Fructose intolerance (fruit sugar intolerance)

Fructose intolerance (fruit sugar intolerance)
International Classification (ICD) E74.-



Intestinal fructose intolerance is caused by a disorder of the transporter GLUT-5. This is located in the small intestine and is responsible for the absorption of fructose and glucose into the bloodstream. In intolerance, fructose is not fully absorbed.

The unabsorbed sugar enters the large intestine where it is broken down by the bacteria located there. During the degradation process, degradation products such as hydrogen, carbon dioxide and short-chain fatty acids are formed. These cause symptoms such as flatulence, diarrhoea, abdominal pain, nausea or a feeling of fullness.

Fructose is not only found in fruit or vegetables, but also in honey and normal household sugar (sucrose). Only in rare cases does fruit have to be completely avoided. Most sufferers usually tolerate small amounts well.

Foods that also contain dextrose (glucose) are better tolerated because the glucose also takes fructose with it when it is absorbed in the intestine. If the ratio between glucose and fructose in a food is correct, it is almost always well tolerated.

In contrast, sorbitol (a sugar substitute) should be avoided. It is also absorbed via GLUT-5 and therefore competes with fructose.

The amount of fructose that can be eaten until symptoms develop is called the tolerance threshold. It is subject to strong fluctuations from person to person. In a healthy person 30g or more, in an intolerant person 1g-5g. In addition, the symptoms are often not immediately attributed to a fructose intolerance, but rather to irritable bowel syndrome.

The exact number of people affected is not known because of the high number of unreported cases. The tendency is estimated to be increasing because fructose is being artificially added to more and more foods.

The intolerance can be acquired or congenital.

Intestinal (acquired) intolerance can occur in childhood, but normalise again later. If it does not occur until adulthood, it is usually a lifelong companion.

Hereditary (congenital) fructose intolerance is already noticeable in infants. This disorder is considered life-threatening, since even small amounts of fructose can cause severe complications.


The symptoms are caused by a disorder of the GLUT-5 transporter. It is responsible for the absorption of fructose and glucose from the intestine into the bloodstream. If the sugar supply exceeds the capacity of the transporters, fructose reaches the large intestine, where it is decomposed by bacteria located there and gases are produced as a decomposition product. This results in symptoms such as flatulence, diarrhoea or abdominal pain.

Exactly why GLUT-5 has a lower capacity in some people has not yet been clarified. The only clue so far is the familial accumulation, which is why genetic factors are being considered.

A fructose intolerance can be congenital, lose itself in childhood, or develop only in adults.


  • Flatulence
  • Diarrhea
  • Abdominal pain
  • Nausea
  • Bloating
  • Dizziness

The disturbance of the intestinal function often causes a deficiency of folic acid or zinc. Affected individuals are much more susceptible to infections as a result.

Intensity of symptoms is caused by several factors:

  • Absorption capacity for fructose
  • Amount of fructose consumed
  • Intestinal flora (amount and type of bacteria in the intestine)
  • Glucose content of food (improves fructose absorption)
  • Sorbitol content (Reduces fructose absorption)


Fructose intolerance is diagnosed on the basis of the hydrogen content in the exhaled air. Hydrogen is found in the breath as soon as intestinal bacteria break down fructose.

Detection is similar to an alcohol test, by exhaling forcefully into a device. This test has to be done fasting, several times in 20-minute intervals, after the patient has drunk 50g of fructose.

This procedure is not suitable for all people. Therefore, the methane content of the breath is also measured, another indication of fructose breakdown in the large intestine.


To alleviate the symptoms, fructose must be avoided. Especially at the beginning of the therapy, it is important to consume as little fructose as possible. This is usually a challenge for those affected, as fructose is not only found in fruit and juices, but also in vegetables, household sugar (sucrose), many sweets, soft drinks, ready-made sauces and honey.

Sorbitol and other sugar substitutes should also be avoided because they inhibit fructose absorption in the intestines. They can be found in sugar-free sweets and fruits such as plums or apricots.

The following foods can be consumed without concern:

  • Meat
  • Pasta
  • Rice
  • Potatoes (watered)
  • Cereal products (most)
  • Dairy products (most)
  • Vegetables such as spinach, peas, green asparagus, sauerkraut, cabbage, chicory, corn, lettuce, mushrooms.

An exact diet plan should be worked out with a specialized doctor or nutritionist and followed for at least 2-4 weeks.

If the therapy is successful, the tolerance threshold can be tested. Other types of vegetables and fruit can also be tried.

The tolerance of fructose increases in many patients if at least the same amount of glucose is consumed at the same time. This effect can be used with foods that already contain both sugars in appropriate proportions, or by sprinkling some glucose over foods containing fructose.

If the symptoms do not subside, other intolerances should be considered, e.g. lactose intolerance.


If the change or adjustment of the diet is successful, a symptom-free life is very likely.

The less pronounced the intolerance, the better the chances.

If children are affected, the symptoms may improve on their own after some time. However, if the intolerance only occurs in adulthood, it usually remains a lifelong companion.

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Danilo Glisic

Danilo Glisic

As a biology and mathematics student, he is passionate about writing magazine articles on current medical topics. Due to his affinity for facts, figures and data, his focus is on describing relevant clinical trial results.

The content of this page is an automated and high-quality translation from DeepL. You can find the original content in German here.



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