Biochemistry Lecture 1: Digestion & Absorption

Nutrient Types

• Digestible nutrients include proteins, carbohydrates, fats (lipids), and nucleic acids.
• Indigestible nutrients, primarily fibers, lack necessary enzymes for digestion in humans (e.g., cellulose, pectin).

Digestion, Absorption, and Transport of Carbohydrates

• Carbohydrates are absorbed only if they can be broken down by specific enzymes; undigested carbohydrates remain unabsorbed.
• Key carbohydrates: starch, lactose, and sucrose.
• Begins with polysaccharides broken down into dextrins by salivary and pancreatic amylase, followed by disaccharidases (like maltase) converting them into monosaccharides for absorption.

Dietary Carbohydrates

• Contribute 40-45% of caloric intake in the USA, with 50-60% primarily from starch.
• Animal-derived carbohydrates include lactose from dairy products.
• Sucrose and small amounts of glucose and fructose serve as primary natural sweeteners in fruits, honey, and vegetables.

Monosaccharide Inter-conversion

• Glucogenic amino acids convert to glucose, generating pyruvate, which can be transformed into glucose via gluconeogenesis.

Digestion of Dietary Carbohydrates

• Glycosidases show specificity for the glycosidic bond and the number of residues in carbohydrates.
• Undigested carbohydrates ferment in the gut, potentially leading to increased fecal water content, contributing to diarrhea, especially after food poisoning or intestinal injury.

Salivary and Pancreatic Amylase

• Breaks down starch into maltose, isomaltose, limit dextrins, and trisaccharides, each needing specific enzymes for further breakdown.
• Daily production: 1 L of saliva and 1.5 L of pancreatic juice.

Disaccharidases of the Intestinal Brush-Border Membrane

• Enzymes work on various substrates through complexes:
  • Maltase-Glucoamylase: Exoglucosidase targeting α–1,4 bonds.
  • Sucrase-Isomaltase Complex: Similar to glucoamylase, acts on disaccharides.
  • Trehalase: Specific for α-1,1 bonds in trehalose; notable for trehalase deficiency leading to health issues after mushroom consumption.
  • Lactase-Glucosylceramidase Complex: Breaks down lactose; significant lactose intolerance prevalence (25% in Jordan).

Intestinal Locations of Activity

• Pancreatic amylase produces maltose and limit dextrins primarily in the duodenum.
• Sucrase–isomaltase and β-Glycosidase are most active in the jejunum, while glucoamylase activity peaks in the ileum.

Metabolism of Sugars by Colonic Bacteria

• Starches high in amylose or poorly hydrated are resistant to digestion and enter the colon.
• Dietary fibers and undigested sugars also reach the colon, where they are fermented by gut bacteria.

Indigestible Carbohydrates

• Humans lack enzymes to break down certain carbohydrates, contributing to lactose intolerance:
  • Low lactase levels in adults compared to infants (less than 10%).
  • Lactase persistence allows some adults to maintain higher lactase levels beyond infancy.