Carbohydrate Digestion

• Carbohydrate digestion starts in the mouth through α-amylase in salivary secretions.
  • α-amylase catalyzes starch hydrolysis and is highly specific.
  • α-(1→4) glucan links in polysaccharides containing three or more α-(1→4)-linked D-glucose units are hydrolyzed.
  • Pig saliva has a pH of about 7.3, which is slightly above optimal for α-amylase activity.
  • Salivary amylase continues to act until the food mixes with gastric acid in the stomach, which inactivates the enzyme.

Principal products of starch digestion

• Maltose
• Maltotriose
• Malto-oligosaccharides
• Isomaltose (α-dextrins) with branched oligosaccharides
• No enzymes are secreted in the stomach for carbohydrate digestion.

Digestion in the Small Intestine by Pancreatic α-Amylase

• Small intestine is the primary site for starch digestion.
• Pancreatic α-amylase is most concentrated in the duodenum.
  • Similar product as salivary amylase, but total activity is greater.

Enzymatic Breakdown in Brush Border Membrane of the Epithelium (Duodenum & Jejunum)

• Enzymes secreted in the small intestine aid carbohydrate digestion.
• Major brush border oligosaccharidases:
  • Lactase splits lactose into glucose and galactose.
  • Sucrase splits sucrose into glucose and fructose.
  • Isomaltase (oligo1,6-glucosidase)—debranching enzyme; cleaves alpha 1,6 glycosidic bonds.
  • Maltase splits maltose into 2 glucose units.

Absorption of Carbohydrates

• The duodenum and upper jejunum have the highest capacity for sugar absorption.
• Monosaccharides mainly absorbed through facilitated diffusion:
  • Glucose
  • Galactose
  • Fructose
• Sodium-dependent hexose transporter (SGLUT-1) carries glucose and galactose into the enterocyte.
• GLUT5 transports fructose from the intestinal lumen into the enterocyte; it is not sodium-dependent.

Fate of Absorbed Monosaccharides

• They are released from the enterocyte on the basolateral side.
• Glucose, galactose, and fructose are transported out of the enterocyte
• Another hexose transporter (GLUT-2) transports glucose, galactose and fructose out of the enterocyte
• They are taken up by blood vessels in the villi and transported to the liver via the hepatic portal vein.

Digestion of CHO in the Large Intestine

• Products entering the large intestine:
  • Non-starch polysaccharides
  • Starch that escaped digestion in the SI
  • Non-digestible oligosaccharides
• No enzymes are secreted by the large intestine, and no monosaccharides are absorbed there like it is in the small intestine.
• Microbial enzymes can digest carbohydrates containing β–1,4 bonds.
• Major end products: Volatile fatty acids
  • Acetic (2C), propionic (3C), butyric (4C) acid.
  • VFA can be absorbed in the large intestine via simple diffusion.
  • VFA can be used for energy production.

Non-starch polysaccharides (NSP)

• NSP equals total dietary fiber minus lignin forming a part of dietary fiber
• NSP are structural elements in plants, can be homo- or hetero-polysaccharides
• NSP has a low feeding value for non-ruminants.
• There is no NSP digestion in the small intestine due to lack of cellulase, the NSP is passed to the large intestine.

Types of NSP by Location

• Cell Wall NSP: cellulose, arabinoxylans, galactans, beta-glucans
• Non-Cell Wall NSP: fructans, mannans, pectins, galactomannans
• Resistant Starch i.e. inaccessible, native, retrograded.
  • Portion of starch that cannot be hydrolyzed by enzymes in the small intestine and is passed into the large intestine for fermentation by gut microflora
  • Physically inaccessible starch is entrapped in protein matrix, cell-wall materials, and other physical barriers that reduce starch accessibility.
  • The compact protein matrix of pasta results in higher resistant starch content.
• Resistant starch provides potential benefits: promotes growth of beneficial microorganisms.

Types of NSP Based on Water Solubility

• Soluble NSP includes pectin, gums, mucilages, and some hemicelluloses.
  • They have high water holding and binding capacity
  • Soluble NSP delays gastric emptying
  • Soluble NSP increases intestinal transit time
  • Soluble NSP decreases nutrient absorption.
• How soluble NSP affect nutrient digestibility?
  • Soluble NSP increases digesta viscosity which limits the interaction between nutrients and enzymes
  • Soluble NSP results in the formation of unstirred water layer in intestinal surface thus decreasing nutrient absorption.
• Insoluble NSP includes cellulose and some hemicelluloses
  • Insoluble NSP decreases intestinal transit time and increases fecal volume.

Carbohydrate Digestion in Ruminants

• The major route of CHO digestion goes through ruminal fermentation and subsequent ruminal absorption of VFA.
• Dietary carbohydrates are degraded by rumen microbes, which secrete enzymes for cellulose breakdown.
• The majority of monosaccharides are broken down into volatile fatty acids.
• Post-ruminal digestion and absorption is the route by which rumen-escaped α-linked CHO becomes available to the host animal.
• Pancreatic α-amylase and brush border disaccharidases are present in the small intestine to degrade rumen-escape α-linked CHO, like in monogastrics.
• There is also microbial fermentation of undigested CHO in the large intestine and VFA from CHO digestion will be absorbed.

Glucose Metabolism

• Fate of dietary glucose:

1. Short term storage (as glycogen)
2. Long term storage (as fatty acids)
3. Supply energy under aerobic conditions
4. Supply energy under anaerobic conditions
5. Synthesis of dispensable amino acids
6. Synthesis of lactose

Mechanisms for the Conversion of Glucose into Energy

1. Glycolysis

• Glycolysis definition: “Glycos” = sugar; “Lysis” = dissolution
• Glycolysis Function: Sequence of reactions that converts glucose into pyruvate with ATP production
• Glycolysis Occurence: Occurs in all tissues
• Glycolysis Location: Occurs in the cytosol of the cell
• Glycolysis Conditions: Can be aerobic (with O₂) or anaerobic (without O₂)
• Glycolysis Phases: Two phases of glycolysis: Ο Preparatory phase is the first 5 reactions, forming 2 molecules of glyceraldehyde 3-phosphate Payoff phase is when glyceraldehyde 3-phosphate will enter the payoff phase turning into pyruvate.
• Each glucose molecule entering glycolysis will consume 2 ATP in the preparatory phase and produce 4 ATP in the payoff phase resulting to net production of 2 ATP.

Transition Reaction (Transition Between Glycolysis and TCA Cycle)

• Enzyme involved: Pyruvate dehydrogenase enzyme complex
• Each pyruvic acid molecule is broken down to form CO2 and a two-carbon acetyl group, which enters the TCA cycle.
• Coenzymes are required vitamins such as:
  • Thiamine
  • Riboflavin
  • Niacin
  • Pantothenic acid

Citric Acid Cycle (TCA Cycle, Krebs Cycle)

• The final common pathway for the oxidation of nutrient molecules (CHO, lipids, and proteins).
• Provides intermediates for biosynthesis of essential molecules.
• Requires oxygen, and takes place in the mitochondria of cells.
• One cycle yields 1 ATP, 3 NADH, 1 FADH2.

Oxidative Phosphorylation (Coupled Reaction Happening in Electron Transport System)

• Oxidative Phosphorylation function is to convert the energy from reducing agents (NADH and FADH2) into ATP.
• Takes place in the mitochondrial membrane.
• NADH and FADH2 are oxidized and produces protons.
• As proton concentration builds up in the intermembrane space, a gradient is created, so protons are transported from high to low concentration.
• The energy from the transfer of protons is used to change ADP into ATP through phosphorylation.
• Formation of ATP via oxidative phosphorylation:
  • 1 NADH yields 3 ATP
  • 1 FADH2 yields 2 ATP
  • 10 NADH yields 30 ATP
  • 2 FADH2 yields 4 ATP
• Total yield via oxidative phosphorylation = 34 ATP.
• NADH produces 3 ATP during the ETC while FADH2 only produces 2.
• NADH gives up its electron to Complex I, which is at a higher energy level than the other Complexes.
• Since NADH started with Complex I, there are more chances to pump more protons across the gradient. Thus more energy is derived from the transfer of protons
• On the other hand, FADH2 gives up its electron to Complex II, bypassing Complex I. Less protons are produced by the time it reaches Complex IV.
• Less energy is derived from the transfer of protons available to power up the ATP synthase (producing only 2 ATP).

Total Energy Production From Glucose

• Glycolysis: 2 ATP
• TCA Cycle: 2 ATP
• ETC (oxidative phosphorylation): 34 ATP
• TOTAL: 38 ATP
• NOTE: For skeletal muscles and brain, less ATP is produced.
• Cytosolic NADH (produced from glycolysis) only produces 2 ATP.
• Instead of the most active NADH shuttle in the liver, kidney, and heart.
• Glycerol 3-phosphate shuttle is active in skeletal muscles and in the brain;
• Glycerol 3-phosphate shuttle bypasses Complex I and II of ETC (only passes III and IV), thus less energy is produced.