FODMAPs are characterised as fermentable but poorly absorbed carbohydrates which enter the colon and are utilised by colonic bacteria. During fasting colonic nutrients are scarce but ingesting FODMAPs causes a rapid increase in carbohydrate which can overwhelm the microbiota’s ability to utilise substrate. The excess reducing equivalents will generate hydrogen or methane. Patients with Irritable bowel syndrome appear to benefit by restricting intake, possibly because they are hypersensitive to intestinal distension. There is no evidence that IBS patients malabsorb more than normal but fermentation of poorly absorbed carbohydrates such as lactulose does appear to differ with more hydrogen being produced1. Regular intake of lactulose has been shown to increase the b-galactosidase enzymes needed to ferment it but whether this is true of all FODMAPs is unclear. The best recognised FODMAPs are small molecules (fructose , lactulose, sorbitol) which are osmotically active. Magnetic resonance imaging studies have shown fructose2 and mannitol 3 cause distension of the small bowel with accelerated transit to the caecum. Larger molecules such as fructans and inulin exert less osmotic force and their main impact is on the colon where they generate large amounts of gas. Studies in IBS patients confirm the significance of these differences since the symptoms developing after FODMAP challenge are correlated with small bowel water volumes after fructose and colonic gas volumes after fructans. It is likely that the physical form of FODMAPs and their distribution throughout the day will alter their impact . Thus liquid forms taken in large amounts e.g. milk, fruit juice are likely to empty quicker and cause large fluid flux and symptoms than solid forms evenly distributed throughout the day . However this has yet to be confirmed by a placebo controlled trial.