Symptoms can occur immediately or up to several hours after ingesting onions. Symptoms can be mild or severe. Being sensitive to onion (intolerance) is more common than a true allergy. Intolerance of nonallergic food hypersensitivity is caused by an inability to process or digest certain foods.
When a person eats high-fiber foods, it is common for some undigested material to appear in the stool because the body cannot fully break down the tough material. Fiber also speeds up a person’s bowel movements by adding bulk to the stool, which encourages the intestinal walls to move.