Biochemistry · MU Pleven

12. Dietary Carbohydrates. Digestion and Absorption of Carbohydrates. Glucose Transport by GLUTs.

Covers the classification of dietary carbohydrates and their main food sources, then walks through digestion from mouth to small intestine, listing each enzyme and the monosaccharides produced. It goes on to absorption in the jejunum and upper ileum, contrasting the SGLT1 and GLUT5 entry routes with GLUT2 exit into the portal blood. The GLUT family is then set out isoform by isoform with tissue distribution and insulin regulation of GLUT4, followed by short clinical correlations such as lactose intolerance and GLUT1 deficiency.

Inside: 6 sections · 8 labelled pictures · 14 flashcards · 5 min

Topic 12 of 76 in the MU Pleven syllabus for Biochemistry (Medicine, Year 2). The free AceMed note covers it in 6 sections, about 749 words (5 min of reading), with 14 flashcards.

In this note

  1. Dietary Carbohydrates. Digestion and Absorption of Carbohydrates.
  2. Dietary Carbohydrates
  3. Digestion of Carbohydrates
  4. Absorption of Carbohydrates
  5. Glucose Transport by GLUTs (Glucose Transporters)
  6. Clinical Correlations

Dietary Carbohydrates

  • Definition:
  • Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen (CₙH₂ₙOₙ).
  • They are the main source of energy in the human diet, providing about 50–60% of total daily calories.
  • Classification (based on complexity):
  • Monosaccharides: Single sugar units (e.g., glucose, fructose, galactose).
  • Disaccharides: Two monosaccharides linked by a glycosidic bond (e.g., sucrose, lactose, maltose).
  • Oligosaccharides: 3–10 monosaccharide units.
  • Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

Major Dietary Sources:

  • Starch: From cereals, potatoes, legumes, and vegetables.
  • Disaccharides: Sucrose (cane sugar), lactose (milk), maltose (malt).
  • Monosaccharides: Glucose and fructose (in fruits and honey).
  • Non-digestible carbohydrates: Cellulose and some fibers act as dietary roughage aiding bowel movement.
  • Physiological Role:
  • Primary energy source (1 g of carbohydrate = 4 kcal).
  • Essential for brain and red blood cell metabolism, both of which rely heavily on glucose.
  • Serve as precursors for amino acids, nucleotides, and fatty acids.

Digestion of Carbohydrates

  • Purpose:
  • Dietary polysaccharides and disaccharides are hydrolyzed into monosaccharides before absorption.
  • Enzymes responsible for this are collectively called carbohydrases or glycosidases.
  • Site 1 – Mouth (Salivary Digestion):
  • Enzyme: Salivary α-amylase (ptyalin).
  • Action: Begins hydrolysis of starch into maltose, maltotriose, and α-limit dextrins.
  • Note: This step is brief because the enzyme is inactivated by stomach acid.
  • Site 2 – Stomach:
  • No significant carbohydrate digestion occurs here due to the acidic pH which inactivates amylase.
  • Food is mixed into chyme and passed into the small intestine.
  • Site 3 – Small Intestine (Major Site):
  • Pancreatic α-amylase: Secreted by the pancreas into the duodenum; continues starch digestion to produce maltose and oligosaccharides.
  • Brush Border Enzymes (of intestinal mucosa):
  • Maltase: Hydrolyzes maltose → 2 glucose.
  • Sucrase: Hydrolyzes sucrose → glucose + fructose.
  • Lactase: Hydrolyzes lactose → glucose + galactose.
  • Isomaltase (α-dextrinase): Hydrolyzes α-limit dextrins → glucose.
  • End Products of Carbohydrate Digestion:
  • Glucose, Galactose, and Fructose are the final absorbable monosaccharides.

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