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