Fructose intolerance
and Martina Feichter, medical editor and biologist Updated onRicarda Schwarz studied medicine in Würzburg, where she also completed her doctorate. After a wide range of tasks in practical medical training (PJ) in Flensburg, Hamburg and New Zealand, she is now working in neuroradiology and radiology at the Tübingen University Hospital.
More about the expertsMartina Feichter studied biology with an elective subject pharmacy in Innsbruck and also immersed herself in the world of medicinal plants. From there it was not far to other medical topics that still captivate her to this day. She trained as a journalist at the Axel Springer Academy in Hamburg and has been working for since 2007 - first as an editor and since 2012 as a freelance writer.
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People with fructose intolerance cannot tolerate fructose. Most of the time, this metabolic disorder is acquired, and those affected can usually eat at least small amounts of fructose. Then doctors speak of a so-called fructose malabsorption. This is not the case with the rare congenital fructose intolerance: the diet here must be strictly fructose-free. By the way, fructose is not only found in fruit, but also in vegetables, beer and sweets, for example. Read more about fructose intolerance here!
ICD codes for this disease: ICD codes are internationally recognized codes for medical diagnoses. They can be found, for example, in doctor's letters or on certificates of incapacity for work. E74
Fructose Intolerance: Description
Fructose intolerance is a form of food intolerance. Affected people tolerate fruit sugar (fructose) only to a limited extent or not at all. There are two forms of metabolic disorder - fructose malabsorption and hereditary fructose intolerance:
Different forms of fructose intolerance
Fructose malabsorption
It is the more common form of fructose intolerance and is also called intestinal fructose intolerance. The term fructose malabsorption indicates where the problem lies for those affected: The fructose cannot be properly absorbed from the small intestine into the blood. Instead, it goes on to the large intestine, where bacteria break it down. This creates gases that can lead to abdominal pain, flatulence and / or diarrhea.
According to the Allergy Information Service, fructose malabsorption is relatively common: in Europe and North America, around one in three adults and two in three small children are affected.
The fructose malabsorption can disappear again in the course of life. Or it can be brought under control with an adapted diet (little fructose depending on individual tolerance).
Hereditary fructose intolerance (HFI)
Hereditary fructose intolerance (HFI) is much less common. In those affected, the fructose enters the blood from the intestines. The liver, however, cannot break it down properly because the enzyme required for this (fructose-1-phosphate aldolase) is not available in sufficient quantities.
This potentially life-threatening defect in fructose metabolism occurs as early as infancy. Even small amounts of fructose can cause serious health complications (e.g. kidney and liver damage) in those affected.
Hereditary fructose intolerance persists for life and requires a special diet for life.
Fructose Intolerance: Symptoms
Most commonly, fructose malabsorption (intestinal fructose intolerance) causes gas and diarrhea to eat fructose. These complaints are uncomfortable and often stressful for those affected in everyday life, but are harmless in themselves.
Hereditary fructose intolerance has more serious consequences: In addition to nausea and vomiting, the consumption of fructose can also cause confusion, dizziness, sweating and even seizures and coma. Liver and kidney damage are also possible.
You can read more about possible symptoms of fructose intolerance in the article Fructose Intolerance - Symptoms.
Fructose Intolerance: Causes and Risk Factors
Even people without fructose intolerance can only tolerate a certain amount of fructose. Typically, the body can absorb around 25 grams of fructose at a time. As a guide: 100 grams of fresh dates contain approximately 31 grams of fructose. An apple weighing 125 grams provides around nine grams of fructose.
If the natural capacity for fructose is exceeded, the excess fructose ends up in the large intestine. There it is split by bacteria, producing gases (including hydrogen, carbon dioxide and methane) and short-chain fatty acids. These can lead to symptoms such as gas, diarrhea or constipation.
If the meal you eat contains little fat or protein besides the fructose, the fructose moves faster through the small intestine. Then there is less time for them to be absorbed, so that more fructose is transported further into the large intestine - and then possibly causing discomfort.
Intestinal fructose intolerance (fructose malabsorption) is only present if someone develops unpleasant symptoms such as flatulence or diarrhea after ingesting just 25 grams of fructose (or even less). People with hereditary fructose intolerance cannot tolerate fructose at all.
Fructose malabsorption - disturbed sugar transport
Fructose is a simple sugar. The body does not have to break it down into its components, but can absorb it directly into the blood through the mucous membrane of the small intestine. Certain membrane proteins - called glucose transporter 5 (GLUT 5) - are responsible for transporting the fructose from the inside of the intestine to the cells of the intestinal wall.
In the case of fructose malabsorption, there is a defect in this transporter. This restricts the absorption of fructose from the intestine. As a result, even a relatively small amount of fructose can no longer be dealt with and continues into the colon.
The transporter disorder can be temporary (e.g. in the case of an acute gastrointestinal inflammation) or permanent or congenital (e.g. in people with Crohn's disease).
What makes the fructose transport easier and more difficult
Fructose is not only found as a single sugar in fruits, but also in ordinary table sugar (sucrose): This is a double sugar, consisting of fructose and glucose (grape sugar).
So when we eat table sugar, it is first broken down in the small intestine into fruit and grape sugar (the sucrose itself cannot pass through the intestinal wall). The intestines can then absorb these individual sugars. Dextrose stimulates the glucose transporter GLUT 5, which also improves fructose absorption. For this reason, even people with intestinal fructose intolerance can usually tolerate household sugar well, even though it contains fructose.
Physical training, on the other hand, is said to worsen the transport capacity of GLUT 5. Sorbitol (sorbitol) - a sugar alcohol that is often added to foods as a sugar substitute or humectant (E420) - also has an adverse effect. For the passage through the intestinal wall it uses the same transporter as the fructose and can therefore compete with it.
Hereditary fructose intolerance - congenital enzyme deficiency
In hereditary fructose intolerance, there is a lack of fructose-1-phosphate aldolase (also called aldolase-B) from birth. This enzyme is involved in the fructose metabolism: the body gradually breaks down the fructose in order to generate energy from it.
The enzyme fructose-1-phosphate aldolase is necessary for one of the degradation steps. If it is not available in sufficient quantities, an intermediate product of fructose breakdown will accumulate (fructose-1-phosphate). This inhibits enzymes that the body needs to burn its most important energy supply - dextrose (glucose) - to generate energy (glycolysis) or to produce new glucose when there is an increased energy requirement (gluconeogenesis).
This can lead to low blood sugar (hypoglycaemia) with corresponding symptoms such as dizziness, cold sweating and tremors. In addition, the resulting metabolic products damage the liver, kidneys and intestinal mucosa.
Fructose Intolerance: Investigations and Diagnosis
The right contact person for suspected fructose intolerance is a specialist in internal medicine who specializes in diseases of the digestive tract (gastroenterologist).
First, the doctor collects your medical history by asking you about your current symptoms and any previous illnesses (anamnesis). Possible questions are, for example:
- Have you been suffering from gas, abdominal pain or bloating a lot recently?
- Do you experience irregular changes in your stool such as diarrhea or constipation?
- Have you discovered a connection with certain foods?
- Do the symptoms get better if you cut out these foods?
- Does a related family member have fructose intolerance?
- Do you suffer from milk sugar intolerance (lactose intolerance)?
This patient interview is followed by a physical examination. The doctor will listen to your stomach with the stethoscope to check for intestinal noises. He also pats and feels the stomach. In this way, he or she can detect increased accumulations of air in the digestive tract.
Since the intestine can only be assessed to a limited extent from the outside, further examinations are usually necessary for a fructose intolerance diagnosis. The fructose intolerance test, in which the hydrogen content in the breath is measured, is a central part of the diagnosis. A blood sample can also be taken to identify hereditary fructose intolerance and rule out other diseases.
Before a fructose intolerance test, the doctor must absolutely rule out hereditary fructose intolerance, as otherwise threatening hypoglycaemia can arise.
Fructose intolerance test
If there is a suspicion of intestinal fructose intolerance (fructose malabsorption), the doctor usually carries out a fructose intolerance test. For a meaningful result, the patient must be sober. At the start of the test, he blows into a device that measures the hydrogen concentration in his exhaled air. Then he drinks a defined fructose solution (usually: 25 grams of fructose in 250 milliliters of water). Thereafter, further breath measurements are made every half hour. Overall, the doctor should take readings for at least two hours for a fructose intolerance test.
What does the test result say? An increased concentration of hydrogen in the exhaled air indicates fructose malabsorption. Because when bacteria in the large intestine break down the fructose because the transport via the small intestine does not work or only works to a limited extent, hydrogen (H2) is produced. This travels into the lungs and from there into the exhaled air.
Some people with intestinal fructose intolerance produce methane in addition to hydrogen. This consumes hydrogen, which is why it can no longer be detected in the air we breathe.In order to be able to diagnose fructose malabsorption in these people as well, the doctor can also measure the methane content of the exhaled air.
Hereditary fructose intolerance test
If symptoms of fructose intolerance appear shortly after birth or if close relatives suffer from hereditary fructose intolerance, the doctor can carry out a genetic test based on a blood analysis: Characteristic genetic changes prove hereditary fructose intolerance.
The genetic enzyme deficiency can be demonstrated by analyzing tissue samples from the liver, kidneys or small intestine.
Fructose Intolerance: Treatment
The only effective starting point in the therapy of fructose intolerance is diet. Those affected have to adhere to a certain diet in order to avoid discomfort.
With hereditary fructose intolerance, foods with fructose are completely taboo. This not only applies to solid food, but also to drinks. For example, with hereditary fructose intolerance, alcohol and lemonades have to be eliminated from the menu, as they often contain fructose.
In the case of acquired fructose intolerance (fructose malabsorption), the diet is not quite as strict. In this case it is even wrong to completely avoid fructose. A fructose-free diet would initially improve the symptoms. However, with prolonged abstinence from fructose, the number of relevant sugar transporters in the intestine is reduced. Then even small amounts of fructose, which were initially unproblematic, can cause symptoms.
Instead, people with fructose malabsorption are recommended multi-step nutritional therapy in collaboration with a nutritionist:
- First of all, the consumption of fructose-containing foods is restricted for a limited period of time (around two weeks).
- As soon as the symptoms have subsided, the food selection is slowly expanded again (for example with small amounts of fruit). The aim of this phase, which can last up to six weeks, is to find out how much fructose the individual can tolerate. You may be able to eat a little more protein and fat-rich foods to increase fructose tolerance.
- Once the individual fructose tolerance has been determined, permanent and individual nutritional recommendations can be worked out and implemented together with the nutritionist. The aim is a complete diet that provides all the necessary nutrients in sufficient quantities and takes into account individual preferences, habits and tolerances as far as possible.
Fructose Intolerance - Table
What do people eat on this diet? You can read about which foods are problematic for fructose intolerance and which are okay in the article Fructose Intolerance Table.
Fructose intolerance: disease course and prognosis
Hereditary fructose intolerance persists for life. So that no symptoms and serious damage (such as liver and kidney damage) occur, those affected must always and permanently adhere to a fructose-free diet. Sorbitol and sucrose (household sugar) are also prohibited.
The fructose malabsorption sometimes occurs only intermittently or disappears on its own. In other cases it lasts for life. As a rule, however, it can be brought under control with an individually adapted diet: a diet with little fructose and sorbitol, depending on how well it is tolerated, can prevent the unpleasant but harmless symptoms of intestinal fructose intolerance.
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