Fructan (fructo-oligosaccharide) intolerance

FOS: Fructans, Inulins, Levans

Fructan (fructo-oligosaccharide) intolerance


Fructo-oligosaccharides (FOS) are chains of fructose molecules that are generally not broken down by digestion in humans, but can be metabolized by some gut bacteria. There is emerging evidence, that some individuals may have the capacity to digest FOS. Depending on the length and composition of the FOS chains they are described as fructans (shorter chains), inulins (longer chains) or levans. They are natural components of food, including cereals, onion, banana, garlic, leek and chicory and daily consumption varies between 1 and 20g. FOS are prebiotics and are especially consummed in vegetarian and vegan diets. Generally, the 'healthier' the food, the more FOS we eat.

See Food tables Fructan and fructooligosaccharide: Fructans, inulins, levans

FOS are widely and increasingly added to manufactured food.

FOS are non-digestible by the human upper intestine, fermented by the gut flora and possibly some individuals in the colon and classified as dietary fiber and prebiotic (promoting the growth of beneficial intestinal microorganisms).

Animal and emerging human research have demonstrated the following FOS effects: modification of composition and activity of the gut flora, improved stool characteristics, reduction of appetite, enhanced absorption of calcium and other minerals, regulation of gastrointestinal endocrine peptides, increased immunity and resistance to infections, improved fat metabolism and the likelihood of reducing the risk of intestinal infections, functional bowel diseases, colon cancer, osteoporosis and obesity. FOS are thus considered beneficial functional food ingredients.


The fermentation of unabsorbed FOS may produce large amounts of gas, depending on the speed of passage through the intestine of the FOS and the composition of the gut flora. The quantity and type of gas and substances (including short-chain fatty acids) produced and the sensitivity to the resultant distension and the substances varies widely.

As the gut flora can change after intestinal infections or antibiotic use, the new appearance of intolerance symptoms is observed in adults.

Individuals with FOS intolerance may also be intolerant to fructose. Fructose and FOS intolerances are not synonymous and some individuals may only be sensitive to FOS or fructose, and in other cases the symptoms may be additive.

Frequency in population and natural history

The frequency of significant symptoms after FOS ingestion is unknown, but in our experience is common. As humans cannot digest FOS, everyone will develop symptoms with high enough doses.

Consequently, individuals with FOS intolerance are those sensitive to even low amounts of FOS.


Bloating, fullness, flatulence, abdominal pain or cramps, increased intestinal sounds, nausea and changes in stool consistency (diarrhea, constipation or both) are most common.

The symptoms resemble those of functional disorders or Irritable Bowel Syndrome (IBS).

Symptoms may appear several hours after meals and last for over 24 hours. This slow onset and protracted symptom profile is helpful to distinguish symptoms related to fructose, or lactose, where the onset is generally more rapid and the duration of sympoms is shorter.

Testing and diagnosis

A careful history should be taken in every case, A fructan breath test is available for diagnosis.

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Self-diagnosis is often difficult due to the presence of FOS in a range of foods, including vegetables and cereals, and the slow onset of symptoms.

The overlap in foods containing fructans and gluten is extensive and must be remembered when a diagnosis of celiac’s disease or non-celiac’s gluten sensitivity is considered (both less common than FOS intolerance).

The breath tests should not be used in infants and reduced doses of test compound are used in children. The tests procedures are identical for all the intolerances.

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  • The following are important details regarding the test procedure.
  • Dietary restrictions, no smoking or excessive exercise from the day before testing and no antibiotics or colonoscopy in the last week pre-test.
  • Ingestion of a specific amount of the sugar or sugar alcohol to be tested for intolerance.
  • Regular breath samples taken for a standardized period.
  • Diary of symptoms during the next day for calculation of the symptom index.
  • Measurement of specific gases, e.g. hydrogen and methane, in the breath samples.

An intolerance is diagnosed based on the symptoms following the test and the gas concentrations in the breath samples. The diagnosis is confirmed by a significant decrease in symptoms whilst on a diet low in the poorly tolerated sugar or sugar alcohol. Expert dietary advice is very helpful for the identification of food and drink containing the difficult-to-spot offending ingredient. Examples of such ingredients are so-called hidden lactose or fructans in vegetables or whole-meal products.

See Food tables

No genetic tests are available.


Reduction of the intake of fructans and other fructo-oligosaccharides to individually tolerated levels will lead to symptom relief in most individuals within a few days.

The identification of fructan content is not intuitive and requires specialist advice to ensure adequate vitamin and vegetable intake. Generally, many foods identified as ‘healthy’ contain fructans and replacement with other equally healthy alternatives or the addition of supplements constitute the dietician’s challenge. Often fructose intolerance co-exists, necessitating a simultaneous reduction in fructose. In some patients the overlap of intolerances to several classes of poorly absorbed carbohydrates (FODMAP’s) necessitates a general reduction of these carbohydrates.

The use of probiotics without prebiotics, such as inulins, can be very useful.

See Food tables for a listing of fructan food content

FOS are not sweet, and are natural components of food, including cereals and vegetables.

Information nuggets

Fructans stimulate the growth of beneficial bacteria (e.g. Bifidobacteria, Lactobacilli) and production of beneficial substances in the gut and act as antioxidants. They are also part of the plant’s immune system.