How to evaluate osmotic diarrhea?
- Osmotic diarrheas are typically due to ingestion of poorly absorbed cations, such as magnesium, or anions, such as sulfate.
- In addition, carbohydrate malabsorption, such as that caused by ingestion of lactose in a patient with lactase deficiency, and ingestion of poorly absorbable sugar alcohols, such as sorbitol, can lead to an osmotic diarrhea.
- Measuring stool pH can help to distinguish between osmotic diarrheas caused by poorly absorbed cations and anions and those caused by ingestion of poorly absorbed carbohydrates and sugar alcohols.
- Carbohydrates and sugar alcohols are fermented by colonic bacteria, reducing fecal pH below 5 because of the production of short-chain fatty acids. In contrast, ingestion of poorly absorbed cations and anions does not affect stool pH much and stool pH is typically 7 in these circumstances.
- Once acidic stools have been discovered, check the diet and inquire about food additives and osmotic laxative ingestion.
- Specific testing for magnesium and other ions in stool is readily available to confirm any suspicions
