What is fructose intolerance?
Letzte Aktualisierung am 24. August 2026 von Dr. Michael Zechmann-Khreis
There are basically two forms of fructose intolerance. However, these two forms have nothing to do with each other and are very different from a medical point of view. Hereditary fructose intolerance is a hereditary disease characterized by a congenital deficiency of the enzyme fructose-1-phosphate aldolase. The fructose is absorbed into the body via the intestine without any problems, but cannot be broken down properly in the liver. This form is extremely rare and leads to liver damage, kidney damage and hypoglycemia even in childhood. This serious illness occurs from the first complementary feeding. A healthy adult can no longer develop this disease. The term hereditary means “hereditary, inherited”.
Intestinal fructose intolerance(fructose malabsorption, fructose intolerance), on the other hand, is an acquired disease that is probably due to a defective and reduced transport system (GLUT-5 or GLUT-7) in the small intestine, i.e. fructose cannot be sufficiently absorbed into the body. However, once it has made it into the body, it can be broken down without any problems. This form affects about 15-30% of the population. The term intestinal means “belonging to the intestine”.
The simplified term“fructose intolerance” is used in public almost exclusively for intestinal fructose intolerance. We therefore also use this term here on the nmi portal. However, hereditary fructose intolerance is never meant (unless explicitly mentioned)!
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Another difference: intolerance or malabsorption?
Maldigestion: means “poor digestion”—the substance involved ( ) is not properly broken down in the stomach or small intestine and therefore cannot be properly digested
Malabsorption: means “poor absorption”—the substance in question cannot be absorbed well, or at all, by the digestive system and into the body
Intolerance: the malabsorbed substance causes symptoms
The difference between these terms lies in the detail and is not normally relevant for those affected. This is why the terms are usually used interchangeably. This means that fructose malabsorption is intestinal fructose intolerance. The same applies to the German term for intolerance. Fructose malabsorption is therefore also a fructose intolerance. The same applies to lactose intolerance, a lactose maldigestion is a lactose intolerance . From a medical point of view, things are different. For an exact diagnosis, the terms must of course be separated. Maldigestion can lead to malabsorption, which in turn can – but does not necessarily have to – lead to intolerance.
This means that there may be patients who have fructose malabsorption, but since they do not describe any noticeable symptoms, they do not suffer from intolerance. In medical practice, however, this will rarely occur, as patients usually come because of symptoms, i.e. they already have an intolerance. Malabsorption should also be treated, as this will also lead to problems in the future, even if the patient is unaware of it. This should certainly be taken into account in the case of multiple intolerances or patients with impaired self-perception!
In the diagnosis, the terms should therefore be broken down correctly wherever possible, but clear definitions used in the non-medical field should be used in communication with the patient. The diagnosis should be broken down even more clearly than stated here.
When do we start talking about fructose intolerance?
It is currently believed that if consuming less than 25 grams of fructose during an H2 breath test leads to symptoms, the person has fructose intolerance. Normally, a person can tolerate up to 50 grams of fructose without any problems. If a healthy person consumes more than 50 grams, or if their absorption mechanisms are affected by medications or, for example, sugar alcohols, they may also experience temporary symptoms (temporary fructose intolerance).
Intestinal fructose intolerance: what happens in the body
Exactly how fructose intolerance works—that is, what exactly goes wrong in the gut—is not yet fully understood. This is partly because we do not yet know exactly how the absorption of sugars works in the first place. It is believed that sugars are absorbed in the gut with the help of transport proteins (GLUT, SGLT). The GLUT-5 transporter is responsible for fructose, but it can also be absorbed via GLUT-2 and GLUT-7 (5). These transporters carry fructose from the intestine into the small intestinal cells; some of these transporters then transport the sugars out of the cell on the other side and into the bloodstream. However, such transporters also respond to other substances. For example, some sugar alcohols, such as sorbitol, block the GLUT-5 transporter, while glucose may stimulate its activity. Glucose, however, has another effect. If there is a lot of it in the intestine, the GLUT-2 transporters migrate to the intestinal side of the cell wall. Since they also absorb fructose, this allows more fructose to be digested. It used to be said: “Every molecule of glucose enables one molecule of fructose to be absorbed.” This statement is probably not physiologically accurate, but in practice, this rule of thumb still works.
This is why people with fructose malabsorption should avoid most sugar alcohols such as sorbitol, mannitol, isomalt, etc. and can eat glucose if they need to consume a lot of fructose (after the elimination period). But be careful: you shouldn’t do this too often and certainly not permanently, otherwise you run the risk of developing insulin resistance.
It is not yet clear why some people develop fructose intolerance.
Sources
(1) Rainer Klinke, Hans-Christian Pape, Stefan Silbernagl (eds.): “Textbook of Physiology,” 5th ed. Thieme, Stuttgart 2005
(2) Ledochowski M, Widner B, Fuchs D. “Fructose Malabsorption.” J Ernährungsmed 2000; 2: 10–14
(3) Born Peter, World J Gastroenterol November 21, 2007; 13(43): 5687-5691 “Carbohydrate malabsorption in patients with nonspecific abdominal complaints”
(4) Manir Ali, Peter Rellos, Timothy M Cox, ItMed Genet 1998;35:353-365, “Hereditary fructose intolerance”
(5) Li Q, Manolescu A, Ritzel M, Yao S, Slugoski M, Young JD, Chen XZ, Cheeseman CI. Cloning and functional characterization of the human GLUT7 isoform SLC2A7 from the small intestine. Am J Physiol Gastrointest Liver Physiol 2004; 287: G236-G242
(6): Pschyrembel’s Clinical Dictionary, 261st edition, 2007; Walter de Gruyter Publishers
(7): H2 Breath Tests, M. Ledochowski, Ledochowski Publishing, 2008
(8): Nutritional Medicine, K. Widhalm (ed.), Verlagshaus der Ärzte, 3rd edition, 2009