- Osmotic Diarrhea occurs due to ingestion of poorly absorbable yet osmotically active solutes that prevent proper GI water and electrolyte absorption.
- Osmotic Diarrheas are typically large volume and watery. They usually subside upon fasting since the ingestion of the poorly absorbable solute is ceased.
- Recall from GI Water and Electrolyte Absorption that intestinal water absorption occurs purely passively. Thus water absorption occurs via osmosis, following resorption of osmotically active electrolytes, principaly sodium. If a high concentration of inabsorbable, osmotically active solutes remains in the intestinal lumen, then water absorption cannot occur. Any water that remains in the intestinal lumen will simply be excreted as a very watery diarrhea.
- The most common cause of osmotic diarrhea is due to malabsorption of lactose derived from dairy products, known as lactose intolerance. As described in Carbohydrate Digestion and Absorption lactose must be cleaved into its component monosaccharides by lactase before their absorption. In the absence of lactase, osmotically active lactose cannot be absorbed and remains in the intestinal lumen, thus interfering with water resorption. Additionally, certain laxatives are intentionally composed of poorly absorbable osmolytes, thus explaining their mechanism of action.