Fructans are a group of food fibers. As you might guess from the name, fructans are chains of ring-shaped fructose sugars. Humans don't have what it takes to break these chains apart, so the fructose in the chains is not absorbed in the small intestine where other sugars are absorbed. When these dietary fibers reach the large intestine, they are fermented by the bacteria that live there, producing gas. This is no problem for many people, and sometimes fructans are even deliberately added to foods or probiotic supplements for the specific purpose of feeding the "good" bacteria. But for some people with IBS, the gas causes painful bloating.
By far the largest source of fructans in the Western diet is wheat.It accounts for 70% of our fructans intake. The second biggest source is onions, followed by garlic and unripe bananas. You can recognize added fructans in your food or supplements by the names "inulin" or "FOS".
It is important to note that even though wheat happens to be the primary source of fructans in our diets, intolerance of this kind has nothing to do with the wheat protein, gluten, which those with celiac disease must avoid. For a more complete discussion of fructans intolerance versus gluten intolerance, please see my recent guest post on the Kitchen Therapy blog.