Digestion, Absorption of Dietary Carbohydrates PDF

Digestion, Absorbtion of Carbohydrates Dr. Öğr. Üyesi Gökhan BAĞCI Introduction to Carbohydrates digestion • In developed countries, an adult person provides about 40-50% of his daily calorie requirement from carbohydrates. • Carbohydrates make up a large part of the daily diet. • Approximately 30...

Digestion, Absorbtion of Carbohydrates Dr. Öğr. Üyesi Gökhan BAĞCI Introduction to Carbohydrates digestion • In developed countries, an adult person provides about 40-50% of his daily calorie requirement from carbohydrates. • Carbohydrates make up a large part of the daily diet. • Approximately 300 g of carbohydrates are taken per day. • Most of this is starch (∼160 g) and sucrose (∼120 g). • In addition, some lactose (∼30 g) and glucose and fructose (∼10 g) are also taken. • Plenty of cellulose, starch and sucrose are taken with plant foods; glycogen and lactose are taken with animal foods. • Glycogen, an animal polysaccharide, is present in small amounts in the diet. • There is no specific sugar that must be taken in the diet. • Glucose, which is at the center of carbohydrate metabolism, can be synthesized from some non-carbohydrate compounds in the body. • In addition, fructose, galactose, xylose and all sugars necessary for metabolic processes can be synthesized from glucose in humans. Digestion, absorption of carbohydrate • The goal is to break sugar and starches into small molecules-chiefly glucose-that the body can absorb and use. • A large starch molecule require extensive breakdown. The disaccharide needs only to be broken once. And the monosaccharide not at all. • The initial splitting begins in the mouth, and the final splitting and absorption occur in small intestine, and the conversion to common energy (glucose) takes place in the liver. Carbohydrate digestion In the mouth: →Mastication: the process by which food is crushed and ground by teeth .The chewing of high-fiber food slows eating and stimulate the flow of saliva. →Starch: salivary glands secretes saliva into the mouth to moisten the food. The salivary enzyme amylase (ptyalin) begins digestion. Starch Amylase small polyshcarrides, maltose(disacharride) →Fibers: the mechanical action of the mouth crushes and tears fiber in food and mixes it with saliva to moisten it for swallowing. Digestion of carbohydrate by salivary α -amylase (ptylin) in the mouth: • This enzyme is produced by salivary glands. Its optimum pH is 6.7. • It is activated by chloride ions (cl-). • It acts on cooked starch and glycogen breaking α 1-4 bonds, converting them into maltose [a disaccharide containing two glucose molecules attached by α 1-4 linkage]. This bond is not attacked by -amylase. • Because both starch and glycogen also contain 1-6 bonds, the resulting digest contains isomaltose [a disaccharide in which two glucose molecules are attached by 1-6 linkage]. • Because food remains for a short time in the mouth, digestion of starch and glycogen may be incomplete and gives a partial digestion products called: starch dextrins (amylodextrin, erythrodextrin and achrodextrin). • Therefore, digestion of starch and glycogen in the mouth gives maltose, isomaltose and starch dextrins. 7 ln the stomach: carbohydrate digestion stops temporarily due to the high acidity which inactivates the salivary - amylase. Digestion of carbohydrate by the pancreatic - amylase small intestine in the small intestine. A. α-amylase enzyme is produced by pancreas and acts in small intestine. Its optimum pH is 7.1. B. It is also activated by chloride ions. C. It acts on cooked and uncooked starch, hydrolysing them into maltose and isomaltose. Final carbohydrate digestion by intestinal enzymes: A. The final digestive processes occur at the small intestine and include the action of several disaccharidases. These enzymes are secreted through and remain associated with the brush border of the intestinal mucosal cells. In the stomach: →Peristalsis: wave like muscular contraction. →The swallowed bolus(a portion of food swallowed at one time) mixes with stomach acids and protein digesting enzymes, which inactivate salivary amylase. →Starch: stomach acid inactivates salivary enzymes, halting starch digestion. To small extent ,the stomach acid continue breaking the starch, but it juices contain no enzymes to digest CHO. →Fibers: is not digested in stomach and delays gastric emptying thereby provide feeling of fullness and satiety. In the small intestine : →In it most of the work of CHO digestion . →Starch: the pancreas produces an amylase that is released through pancreatic duct into the small intestine. The major CHO-digesting enzyme is the pancreatic amylase which will continue breaking polysaccharides. Starch pancreatic amylase small polysaccharides, maltoses →the pancreatic amylase in the duodenum breaks down all small polysaccharides into disaccharides. →The final step takes place in the outer membrane of the intestinal cells. The disaccharide digestion begins at this point. There specific enzymes secreted from the intestinal glands breaks down specific disaccharides. The disaccharide enzymes on the surface of the small intestinal cells hydrolyze the disaccharides into monosaccharaides Maltose maltase Glucose +Glucose Sucrose sucrase Fructose+Glucose Lactose lactase Galactose+Glucose ==>intestinal cells absorb these monosaccharaides. →Fibers: is not digested ,and delays the absorption of other nutrients. Large intestine: • • • • • Within 1-4 hours after a meal, all of the sugars and most of the starches have been digested. Only fibers remain in the digestive tract. Fibers in large intestine attracts water, which softens the stool for passage without straining. Also, bacteria in the GI tract ferment some fibers. Most fiber passes intact through digestive tract to the large intestine. Here, bacterial enzyme digest fiber. Some fibers Bacterial Enzymes short-chain fatty acids, gas & water • Colon uses these small fat molecules for energy. • Metabolism of short chain fatty acids occurs in the cells of liver. fiber therefore can contribute some energy. • Depending on the extent to which they are broken down by bacteria and the fatty acids are absorbed. • Fiber holds water ,regulate bowel activity, and bind substances such as bile ,cholesterol, and some minerals, carrying them out of the body. I. Introductıon: ➢More than 60% of our foods are carbohydrates. Starch, glycogen, sucrose, lactose and cellulose are the chief carbohydrates in our food. ➢Before intestinal absorption, they are hydrolysed to hexose sugars (glucose, galactose and fructose). ➢A family of a glycosidases that degrade carbohydrate into their monohexose components catalyzes hydrolysis of glycocidic bonds. ➢These enzymes are usually specific to the type of bond to be broken. 16 Carbohydrate absorption • Peristaltic movement moves the monosaccharaides into the jejunum where digestion is completed and absorption begins. • Absorption is increases as a result of the intestinal villi (small mucus projections lining the small intestine). • Each villi contain blood capillary into which the monosaccharaides passes via diffusion or active transport. • Glucose is unique that it can be absorbed to some extent through the lining of mouth, but for most part nutrient absorption takes place in the small intestine. • Glucose and galactose traverse the cell lining the small Intestine by active transport. • Fructose is absorbed by facilitated diffusion, which slows its entry and produces a smaller rise in blood glucose. • Likewise, unbranced chains of starch are digested slowly and produce a smaller rise in blood glucose than branched chains, which have many more places for enzymes to attack and release glucose rapidly. o As the blood from the intestine circulates through the liver, cells there take up fructose and galactose and convert them to other compounds ,most often to glucose . o Thus all disaccharides provide at least one glucose molecule directly ,and they can provide another one indirectly –through the conversion of fructose and galactose to glucose. Absorption of Carbohydrates • •Carbohydrates are absorbed as monosaccharides from the intestinal lumen. • •Two mechanisms are responsible for the absorption of monosaccharides: • 1.Active transport against a concentration gradient, i.e. from a low glucose concentration to a higher concentration. • 2.Facilitative transport, with concentration gradient, i.e. from a higher concentration to a lower one. Active Transport ➢ •The transport of glucose and galactose across the brushborder membrane of mucosal cells occurs by an active transport. ➢ •Active transport is an energy requiring process that requires a specific transport protein and the presence of sodium ions ➢ A sodium dependent glucose transporter (SGLT-1) binds both glucose and Na+ at separate sites and transports them both through the plasma membrane of the intestinal cell. ➢ The Na+ is transported down its concentration gradient (higher concentration to lower concentration) and at the same time glucose is transported against its concentration gradient. ➢ The free energy required for this active transport is obtained from the hydrolysis of ATP linked to a sodium pump that expels Na+ from the cell in exchange of K+ Facilitative Transport • •Fructose and mannose are transported across the brush border by a Na+ independent facilitative diffusion process, requiring specific glucose transporter, GLUT-5. • •The same transport can also be used by glucose and galactose if the concentration gradient is favorable. Transport of Carbohydrates The sodium independent transporter, GLUT-2 that facilitates transport of sugars out of the mucosal cells, thereby entering the portal circulation and being transported to the liver. Transport of Carbohydrates Summary of types of functions of most important glucose transporters: Function Site SGLT-1 Absorption of glucose by active transport (energy is derived from Na+- K+ pump) Intestine and renal tubules. GLUT -5 Fructose transport and to a lesser extent glucose and galactose. Intestine and sperm GLUT - 2 Transport glucose out of intestinal and renal cells → circulation -Intestine and renal tubule -β cells of islets-liver 23 GLUCOSE TRANSPORTERS Lactose intolerance • A condition that results from inability to digest the milk sugar lactose, characterized by bloating, gas, abdominal discomfort, and diarrhea. • Lactose intolerance differs than milk allergy, which is caused by an immune reaction to the protein in milk. • It is a common condition that occurs when there is insuffient lactase to digest the disaccharide lactose found in milk and milk products. • Because treatment requires limiting milk intake, other sources of riboflavin, vitamin D , and calcium must ne included in diet. Galactosemia • Galactosemia (is a rare genetic metabolic disorder that affects an individual's ability to metabolize the sugar galactose properly. Although the sugar lactose can metabolize to galactose, galactosemia is not related to and should not be confused with lactose intolerance .Lactose in food (such as dairy products) is broken down by the enzyme lactase into glucose and galactose. • In individuals with galactosemia, the enzymes needed for further metabolism of galactose are severely diminished or missing entirely, leading to toxic levels of galactose 1-phosphate in various tissues as in the case of classic galactosemia, →resulting in hepatomegaly (an enlarged liver), cirrhosis, renal failure, cataracts, brain damage, and ovarian failure. Sucrase deficiency: A rare condition, showing the signs and symptoms of lactase deficiency. It occurs early in childhood. Monosaccharide malabsorption: • • This is a congenital condition in which glucose and galactose are absorbed only slowly due to defect in the carrier mechanism. Because fructose is not absorbed by the carrier system, its absorption is normal. 28 Fate of absorbed sugars: Monosaccharides (glucose, galactose and fructose) resulting from carbohydrate digestion are absorbed and undergo the following: A. Uptake by tissues (liver): After absorption the liver takes up sugars, where galactose and fructose are converted into glucose. B. Glucose utilization by tissues: Glucose may undergo one of the following fate: 1. Oxidation: through a) Major pathways (glycolysis and Krebs' cycle) for production of energy. b) Hexose monophosphate pathway: for production of ribose, deoxyribose and NADPH + H+ c) Uronic acid pathway, for production of glucuronic acid, which is used in detoxication and enters in the formation of mucopolysaccharide. 2. Storage: in the form of: a) Glycogen: glycogenesis. b) Fat: lipogenesis. 3. Conversion: to substances of biological importance: a) Ribose, deoxyribose → RNA and DNA. b) Lactose → milk. c) Glucosamine, galactosamine → mucopolysaccharides. d) Glucoronic acid → mucopolysaccharides. e) Fructose → in semen. 30 Digestion of cellulose: ❖ Cellulose contains β(1-4) bonds between glucose molecules. ❖ In humans, there is no β (1-4) glucosidase that can digest such bonds. So cellulose passes as such in stool. Cellulose helps water retention during the passage of food along the intestine producing larger and softer feces preventing constipation. 31