Nutrient Absorption Journey: From Mouth to Large Intestine

Nutrient absorption is a sequential, site-specific process that begins the moment food enters the mouth and concludes in the large intestine. Each digestive organ plays a specialized role in breaking down macronutrients and extracting vitamins, minerals, and water.

1. Mouth: Mechanical and Chemical Initiation

• Mastication increases surface area.
• Salivary amylase (ptyalin) begins starch hydrolysis.
• Lingual lipase (minor in adults) starts fat digestion.

2. Stomach: Churning and Protein Denaturation

• Gastric peristalsis mixes food with acid/pepsin → chyme.
• Pepsin (activated by HCl) cleaves proteins to peptides.
• Intrinsic factor binds B12 for later ileal absorption.
• Minimal absorption: water, alcohol, some lipid-soluble drugs (aspirin).

3. Small Intestine: The Absorption Powerhouse

The duodenum, jejunum, and ileum collectively provide ~250 m² surface area via circular folds, villi, and microvilli (brush border).

• Duodenum: Receives chyme + bile + pancreatic juice. Iron, calcium, folate, fat-soluble vitamins absorbed here.
• Jejunum: Major site for monosaccharides, amino acids, water-soluble vitamins, most minerals.
• Ileum: Bile salts (enterohepatic circulation), B12-intrinsic factor complex, remaining nutrients.

Pancreatic enzymes (amylase, lipase, proteases) and bile micelles are essential for luminal digestion.

4. Large Intestine: Water, Electrolytes, and Microbiome

• Absorbs remaining water (1–1.5 L/day) and electrolytes (Na⁺, K⁺, Cl⁻).
• Colonic microbiota ferment undigested fiber → short-chain fatty acids (butyrate, propionate, acetate) absorbed for colonic energy; synthesize vitamin K₂, biotin, folate.
• Forms and stores feces; defecation via rectum/anal canal.

Clinical Relevance

Disruption at any stage—achlorhydria, pancreatic insufficiency, celiac disease, ileal resection, dysbiosis—causes specific malabsorption syndromes. Targeted diagnosis (breath tests, stool studies, endoscopy) guides replacement therapy.