Fructan intolerance: what is it, symptoms and how to eat safely

A fructan is a molecule consisting of a chain of fructose molecules joined together, and with a glucose molecule at the end. Fructans are sugars that are included in the FODMAP group of carbohydrates that can cause digestive disorders.  

Limiting intake of fructans may help in some cases with conditions such as irritable bowel syndrome (IBS), where the build-up of fermentation in the large intestine is a key cause of symptoms such as abdominal bloating and distension, bellyache, flatulence and nausea, as well as diarrhoea, constipation or both.   

Present in food such as wheat, onions, garlic, asparagus, beans, watermelon and many other foods, fructans are difficult to avoid completely and the foods fructans are found in form part of a healthy and balanced diet.  

The human digestive system lacks capacity to fully break apart the molecules that make up fructans to release the fructose and glucose. Studies show that people absorb between 5% and 15% of fructans in the small intestine, allowing the majority to pass through into the large intestine, or colon. Here, naturally occurring bacteria ferment the fructans. This process can cause different levels of discomfort in different people, including painful and unpleasant symptoms such as bloating and diarrhoea. 

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, and others experience stronger symptoms than others as a result of the fermenting of the unabsorbed sugars.  

If you have fructan intolerance, you may experience symptoms including flatulence, abdominal bloating, belching, constipation or diarrhoea. 

If you experience such symptoms on a regular basis, it is a good idea to discuss them with a doctor. Diagnosis of fructan intolerance is still in its infancy, but, as with other intolerances of sugars such as fructose intolerance, breath tests can sometimes be used to measure the content of gases that are produced when fructans are fermented in the gut, but the results are no guarantee of a correct diagnosis. 

The symptoms and the causes of fructan intolerance bear many similarities with irritable bowel syndrome (IBS). IBS is a disorder that affects the large intestine, and may be triggered by certain foods, stress, hormonal changes or other illnesses affecting the gastrointestinal tract. Patients who have food intolerance-related IBS may experience difficulty tolerating fructans as well as other fermentable short-chain carbohydrates. Some people with IBS can eat various foods containing fructans, but it is interesting to bear in mind that the two conditions may be related in some people.  

In trying to determine what foods are triggering the symptoms in cases of suspected IBS and fructan intolerance, a doctor or dietitian is likely to suggest an analysis of how your body is responding to FODMAPs, foods high in the sugars that lead to higher levels of fermentation in the intestine. The best way is to observe the impact of eliminating them and what happens when they are returned to the diet to find the best balance for you. 

Gluten is a protein found in certain cereals, especially wheat, barley and rye. Digestive disorders linked to gluten include coeliac disease, an autoimmune disorder triggered by the presence of gluten in the gut, and gluten intolerance, often called non-coeliac gluten sensitivity. Given that wheat, spelt, rye and barley are high-fructan foods as well as containing gluten, it can be difficult to know whether the symptoms are caused by gluten or another component.  

Coeliac disease can be tested for and diagnosed with a high degree of certainty. But non-coeliac gluten sensitivity is difficult to diagnose, so anyone who thinks they have this gluten intolerance should bear in mind there is no way to know if the symptoms that emerge after eating gluten-containing products are caused by the gluten or another component present in wheat, barley and rye, such as fructans. 

If you or your doctor think you may have fructan intolerance, you will need a fructans food list to guide your dietary choices, seeking a low-fructan diet or to identify what is triggering your symptoms. High-fructan food include: 

Cereals: wheat, spelt, rye and barley 

Fruit:

watermelon, nectarine, grapefruit, persimmon, pomegranate, plums, ripe bananas, dates, prunes and raisins 

Vegetables: 

onions, shallots, garlic, leeks, artichoke, asparagus, beets, Brussels sprouts, savoy cabbage, fennel and snow peas 

Pulses: 

Kidney beans, black beans, mung means, navy beans, lima beans and split peas 

Nuts: 

cashews and pistachios 

Others:

Chicory root and some soy products 

How to avoid fructans? Although some of the high-fructan foods are very common, there are ways to substitute them or seek lower-fructan alternatives. For example, the cereals that contain fructans can be replaced by gluten free cereals, such as buckwheat, oats, quinoa, tapioca and brown rice. Onions and most obvious alternatives contain fructans, but Spanish onions are lower-fructan than the rest.  

If you suspect you may have a fructan intolerance, you should consult with a gastroenterologist and/or dietitian. A medical professional may design an exclusion diet in which you avoid food containing fructans for a limited period to see whether the symptoms go away.  

After the initial elimination of fructans, you should gradually add fructan-containing foods back into your diet one at a time to determine your individual intolerance. You should keep a diet and symptoms diary so that you can analyse the results obtained from eating different foods. The low-FODMAP diet approach developed by Melbourne’s Monash University works in the same way. The idea is not to eliminate all FODMAPs from the diet forever, but ascertain the impact of each one and find a comfortable intake level as part of a balanced diet. 

Before undertaking major changes to your diet, it is important to seek advice from your doctor and/or a dietitian to ensure that you are always eating a diet that is right for your nutritional needs.