Biochemistry Lecture 1: Digestion & Absorption

Study Notes

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).

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.

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.

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

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.

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.

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).

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.

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.

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.