Osmotic Diarrhoea

Definition and Mechanism

1. Ingestion of non-absorbable substances (osmotic laxatives, sugar alcohols)
2. Maldigestion - failure to break down substrates (disaccharidase deficiency)
3. Malabsorption - failure to absorb breakdown products (mucosal disease, pancreatic insufficiency, SIBO)

• Sleisenger and Fordtran's Gastrointestinal and Liver Disease, p. 226

Causes

1. Poorly Absorbed Ions / Medications

• Magnesium salts - magnesium citrate, magnesium phosphate, milk of magnesia, magnesium sulfate (used as laxatives or antacids - a common accidental cause in patients overusing Mg-antacids for dyspepsia/reflux)
• Phosphate/sulfate salts, polyethylene glycol (PEG), sodium phosphate bowel prep
• Lactulose - a synthetic disaccharide not hydrolyzable by human disaccharidases
• Vitamin C (high doses)
• Other drugs: antibiotics (ampicillin, clindamycin), methyldopa, quinidine, propranolol, hydralazine, ACE inhibitors, procainamide

2. Poorly Absorbed Carbohydrates (Disaccharidase Deficiency)

3. Sugar Alcohols (Polyols)

4. FODMAPs

5. Generalized Malabsorption States

• Sleisenger and Fordtran's, p. 228-229; Frameworks for Internal Medicine, p. 205-206; Yamada's Textbook of Gastroenterology, p. 720

Clinical Features

• Stops with fasting - the hallmark of osmotic diarrhea; differentiates it from secretory diarrhea (which continues during fasting)
• No nocturnal diarrhea - diarrhea improves or stops overnight when patient is not eating; patients may avoid eating before long trips
• Watery stools with low electrolyte concentrations
• Steatorrhea (greasy, bulky, malodorous, hard-to-flush stools) if fat malabsorption is also present
• Weight loss when underlying malabsorption is present
• Bloating, flatulence, borborygmi when carbohydrate fermentation is the mechanism

Stool Osmotic Gap - The Key Diagnostic Test

Stool Osmotic Gap = 290 mOsm/kg - 2 × (stool Na⁺ + stool K⁺)

Stool Osmotic Gap = Measured osmolality - 2 × (stool Na⁺ + stool K⁺)

• Yamada's Textbook of Gastroenterology, p. 720; Frameworks for Internal Medicine, p. 205

Osmotic vs. Secretory Diarrhea - Comparison

Diagnostic Approach

1. Confirm osmotic diarrhea: stool Na⁺ <60 mmol/L + osmotic gap >100 mOsm/kg
2. Trial of lactose-free diet - if no improvement:
3. Check stool pH:
   • pH <6 → Hydrogen breath testing (positive = carbohydrate malabsorption)
   • pH >6 → Trial of elimination of products with alcohol sugars (sorbitol, mannitol, xylitol)
     • Improves → sugar alcohol ingestion confirmed
     • No improvement → evaluate for factitious causes

• D-xylose absorption test: Detects small intestinal mucosal malabsorption (not useful for pancreatic insufficiency, as xylose doesn't need pancreatic enzymes); false positives in renal dysfunction and SIBO
• Hydrogen breath test: Patient fasts 8-12 h, ingests lactose or fructose, expired H₂/CH₄ measured every 15-30 min for 2-3 hours - elevation indicates bacterial fermentation of unabsorbed carbohydrate
• Stool magnesium: >15 mmol/day (or >44 mmol/L) strongly suggests magnesium-induced diarrhea
• Stool phosphate: >15 mmol/day suspicious for phosphate-induced diarrhea

Management

• Dietary modification: lactose-free diet (lactase deficiency), low-FODMAP diet, elimination of sugar alcohols, reduced fructose intake
• Stop the offending medication/supplement (Mg antacids, lactulose, PEG)
• Treat underlying malabsorption:
  • Celiac disease: gluten-free diet
  • Pancreatic exocrine insufficiency: pancreatic enzyme replacement
  • SIBO: antibiotics (rifaximin, metronidazole) + treat underlying anatomic/motility cause + correct nutritional deficiencies
  • Short bowel syndrome: dietary optimization, octreotide, intestinal rehabilitation
• Sleisenger and Fordtran's, p. 228-229; Yamada's Textbook of Gastroenterology, p. 720

1. Osmotic diarrhea stops with fasting - this is the most reliable clinical clue
2. Stool osmotic gap >100 mOsm/kg confirms osmotic mechanism
3. Low stool Na⁺ (<60 mmol/L) and low stool pH (<6 in carbohydrate malabsorption) are supporting findings
4. The three broad causes are: osmotic laxatives/ions, poorly absorbed carbohydrates/polyols, and generalized malabsorption
5. Treatment is always directed at eliminating or correcting the underlying cause

Stool Osmotic Gap

The Core Concept

Are the solutes in stool mostly electrolytes (Na⁺, K⁺, and their anions), or are there significant non-electrolyte solutes present that we haven't directly measured?

Formula

Stool Osmotic Gap = 290 mOsm/kg − 2 × (Stool Na⁺ + Stool K⁺)

• 290 mOsm/kg is used as a fixed assumed value for stool osmolality (equal to plasma), because stool is always in osmotic equilibrium with body fluids
• 2 × accounts for the paired anions (Cl⁻, HCO₃⁻, etc.) that accompany each cation - stool fluid is electrically neutral, so each cation has an accompanying anion
• The result represents the osmolality not explained by electrolytes - i.e., the "gap" filled by unmeasured, non-electrolyte solutes

• Sleisenger and Fordtran's Gastrointestinal and Liver Disease, p. 235

Interpretation

Note: Different textbooks use slightly different cutoffs (50, 100, or 125 mOsm/kg) for the upper threshold. The Goldman-Cecil Medicine states the normal gap is 50-125 mOsm/kg. The key principle is directional: high gap = osmotic; low gap = secretory.

Why the Gap Differs Between Osmotic and Secretory Diarrhea

Secretory Diarrhea

• Stool Na⁺ usually >90 mmol/L
• Stool K⁺ usually <10 mmol/L
• Stool HCO₃⁻ usually >50 mmol/L
• 2 × (Na⁺ + K⁺) approaches 290 mOsm/kg → gap is small (<50)

Osmotic Diarrhea

• Stool Na⁺ is low (<60 mmol/L)
• The "unmeasured osmoles" (non-electrolyte solutes) explain the gap
• 2 × (Na⁺ + K⁺) is far less than 290 → gap is large (>100)

Worked Example

• Stool Na⁺ = 40 mmol/L
• Stool K⁺ = 25 mmol/L

Osmotic Gap = 290 − 2 × (40 + 25) = 290 − 130 = 160 mOsm/kg

Additional Clues from Stool Chemistry

• Tietz Textbook of Laboratory Medicine 7th Edition, p. 1651; Goldman-Cecil Medicine, p. 1466

Summary Diagram (Conceptual)

Total stool osmolality = 290 mOsm/kg (fixed)
         │
         ├── Explained by electrolytes: 2 × (Na⁺ + K⁺)
         │
         └── Unexplained = OSMOTIC GAP
                  │
                  ├── Gap HIGH (>100):   Non-electrolyte solutes dominant
                  │                      → OSMOTIC diarrhea
                  │
                  └── Gap LOW (<50):     Electrolytes dominant
                                         → SECRETORY diarrhea