Carbohydrate Digestion

Overview 

• Overview
  • dietary carbohydrates include polysaccharides, disaccharides, and monosaccharides
    • intestinal epithelial cells absorb only monosaccharides
      • glucose, galactose, and fructose
• Digestion of dietary polysaccharides
  • salivary secretions
    • salivary α-amylase initiates digestion of polysaccharides in mouth
      • digests interior α-1,4 glycosidic linkages, yielding 3 disaccharides
        • α-limit dextrins, maltose, and maltotriose
    • salivary α-amylase plays a minimal role in digestion of carbohydrates
      • low pH of gastric contents quickly inactivates salivary α-amylase
  • pancreatic secretions
    • pancreatic juices are secreted into lumen of duodenum of small intestine
    • pancreatic α-amylase initiates digestion of polysaccharides in the small intestines
      • digests interior α-1,4 glycosidic linkages, yielding 3 disaccharides
        • α-limit dextrins, maltose, and maltotriose
  • intestinal mucosa
    • intestinal brush border enzymes initiate digestion of disaccharides to monosaccharides
      • α-dextrinase catalyzes α-limit dextrins → glucose
      • maltase catalyzes maltose → glucose
      • sucrase catalyzes maltotriose → glucose
    • intestinal epithelial cells absorb glucose, a monosaccharide
• Digestion of dietary disaccharides
  • dietary disaccharides include trehalose, lactose, and sucrose
  • dietary disaccharides do not require α-amylase digestion
  • intestinal brush border enzymes initiate digestion of disaccharides to monosaccharides
    • trehalase catalyzes trehalose → glucose + glucose
    • lactase catalyzes lactose → glucose + galactose  
    • sucrase catalyzes sucrose → glucose + fructose
  • intestinal epithelial cells absorb glucose, galactose, and fructose, all monosaccharides