide comes from the Greek saccharus meaning sugar and the important ones for us are shown in brackets above. Sugar molecules exhibit optical isomerism and so are often labeled as D- or L- (standing for their Dextro (right) or Laevo (left) rotation of polarized light) – only the D-isomers are utilized in metabolism. Glycogen, the principal dietary polysaccharide from animal sources, is a polymer of glucose molecules are joined by α1-4 glycosidic linkages, with some chain branching by α1-6 linkages. Starches, the term generally used for plant-derived polysaccharides, are polymers of glucose and their derivatives and are similarly structured, but with fewer α1-6 branches.

Digestion Starch is first degraded by ptyalin, the α-amylase of saliva. However, the optimal pH for this is 6.7 and activity is terminated by gastric acidity. Once in the small intestine, pancreatic α-amylase catalyzes α1-4 linkages but not the α1-6 linkages, the α1-4 linkages next to branch points or the terminal α1-4 linkages. Thus, the end products of this digestion are:- a. the disaccharide maltose b. the trisaccharide maltotriose c. larger polymers of glucose with α1-4 linkages d. branched polymers, ~ 8 units, the α-limit dextrins. These are further digested by the oligosaccharidases located at the outer portion of the membrane of the microvilli:- a. maltase b. lactase c. sucrase d. α-limit dextrinase

In many mammals and some races of humans, intestinal lactase activity is high at birth, declines to low levels in childhood and remains low subsequently. Low levels are associated with intolerance of milk, since lactose remains in the GI tract and acts

as an osmotic agent, prior to being broken down by bacteria in the colon. However, most Caucasians retain their lactase activity but most adult blacks are intolerant Absorption Hexoses and pentoses are rapidly absorbed across the intestinal mucosa these then enter the capillaries, which drain to the portal vein. Glucose and Na+ share the same transporter, thus a high Na+ concentration at the mucosal surface facilitates glucose absorption. Due to the action of the basal Na+-K+-pump -secondary active transport out of the enterocytes occurs. Galactose, a glucose isomer, is transported from the lumen by the same channel as glucose. Fructose utilizes a different carrier and its absorption is independent of luminal Na+ - facilitated diffusion.