Fructans are made up of many fructose units (a type of sugar) joined together with a glucose unit on the end fructans are made. Humans are unable to break the bonds between the fructose units as we lack the enzyme needed to do this. The regular human process is for fructans to travel through the bowel undigested and unabsorbed and when in the large bowel the bacteria present ferment them. This process creates some gas/wind and is completely normal but individuals with IBS can have an extra-sensitive gut or a gut that acts differently when some wind is present and bloating and pain may be experienced.
Fructans are found in a range of foods including onion, garlic, leek and wheat-based foods as well as some fruits including watermelon, peaches, bananas and dried apricots.