While they are both FODMAPs, fructose and fructans are quite different.
Fructose is a monosaccharide (mono meaning one, saccharide meaning sugar (carbohydrate) molecule) and usually, our bodies are quite good at absorbing it. There are two ways it does this: 1 – fructose is absorbed on its own and, 2 – it is absorbed alongside glucose.
Some of us are unable to absorb it on its own and, failing the first way, we need glucose to be present in an equivalent amount – think of it as fructose piggy-backing glucose. Without the ability to absorb fructose on its own or sufficient glucose, the fructose travels through our small intestine to our large intestine and is fermented which can be what causes IBS symptoms.
This is why some fruits are not included in the ‘foods to avoid’ on a low FODMAP diet. To be high FODMAP, foods must contain an excess of fructose when compared to the level of glucose (not just simply contain fructose). Strawberries, for example, contain less fructose than glucose and are therefore low FODMAP.
Foods that do have excess fructose include honey, apples, pears and watermelon.
Fructans are actually oligosaccharides (oligo meaning many, and again saccharides meaning sugars) – they are long chains of fructose molecules with one glucose molecule on the end (similar to GOS (galacto-oligosaccharides) which are long chains of galactose with one glucose molecule on the end).
We don’t have an enzyme to break down this chain, meaning it goes undigested, reaching our large intestine and again, is fermented here. In a normally functioning gut, this causes a small amount of tolerable gas. It happens to everyone – remember; small amounts of gas are VERY normal! For those who do have IBS however, this may be what causes symptoms.
Foods that are high in fructans include onion, garlic, wheat and some dried fruits.
So to answer the original question; while both are FODMAPs, it’s important to know they are different and are found in different foods.