Decreased absorption may be caused by mucosal damage, osmotically active agents in the lumen, and, in rare instances, increased motility resulting in decreased time for absorption.
Increased secretion can result from stimulated secretion of chloride from crypt cells.
In addition, fluids and electrolytes may be secreted into the lumen. When luminal contents are hyperosmolar, fluid is secreted into the lumen to restore osmotic equilibrium. Diarrhea can result from unabsorbed carbohydrates, as in lactose intolerance, when the products of bacterial fermentation impose an osmotic load on the colon. When transit is too rapid, absorption time is decreased and diarrhea can result, which explains the diarrhea seen in hyperthyroidism. Diarrhea caused by rapid transit is seen after gastric resection or vagotomy.