Dr. Shraddha's Digestion and Absorption of Carbohydrates PDF

 Lecture series Gastrointestinal tract Professor Shraddha Singh, Department of Physiology, KGMU, Lucknow Carbohydrates  Learning Objectives: Understand the different classes of carbohydrates Understand the enzymes responsible for digestion of...

 Lecture series Gastrointestinal tract Professor Shraddha Singh, Department of Physiology, KGMU, Lucknow Carbohydrates  Learning Objectives: Understand the different classes of carbohydrates Understand the enzymes responsible for digestion of carbohydrates What are sites for absorption Molecular basis of glucose transpotation Learn about the function of carbohydrates  Carbohydrates : provide a substantial energy substrate for metabolism (4 kcals/g). Approximately 45% to 60% of dietary carbohydrate is in the form of starch, which is a polysaccharide BALANCED DIET-Diet consisting of the proper quantities & proportions of foods needed to maintain health or growth Simple sugars Complex (starch and fiber) -Monosaccharide – One unit -Disaccharides – Two units -Oligosaccharides – 10 or fewer units -Polysaccharides – Up to 1000 units  Monosaccharides Glucose Galactose Fructose  Disaccharides Sucrose – Glucose + Fructose Lactose – Glucose + Galactose Maltose – Glucose + Glucose  Polysaccharides Starch – Amylose – Amylopectin Dextrins – Produced when starch molecules are partially broken down by enzymes, acid, or heat. -Less thickening power than starch Glycogen Plant Fiber Components Digestion of Carbohydrates Begins in the Mouth and Stomach. When food is chewed, it is mixed with saliva, which contains the digestive enzyme ptyalin (an α-amylase) secreted mainly by the parotid glands. This enzyme hydrolyzes starch into the disaccharide maltose and other small polymers of glucose that contain three to nine glucose molecules.  However, the food remains in the mouth only a short time, so probably not more than 5 percent of all the starches will have become hydrolyzed by the time the food is swallowed..Activity of the salivary amylase is then blocked by acid of the gastric secretions because the amylase is essentially inactive as an enzyme once the pH of as an enzyme falls below about 4.0  Starch digestion in the lumen of intestine Pancreatic α-Amylase is an hydrolyzes internal α-1,4 linkages α-Amylase does not cleave terminal α-1,4 linkages, α-1,6 linkages (i.e., branch points), or α-1,4 linkages that are immediately adjacent to α-1,6 linkages. As a result, starch hydrolysis products are maltose, maltotriose, and α-limit dextrins. Digestion in the lumen of intestine Digestion at the brush border in the lumen of intestine The intestine cannot absorb these products of amylase digestion of starch, and thus further digestion is required to produce monosaccharides The human small intestine has enzymes on brush border oligosaccharidases: lactase, glucoamylase (most often called maltase), and sucrase-isomaltase.  Digestion at the brush border of intestine The intestine cannot absorb these products of amylase digestion of starch, and thus further digestion is required to produce monosaccharides The human small intestine has enzymes on brush border oligosaccharidases: lactase, glucoamylase and sucrase- isomaltase. Lactose and maltotriose at brush border Absorption of carbohydrate  Absorption of carbohydrate There are three monosaccharide products of carbohydrate digestion—glucose, galactose, and fructose They are absorbed by the small intestine in a two-step process: Their uptake across the apical membrane into the epithelial cell Their coordinated exit across the basolateral membrane The Na/glucose transporter 1 (SGLT1) is the membrane protein responsible for glucose and galactose uptake at the apical membrane the apical step of fructose absorption occurs by the facilitated diffusion of fructose through GLUT5 a single transporter (GLUT2) is responsible for the movement of both monosaccharides across the basolateral membrane.  Absorption of glucose , galactose and sucrose The uptake of glucose across the apical membrane through SGLT1 (secondary active transport,) because the glucose influx occurs against the glucose concentration gradien Glucose uptake across the apical membrane is energized by the electrochemical Na+ gradient, which, in turn, is maintained by the extrusion of Na+ across the basolateral membrane by the Na-K pump The apical step of fructose absorption occurs by the facilitated diffusion of fructose through GLUT5 Absorption of glucose, galactose and sucrose  Lactose Intolerance In most mammals and in many races of humans, intestinal lactase activity is high at birth, then declines to low levels during childhood and adulthood. The low lactase levels are associated with intolerance to milk ( lactose intolerance). When such individuals ingest dairy products, they are unable to digest lactose sufficiently, and so symptoms--such as bloating, pain, gas, and diarrhea are produced The simplest treatment for lactose intolerance is to avoid dairy products in the diet, Commercial lactase preparations, Yogurt is better to administration  APPLIED DIABETES MELLITUS It is a syndrome of impaired carbohydrates,fat and protein metabolism due to either insulin lack or decreased sensitivity of tissue to insulin TYPES- Type 1/IDDM -Type 2/NIDDM  CAUSE Hereditary Autoimmune Viral infection Obesity SYMPTOMS &SIGNS Glucosuria Dehydration Osmotic diuresis Polyurea,polydipsia Hypertension,heart strokes,artherosclerosis  End stage kidney disease Retinopathy Peripheral neuropathy. ANS dysfunctions Ketoacidosis,diabetes coma,death Asthemia(lack of energy) Weight loss  References Lippincott’s Illustrated Reviews: Physiology (2013) Medical Physiology, Updated second edition (walter F. Boron, MD, phd) Berne & levy, physiology, sixth edition, updated edition Ganong’s Review of Medical Physiology, 26 t h e d i t i o n