What is fructose intolerance?

Fructose intolerance means that the body can only process fructose to a limited extent or not at all because digestion in the small intestine is disturbed. The fructose reaches the large intestine undigested and bacteria break it down there. The bacterial metabolic processes produce gases such as carbon dioxide and methane, which can cause unpleasant flatulence . In addition, short-chain fatty acids are formed, which promote the influx of water into the interior of the intestine. This makes the stool thinner and causes diarrhea . Fructose intolerance is also called fructose intolerance or fructose intolerance. It is incurable, but with a change in diet, those affected can lead a largely symptom-free life.

What is fructose?

Fructose is a simple sugar (monosaccharide) that occurs naturally in honey and many types of fruit. This is where the name "fruit sugar" comes from. Nowadays, however, it is also found in numerous industrially produced foods such as ready meals, lemonades and sweets.

Fructose is absorbed in small intestine via special transporter proteins, then reaches the liver via the bloodstream and is broken down there. In contrast to glucose (grape sugar), fructose does not provide any quickly available energy and does not cause the insulin level to rise. The human body does not depend on fructose and the metabolism is quickly overwhelmed with a large amount of it.

Even in peoples who are not fructose intolerant, the small intestine can only metabolize a certain amount of fructose. As a rule, 25 to 50 grams can be used without any problems. If more fructose is consumed in food, the excess amount reaches the large intestine undigested, even in a healthy person - this can lead to flatulence and abdominal pain .

Doctors speak of fructose intolerance if a person:

• can utilize unusually small amounts of fructose in the small intestine, namely less than 25 grams.
• cannot metabolize any fructose at all (the technical term for this is "hereditary fructose intolerance").  

Fructose malabsorption and fructose maldigestion - the differences

Many doctors also refer to "fructose intolerance" as "fructose malabsorption". That means "poor intake of fructose". Both terms are used synonymously, even if some people have fructose malabsorption but do not develop symptoms. Strictly speaking, one should not speak of fructose intolerance in this case.

A fructose maldigestion means "poor or insufficient breakdown of fruit sugar". This indigestion occurs when a person does not properly break down the fructose in the stomach or small intestine and therefore cannot digest it well.

What are intestinal fructose intolerance and hereditary fructose intolerance?

Doctors generally differentiate between two forms of fructose intolerance :

• Congenital (hereditary) fructose intolerance : In this case, fructose intolerance is hereditary. From birth, those affected lack the enzyme fructose-1-phosphate aldolase. The fructose then passes through the intestines without any problems, but the liver cannot break it down properly. Without treatment, severe liver and kidney damage can result. Hereditary fructose intolerance is very rare.
• Acquired fructose intolerance: Doctors also call this form intestinal fructose intolerance because it comes from the intestines. It is the most common. The cause is, for example, defective transport proteins (especially GLUT-5), which are responsible for the fructose. As a rule, the term “fructose intolerance” refers to the acquired fructose intolerance. Experts estimate that around every third adult in Germany is affected.

Fructose intolerance: The causes often lie in the intestines

As part of normal digestive process, the fructose from food is broken down in the small intestine and from there enters the blood. Special transport proteins (GLUT), especially GLUT-5, ensure this chemical process. These proteins are located in the wall of the small intestine.

The causes of fructose intolerance can be:

• There are too few transport proteins.
• The transport proteins are only effective to a limited extent - for some people they naturally work less well.
• The chyme passes through the small intestine too quickly.
• Gastrointestinal diseases, such as a gastrointestinal infection or Crohn's disease - a chronic inflammatory bowel disease that attacks the intestinal wall.

Fructose Intolerance: Consequences

Fructose intolerance can not only cause acute digestive problems, but also have long-term consequences. Because the unused fructose accumulates in the intestine, certain bacteria can multiply more intensely. This changes the composition of the intestinal flora. This in turn can impair the absorption of certain vitamins, minerals and trace elements. Often there is a folic acid and / or zinc deficiency, which in turn manifests itself in symptoms.

Fructose Intolerance: Symptoms

The symptoms of fructose intolerance depend on whether it is acquired (more common) or congenital (rare). The intensity of the complaints can also vary greatly from person to person. And not everyone with fructose intolerance develops all symptoms.

Acquired fructose intolerance: symptoms

• Flatulence
• Feeling of fullness , bloating
• Audible belly noises
• Abdominal pain - especially in the lower abdomen - up to abdominal cramps
• nausea
• Mushy (foul-smelling) stool
• Mucus build-up on the stool
• diarrhea
• Constipation - often after diarrhea
• heartburn
• Increased reflux (acid regurgitation)
• Fatigue, especially after eating
• Nutritional deficiencies, such as zinc or folic acid

Symptoms of a zinc and folic acid deficiency can include:

• A zinc deficiency can makes you more susceptible to infection, impair wound healing, and cause hair loss.

• A folic acid deficiency can leads to anemia. Signs of this are tiredness, exhaustion, lack of energy, pale skin and headaches . Those affected often develop another food intolerance, such as lactose intolerance (milk sugar intolerance).

Fructose intolerance can even affect the psyche. Scientists suspect that a tryptophan deficiency could be to blame: the body absorbs this amino acid more poorly. Since tryptophan is necessary for the formation of serotonin, there is a deficiency in this happiness hormone. People with fructose intolerance are more likely to develop depression than people without fructose intolerance .

In general it can be said:

• In the case of fructose intolerance, the symptoms mainly occur when the person concerned consumes large amounts of foods that are high in fructose. Examples are fruit, fruit juices or honey. When the symptoms of fructose intolerance occur and how long they will last cannot be generally predicted.
• The symptoms are more likely to occur when a meal contains fructose and few proteins and fats . Both nutrients usually slow down the passage of food through the intestines. This allows the body to better absorb fructose in the small intestine.
• On the other hand, it has a positive effect on fructose tolerance if you consume fructose together with glucose.

Hereditary fructose intolerance: symptoms

The symptoms become noticeable early on because this form of fructose intolerance is innate. These symptoms can occur:

• Nausea, vomiting
• Low blood sugar (hypoglycaemia): It manifests itself as a feeling of weakness, sweating, dizziness, headache, concentration problems and - if the course is severe - also through seizures and fainting
• Clotting disorders
• Kidney dysfunction
• Liver disease

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