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Paul Wischmeyer: Nutrient Absorption in the GI Tract
Feb 17, 2026, 15:42

Paul Wischmeyer: Nutrient Absorption in the GI Tract

Paul Wischmeyer, Professor of Anesthesiology and Surgery with Tenure at Duke University School of Medicine, shared a post on LinkedIn:

“Nutrient Absorption in the GI Tract

How and Where Nutrients Are Absorbed

Duodenum (mixing plus minerals)

  • Acid plus bile plus pancreatic enzymes meet here
  • Iron uptake is primarily proximal (risk: low iron if proximal disease/bypass)
  • Calcium/Mg begin early; vitamin D status matters
  • Folate tends to be more proximal

Jejunum (macronutrient workhorse)

  • Carbs: brush-border digestion → glucose/galactose/fructose absorption
  • Protein: amino acids + small peptides via high-capacity transport
  • Fat: micelles deliver lipids for absorption (needs bile + enzymes)
  • Many water-soluble vitamins are absorbed across the small bowel

Ileum (specialists)

  • B12 requires intrinsic factor + terminal ileum receptors
  • Bile acid recycling preserves the bile pool → supports fat + A/D/E/K absorption

Colon (salvage + microbiome)

  • Reclaims water, Na⁺, Cl⁻
  • Fiber → microbiome → SCFAs (e.g., butyrate) that fuel colon cells and support barrier function
  • Some vitamin K/biotin from microbial activity
  • Why this infographic matters in real life:

If someone has:

  • Crohn’s in the terminal ileum to think B12 deficiency + bile acids
  • Celiac (proximal small bowel) to think iron, folate, calcium deficiency
  • Pancreatic insufficiency / cholestasis to think fat plus fat-soluble vitamins (A/D/E/K) deficiency
  • Short bowel or bariatric bypass to think site-specific + global malabsorption risk

Understanding the map helps you understand the labs, symptoms, and fixes.”

Paul Wischmeyer: Nutrient Absorption in the GI Tract

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