Digestion And Absorption Of Carbohydrates 3k6h3y

FACULTY OF MEDICINE BMS 1153 – Cell Metabolism Biochemistry Lecture # 2 Digestion and Absorption of Carbohydrates

Learning Outcomes: Process of Digestion • Salivary enzymes • Pancreatic enzymes Process of Absorption • Glucose transport • Carrier mediated facilitated transport Different types of Dietary fibers Importance of Dietary Fiber • Types and source • Role in colonic motility • Role in absorption of glucose and cholesterol Glycemic index • What is GI • GI of foods

DIGESTION OF FOODSTUFFS  The diet of any animal contains different molecules, but the bulk of the ingested nutrients are in the form of huge macromolecules that cannot be absorbed into blood without first being reduced to simpler and smaller forms.  The most important enzymatic reaction in digestion of foodstuffs is hydrolysis - the breaking of a chemical bond by the addition of a water molecule.

Digestion  Digestion is the chemical breakdown of large food molecules into smaller molecules for use by cells. The breakdown occurs when certain specific enzymes are mixed with the food.  Enzymes involved in Digestion  polysaccharides    maltose    glucose   salivary amylase pancreatic amylase maltase    proteins    peptides    pepsin trypsin Stomach pancreas  fats

  amino acids  peptidase intestine

 fatty acids and glycerol lipases pancreatic

Digestion begins only when there is food in the GIT, beginning in the mouth: A. 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). B. A family of a glycosidases degrade/hydrolyses carbohydrate into their monohexose components. These enzymes are usually specific to the type of bond to be hydrolysed.

Digestion of carbohydrate by salivary αamylase (ptylin) in the mouth: A. This enzyme is produced and secreted by salivary glands. Its optimum pH is 6.7. B. It is activated by chloride ions (cl-). C. 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].

Digestion of starch in mouth ctd. D. 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.

Digestion in STOMACH ln the stomach: carbohydrate digestion stops temporarily due to the high acidity which inactivates the salivary - amylase.

Digestion in the SMALL INTESTINE Final carbohydrate digestion by intestinal enzymes 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. 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.

The Intestinal disaccharidases 1. Lactase (β-galactosidase) which hydrolyses lactose into two molecules of glucose and galactose: Lactase Lactose  Glucose + Galactose 2. Maltase ( α-glucosidase), which hydrolyses maltose into two molecules of glucose: Maltase Maltose  Glucose + Glucose

Intestinal disaccharidases ctd. 3. Sucrase (α-fructofuranosidase), which hydrolyses sucrose into two molecules of glucose and fructose: Sucrase Sucrose  Glucose + Fructose 4. α - dextrinase (oligo-1,6 glucosidase) which hydrolyze (1 ,6) linkage of isomaltose. Dextrinase Isomaltose  Glucose + Glucose

Absorption A. The end products of carbohydrate digestion are monosaccharides: glucose, galactose and fructose. They are absorbed from the jejunum to portal veins to the liver, where fructose and galactose are transformed into glucose. B. 2 mechanisms are responsible for absorption of monosaccharides: active transport (against concentration gradient i.e. from low to high concentration) and ive transport (by facilitated diffusion). C. For active transport to take place, the structure of sugar should have Hexose ring.

1.

The active transporter:

a. In the cell membrane of the intestinal cells, there is a mobile carrier protein called sodium dependent glucose transporter (SGL T-1/GLUT-1) It transports glucose to inside the cell using energy. The energy is derived from sodium-potassium pump (Na-K ATPase). b. The transporter has 2 separate sites, one for sodium and the other for glucose. It transports them from the intestinal lumen across cell membrane to the cytoplasm. Both glucose and sodium are released into cytoplasm allowing the carrier to return for more transport of glucose and sodium.

2. ive transport (facilitated diffusion): Sugars with concentration gradient i.e. from high to low concentration. It needs no energy. Occurs by means of sodium independent facilitative transporter (GLUT -5). Fructose and pentoses are absorbed by this mechanism. Glucose and galactose can also use the same transporter if the concentration gradient is favorable. 3. There is also sodium – independent transporter(GLUT-2) that facilitates transport of sugars out of the cell i.e. into circulation.

Summary of types of functions of most important glucose transporters: Function

SGLT-1 Absorption of glucose by (GLUT-1) active transport (energy is derived from Na+- K+ pump) GLUT -5

Site

Intestine and renal tubules

Fructose transport and to Intestine and a lesser extent glucose sperm and galactose.

GLUT - 2 Transport glucose out of intestinal and renal cells  circulation

Intestine and renal tubule -β cells of islets-

DIETARY FIBRE Dietary fiber consists of Non-starch polysaccharides such as arabinoxylans, cellulose, and Other plant components such as resistant starch, resistant dextrins, inulin, lignin, waxes, chitins, pectins, beta-glucans, and oligosaccharides. Dietary Fibre change the nature of the contents of the gastrointestinal tract and by changing how other nutrients and chemicals are absorbed. .

Dietary fibre - Cellulose  Cellulose the main bulk, contains β (1-4) bonds between glucose molecules.  In humans, there is no β (1-4) glucosidase that can digest such bonds. So cellulose es as such in stool/feces.  Cellulose helps water retention during the age of food along the intestine  producing larger and softer feces  preventing constipation

Dietary fibre or roughage is the indigestible 2 components: portion food Solubleof fiber, which dissolves in water, is readily fermented in the colon into gases and physiologically active by-products, and can be prebiotic and viscous. Soluble fibers tend to slow the movement of food through the system. Insoluble fiber, which does not dissolve in water, is metabolically inert and provides bulking, or it can be prebiotic and metabolically ferment in the large intestine. Bulking fibers absorb water as they move through the digestive system, easing defecation.

Fermentable insoluble fibers  Fermentable insoluble fibers mildly promote stool regularity, not to the extent that bulking fibers do, but they can be readily fermented in the colon into gases and physiologically active by-products.  Insoluble fibers tend to accelerate the movement of food through the system.

Prebiotics is a general term to refer to chemicals that induce the growth and/or activity of commensal microorganisms .  PREBIOTICS are a special form of dietary fiber.  PREBIOTICS nourish the good bacteria.  PREBIOTICS may be helpful for several chronic digestive disorders or inflammatory bowel disease.  PROBIOTICS are live bacteria in yogurt, dairy products.

Importance of DIETARY FIBER  Production of healthful compounds during the fermentation of soluble fiber, and  Insoluble fiber's ability (ive hygroscopic properties) to increase bulk, soften stool, and shorten transit time through the intestinal tract.  Constipation can occur if insufficient fluid is consumed with a high-fiber diet.

Dietary fibre’s effect on cholesterol metabolism by: Slowing of gastric emptying time A glycemic index type of action on absorption A slowing of bile acid absorption in the ileum so bile acids escape through to the cecum Altered or increased bile acid metabolism in the cecum Binding of bile acids to fiber or bacteria in the cecum with increased fecal loss from the entero-hepatic circulation. Fermentable fibers e.g., pectin, provides a medium for bacterial growth - increases the bacterial mass in the colon. The sequestrated bile acids are then excreted in feces. Indirectly by absorbed short-chain fatty acids, especially propionic acid, resulting from fiber fermentation affecting the cholesterol metabolism in the liver.

Soluble fibers are fermented, producing Short-chain fatty acids (SCFA). Importance of SCFA: Stabilize blood glucose levels by acting on pancreatic insulin release and liver control of glycogen breakdown. Stimulate gene expression of glucose transporters in the intestinal mucosa, regulating glucose absorption. Provide nourishment of colonocytes, particularly by the SCFA butyrate. Suppress cholesterol synthesis by the liver and reduce blood levels of LDL cholesterol and triglycerides responsible for atherosclerosis. Lower colonic pH (i.e., raises the acidity level in the colon) which protects the lining from formation of colonic polyps and increases absorption of dietary minerals. Stimulate production of T helper cells, antibodies, leukocytes, cytokines improving immune mechanism. Improve barrier properties of the colonic mucosal layer, inhibiting inflammatory and adhesion irritants, contributing to immune functions.

What is the Glycemic Index?  Glycemic Index (GI) is a measurement carried out on carbohydrate-containing foods and their impact on our blood sugar (effect on a person's blood glucose).  The current validated methods use glucose as the reference food, giving it a glycemic index value of 100 by definition.  This value is used to compare with other foods

Foods appear on the GI index if they contain carbohydrate. Foods ranked as being very low,  High GI are : potato , rice and bread, GI >70 low, medium, or high in their GI value. Medium GI: banana, basmati rice GI>50  Low GI :Peanuts, beans, milk, yogurt, lentils, barley, most fruits and vegetables GI<40

Rise of blood sugar after meal

References: Lippincott’s Illustrated Biochemistry.  Text book of Biochemistry for Medical Students by Dr. Vasudevan  Harper’s Text book of Biochemistry, 26th edition