A fructan is a molecule consisting of a chain of fructose molecules joined together, and with a glucose molecule at the end. Fructans are naturally occurring carbohydrates in the form of linear or branched fructose polymers. The shorter fructose polymers are made up of 2-9 units length and are called oligofructose. The longer fructose chains are made up of more than 10 units and are called inulin.

Because humans don’t have the fructan digestive enzyme, the human digestive system lacks the capacity to fully break drown fructans into fructose and glucose. Studies show that people can only absorb between about 90% of fructans in the small intestine, allowing the majority of fructan intake, about 90%, to pass through into the large intestine (colon). Many beneficial bacteria in the colon take up and ferment fructans producing gases and byproducts. These extra gases can cause different levels of discomfort in different people, including painful or unpleasant symptoms such as bloating, gases, distension, constipation, or diarrhea.

The causes of fructan intolerance are not well understood. Some people seem to have less capacity than others to digest and absorb fructans into their bloodstream. Some people experience stronger symptoms than others as a result of the fermentation process of the unabsorbed sugars in the colon.  

Research investigated patients with symptoms of food intolerance triggered by bread. Bread is well known to have a protein known as gluten which is contained in grain such as wheat, barley, and rye. Therefore, these patients, which had not gluten allergy, had been labelled before as non-celiac gluten sensitivity (NCGS). On a gluten-free diet, most of fructans are also eliminated since bread is the major source of fructans. Actually, most patients had fructan intolerance and benefited from reducing fructans by eliminating bread.

There are no standardised tests to know if you have fructan intolerance. The way to test for this is by following a low FODMAP diet as an initial elimination phase. Later, a challenge phase is initiated by introducing fructan rich food. In the elimination phase, symptoms from fructan intolerance should be eliminated, if you are affected from fructan intolerance. In the challenge phase, symptoms induced by fructans should reoccur.