How do FODMAPs work?

Many patients report symptoms come on after eating, and experience with exclusion diets suggests that certainly poorly absorbed but rapidly fermentable carbohydrates may be responsible. While monomeric and short chain carbohydrate polymers exert osmotic forces and trap water in the small bowel with acceleration of transit, the longer polymer polymers such as inulin pass through the small bowel unaltered but are fermented in the colon where they may cause distension and symptoms. Recent developments in magnetic resonance imaging confirm the importance of fructose in stimulating small bowel water secretion and its negation by the presence of glucose. Fructans have no impact on the small intestine but cause colonic distension and symptoms including bloating.