Many of my clients tell me that even a few small pieces of sugar-free candy or gum cause diarrhea. Ditto for the well-know effect of prunes or cherries. What do these foods have in common? They contain polyols, also known as "sugar alcohols".
Technically, they are FODMAPS carbohydrates, and they are sweet! Most polyols are poorly absorbed, and wind up in the large intestine when they are consumed. In the large intestine, polyols are fermented by gut bacteria. They are also capable of drawing water into the large intestine. Together, the excess gas and excess fluid can cause distension of the colon, which cause bloating, flatulence and looser stools for the IBS sufferer. In a double whammy, eating or drinking sugar alcohols worsens fructose intolerance!
Some sugar alcohols, such as sorbitol, occur naturally in fruits such as apples, pears, and the "stone fruits," which include plums, prunes, apricots, peaches, cherries and so on. Sugar alcohols are often used to sweeten "sugar-free" foods (such as candy, gum, low-carb items, and bubbly water) and liquid medications. They can be recognized on the food label's list of ingredients by the ending "-ol". Examples are xylitol and maltitol.
Note that erythritol seems to be an exception. Studies have shown it is well absorbed and unlikely to cause GI symptoms.