Malabsorption

• Malabsorption = defective absorption of fats, vitamins, proteins, carbohydrates, electrolytes, minerals and water
• Most common presentation: chronic diarrhea; hallmark = steatorrhea (fatty diarrhea)

3 normal digestive functions that can be disturbed:

1. Intraluminal digestion: enzymatic breakdown of proteins, carbohydrates, fats
2. Mucosal absorption: absorption + transport of water, electrolytes, nutrients into the cell
3. Nutrient delivery: transport from intestinal cells into the lymphatics

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A) Defects in Intraluminal Digestion

• Osmotic diarrhea: excessive osmotic forces by luminal solutes; decreases with fasting
• Steatorrhea: excess undigested fat in stool; caused by inadequate pancreatic lipases or inadequate bile solubilization of fat
• Most common causes: pancreatic insufficiency (chronic alcoholism) and Crohn’s disease

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B) Defects of Mucosal Absorption

Lactose Intolerance

• Deficiency of lactase enzyme (brush-border of enterocytes) → inability to digest lactose
• Inherited form (rare): milk intolerance in infants → diarrhea, weight loss, failure to thrive
• Acquired form (common in adults): should avoid milk products

A-beta-lipoproteinemia (Apolipoprotein-B deficiency)

• Apolipoprotein-B required for assembly of chylomicrons
• Deficiency → no chylomicrons → mucosal cells cannot export lipids (lipids stack up in enterocytes)
• Consequences: diarrhea, steatorrhea, systemic lipid membrane abnormalities

Celiac Disease (Gluten-sensitive enteropathy)

• Immunological sensitivity to gluten (wheat/related grains), especially the antigen gliadin
• Gliadin presented by APCs to CD4+ T-cells → immune response → CD8+ T-cells destroy intestinal epithelial cells → loss of villi → ↓ absorptive surface area
• Non-infectious cause of malabsorption from reduced small intestine surface
• Long-term risk: enteropathy-associated T-cell lymphoma or adenocarcinoma of small bowels

Tropical Sprue & Whipple Disease

• Associated with intestinal infections
• Intestinal mucosa contains many PAS-positive macrophages packed with rod-shaped bacilli
• Clinical: steatorrhea, weight loss, abdominal distention, anorexia

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C) Clinical Features

General symptoms: weight loss, anorexia, abdominal distention, steatorrhea

Complications by system:

System	Complication	Cause
Hematopoietic	Anemia	Iron, folate, or B12 deficiency
Hematopoietic	Bleeding	Vitamin K deficiency
Musculoskeletal	Osteopenia + tetany	Ca²⁺, Mg²⁺, vitamin D deficiency
Endocrine	Amenorrhea, impotence, infertility	Generalized malnutrition
Endocrine	Hyperparathyroidism	Protracted Ca²⁺ + vitamin K deficiency
Skin	Purpura, petechia	Vitamin K deficiency
Skin	Dermatitis, hyperkeratosis	Vitamin A, zinc, fatty acids, niacin deficiency
Nervous system	Peripheral neuropathy	Vitamin A + B12 deficiency

💡 High-yield: Hallmark = steatorrhea. Celiac disease = gliadin antigen → CD8+ T-cells destroy villi; risk for T-cell lymphoma + small bowel ADC. Whipple = PAS+ macrophages with rod-shaped bacilli. A-beta-lipoproteinemia = no chylomicrons; lipids stack in enterocytes. Complications: anemia (Fe/B12/folate), bleeding (vit K), osteopenia (Ca/D), peripheral neuropathy (A/B12).