Nutrient Absorption

Overview

• Absorption primarily occurs in the small intestine.
  • 80% of food, electrolytes, and most water.
  • Bile salts reclaimed in the ileum.
• Most absorption requires energy (active transport).
  • Exception: lipids.

Carbohydrate Absorption

• Glucose and Galactose
  • Apical side: Active transport using sodium gradient.
  • Basolateral side: Passive transport (facilitated diffusion).
• Fructose
  • Apical & Basolateral sides: Facilitated diffusion (passive process).
• Movement into capillary beds in villi after entering ISF.

Protein Absorption

• Most amino acids use secondary active transport with sodium ions.
• Dipeptides and tripeptides co-transported with hydrogen ions, further digested inside cells.
• Enter blood through facilitated diffusion.

Lipid Absorption

• Monoglycerides and Fatty Acids
  • Cluster with bile salts and lecithin to form micelles.
  • Diffuse across cell membrane.
  • Inside cell: Combine with lecithin, phospholipids, cholesterol, and proteins to form chylomicrons.
  • Absorbed through lacteals into the lymphatic system.
  • Hydrolyzed to free fatty acids and glycerol by lipoprotein lipase in capillary endothelia.
• Short-chain fatty acids can diffuse directly into portal blood.

Nucleic Acid Absorption

• Absorbed via active transport into the bloodstream.

Vitamin Absorption

• Fat-soluble vitamins (A, D, E, K)
  • Absorbed with micelles like lipids.
• Water-soluble vitamins (C, B)
  • Absorbed by diffusion or transport mechanisms.
• Vitamin B12
  • Must bind to intrinsic factor from the stomach for absorption in the duodenum.
• Some B and K vitamins produced by enteric bacteria absorbed in the large intestine.

Electrolyte Absorption

• General
  • Active absorption along the small intestine.
• Iron and Calcium
  • Absorbed in the duodenum, dependent on need.
  • Iron bound to ferritin for storage and transferrin for transport.
• Sodium & Chloride
  • Sodium coupled with glucose/amino acids (secondary active transport).
  • Chloride transported actively.
• Potassium
  • Diffuses according to osmotic gradients.
  • Poor water absorption affects potassium absorption.
• Calcium
  • Dependent on parathyroid hormone and vitamin D (facilitates absorption).

Water Absorption

• 9 liters absorbed via osmosis in the small intestine.
• Remaining water absorbed in the large intestine.
  • Balance critical to prevent constipation or diarrhea.
• Coupled with solute uptake due to water following solutes.