Carbohydrate Digestion

Carbohydrate Digestion

• Carbohydrates require hydrolysis to monosaccharides before absorption
• Some polymers are digestible, while others are not; non-digestible polymers are classified as fiber
• Dietary fiber is a non-digestible polymer found in fruits, vegetables, and cereals and can be either soluble or non-soluble
• Glycogen is the storage form of carbohydrates in animals, equivalent to starch in plants

Intraluminal Hydrolysis

• Digestion of carbohydrates involves two steps: intraluminal hydrolysis and membrane digestion
• Intraluminal hydrolysis involves the breakdown of starch to oligosaccharides by salivary and pancreatic enzymes
• Salivary amylase initiates starch digestion in the mouth, but is inactivated by gastric acid
• Pancreatic alpha-amylase completes starch digestion in the small intestine, producing disaccharides
• Disaccharides cannot be absorbed by the small intestine and require further digestion to produce absorbable monosaccharides

Membrane Digestion

• Membrane digestion involves the hydrolysis of oligosaccharides to monosaccharides by brush border disaccharidases
• The small intestine has three brush border oligosaccharidases: lactase, maltase, and sucrase-isomaltase
• Each enzyme has a specific hydrolytic function

Carbohydrate Absorption

• The three monosaccharide products of carbohydrate digestion (glucose, galactose, and fructose) are absorbed by the small intestine in a two-step process
• The first step involves uptake across the apical membrane into the epithelial cell
• The second step involves coordinated exit across the basolateral membrane
• The sodium/glucose transporter 1 (SGLT1) is responsible for glucose and galactose uptake at the apical membrane
• Fructose is absorbed by facilitated diffusion through GLUT5

Organisation of the Gastrointestinal System

• The gastrointestinal tract is a tube specialized for sequential processing of food
• The tract consists of hollow organs (mouth to anus) and accessory glands and organs that add secretions to the hollow organs
• Each hollow organ serves a specialized function, separated by sphincters at key locations
• The mouth and oropharynx initiate mechanical breakdown and lubrication of food, and propel food into the esophagus
• The esophagus serves as a conduit to the stomach
• The stomach provides temporary food storage, churns, and secretes proteases and acid to facilitate digestion
• The small intestine continues digestion and is the primary site for nutrient absorption
• The large intestine reabsorbs fluids and electrolytes, but does not absorb nutrients, and stores faecal matter before expulsion
• Accessory glands include salivary glands, pancreas, and liver

Nutrient Digestion and Absorption

• Digestion is the enzymatic conversion of complex dietary substances to a form that can be absorbed
• Most, but not all, digestive processes occur in the small intestine
• Absorption is the process of taking up nutrients into cells or across tissues and organs through diffusion or osmosis

Protein Digestion

• Proteins must be digested into their constituent oligopeptides and amino acids before being taken up by enterocytes
• Digestion-absorption of proteins occurs through four major pathways
• Proteins are digested into peptides and amino acids by luminal enzymes (proteases) from the stomach and pancreas
• Brush border peptidases fully digest some oligopeptides to amino acids, while cytosolic peptidases digest oligopeptides that directly enter the enterocyte
• Both gastric and pancreatic proteases are secreted as proenzymes that require conversion to their active form for protein hydrolysis
• The protein that is digested and absorbed in the small intestine comes from both dietary and endogenous sources
• Nine of the 20 amino acids are essential, meaning they are not synthesized in adequate amounts by the body and must be derived from animal or plant sources