Introduction
Fructose is one of the simple sugars and occurs naturally in fruit and honey. After absorption through the intestines and splitting in the liver, fructose serves to provide energy in the human body. Depending on requirements, the energy gained is either converted directly or stored as an energy depot in the fat metabolism during the production of so-called triglycerides (fats). The symptoms of fructose intolerance range from increased urination to intestinal complaints such as diarrhoea, to symptoms of poisoning in the hereditary form (hereditary form). The therapy of the various fructose intolerances depends on the cause and can consist in the reduced consumption of fructose or the total renunciation of it.
Causes
There are three different causes of fructose intolerance. A malabsorption disorder describes the inability or insufficient absorption of fructose via the intestines. One also speaks of intestinal or alimentary fructose malabsorption.
It results from a disturbed function of the transport mechanisms of the intestinal wall. Special importance is attributed to the so-called GLUT-5 transporter. The transport proteins involved are impaired from birth or experience a loss of function in the course of life.
This can be triggered by antibiotic therapy, certain drugs with gastrointestinal side effects and fungal infections. Fructosemia refers to an increased fructose level in the blood. This is due to the failure of a certain enzyme in the liver that breaks down fructose.
The body tries to excrete the fructose via the kidneys. Hereditary fructose intolerance is based on a genetic enzyme defect. It is a hereditary metabolic disorder.
The enzyme aldolase B normally breaks down fructose-1-phosphate into smaller fragments. Due to the lack of the enzyme, fructose-1-phosphate accumulates mainly in the kidney and liver. The first symptoms already appear in infancy.
Symptoms
The symptoms of intestinal fructose intolerance or malabsorption disorder are due to the remaining fructose in the intestine. In most cases the symptoms occur with a latency to consumption. There can be 24 to 48 hours between intake and symptoms.
Under normal circumstances, fructose only reaches the small intestine, where it is absorbed into the bloodstream. If, on the other hand, it remains in the intestine, the fructose travels as far as the large intestine. There, bacteria process the sugar into hydrogen, carbon dioxide and short-chain fatty acids.
Carbon dioxide causes flatulence and painful stomach cramps. The fatty acids formed, on the other hand, affect the local fluid balance. They cause the increased influx of water into the colon.
This serves to balance the osmotic gradient in the intestine, but at the same time leads to liquid, strong-smelling stool. In some cases, constipation symptoms are observed instead of diarrhea. Nausea and pain when pressure is applied to the abdomen are also common symptoms.
More rarely, vomiting, headaches, increased tiredness and depressive moods are also more common. Heartburn with a burning sensation in the area of the sternum is also observed. If an intestinal fructose intolerance remains untreated for a longer period of time, this has an effect on the colon bacteria.
Hydrogen-producing bacteria multiply and settle not only in the large intestine but also in the lower sections of the small intestine. As a result, even foods without fructose are less well tolerated and cause complaints. Hereditary fructose intolerance leads to symptoms already in infancy.
As soon as fructose is added to the diet, for example in the form of sucrose contained in milk, the metabolic disorder becomes apparent. If fructose-1-phosphate accumulates in the body, symptoms of poisoning occur. In addition to hypoglycemia, low blood sugar levels and acidosis, sweating and altered consciousness are observed. Nausea, vomiting and diarrhoea also occur. In the course of the disease, an enlargement of the liver with subsequent connective tissue remodelling (cirrhosis) can develop, leading to liver failure, a blood clotting disorder and a functional disorder of the kidneys.
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