What type of diarrhea usually improves with fasting?

Osmotic diarrhea occurs when there is an accumulation of non-absorbable solutes in the intestinal lumen, which draws water into the bowel, leading to diarrhea. This type of diarrhea typically improves with fasting because, during fasting, the intake of food and solutes is reduced, allowing for the absorption of the remaining solutes and the cessation of the osmotic load on the intestines.

In conditions that cause osmotic diarrhea, such as malabsorption syndromes (like lactose intolerance), the diarrhea lessens or resolves when the offending agent is removed from the diet, illustrating the direct relationship between dietary intake and the symptoms. Therefore, fasting, which limits the introduction of additional unabsorbed solutes, leads to increased absorption and decreases the volume of stool.

Other types of diarrhea, such as secretory diarrhea and toxin-induced diarrhea, typically do not improve with fasting, as they are often caused by active secretion of electrolytes and water into the intestinal lumen or by the action of toxins, which act independently of food intake. Chronic diarrhea can arise from various underlying issues but doesn't have the same predictable response to fasting that osmotic diarrhea does.