Diarrhea Types: Osmotic and Secretory

Normal Stool Processing

• The GI system processes 8-9 L/day of stool fluids, with 1-2 L/day ingested and 7 L/day created
• The small bowel reabsorbs 6-7 L/day, and the large bowel absorbs 1-2 L/day
• This results in 100-200 g/day of stool created
• A decrease in absorption or increase in secretion by as little as 1% can lead to diarrhea

Nutrient/Vitamin Absorption

• Carbohydrates/simple sugars are absorbed in the duodenum/jejunum
• Vitamin B12 and short-chain fatty acids are absorbed in the ileum
• Fats, bile salts, and vitamin K are absorbed in the colon
• Amino acids, iron, fat-soluble vitamins (A, D, E, K), calcium, magnesium, and other vitamins and minerals are also absorbed in the GI system

Chronic Diarrhea

• Affects 5% of the population
• Most frequent etiologies include:
  • Functional disorders (e.g., irritable bowel syndrome)
  • Inflammatory bowel disease (IBD)
  • Malabsorptive syndromes
  • Chronic infection

Small Bowel Function

• Motility
• Digestion
• Absorption
• Immune defense

Pathophysiology of Chronic Diarrhea

• Types of chronic diarrhea:
  • Osmotic diarrhea
  • Secretory diarrhea
  • Inflammatory diarrhea
  • Motility disturbances

Osmotic Diarrhea

• Causes:
  • Ingestion of poorly absorbed osmotically active substances (non-electrolytes)
  • Carbohydrate malabsorption (e.g., disaccharidase deficiency, glucose-galactose malabsorption, fructose malabsorption)
• Mechanism: retention of unabsorbed ions in the intestinal lumen, leading to water retention to maintain intraluminal osmolality

Secretory Diarrhea

• Causes:
  • Exogenous secretagogues (e.g., cholera toxin)
  • Endogenous secretagogues (e.g., neuroendocrine tumor)
  • Absence of ion transporter (e.g., congenital chloridorrhea)
• Mechanism: increased net flux of ions into the intestinal lumen, leading to secretion of anions (chloride or bicarbonate) and inhibition of sodium absorption

Inflammatory Diarrhea

• Causes:
  • Inflammatory bowel disease (IBD)
  • Ischemic colitis
  • Diverticulitis
  • Radiation colitis
  • Infectious causes (e.g., invasive bacterial, parasitic, or viral infections)
• Mechanism: structural damage to the intestinal mucosa, impairing absorption and stimulating secretion

Diarrhea - Motility Disturbances

• Causes:
  • Abnormal intestinal motility (e.g., hyperthyroidism, dumping syndrome, postvagotomy diarrhea, postsympathectomy diarrhea, diabetic autonomic neuropathy)
• Mechanism: decreased contact between digested food products and the intestinal mucosa, leading to diarrhea

Clinical Classification of Chronic Diarrhea

• Organic vs. functional
• Fatty
• Watery
• Inflammatory

This quiz covers the causes and characteristics of osmotic and secretory diarrhea, including the role of poorly absorbed substances, ions, and carbohydrates in the development of these conditions.