Fructose intolerance is the intolerance of fructose in food. Here you will find information on the causes, symptoms, and nutrition of fructose intolerance. Fructose intolerance is one of the food intolerances. In principle, it is a metabolic disorder. People with fructose intolerance cannot digest or break down fruit sugar (fructose) or only to a very limited extent. According to the cause, doctors differentiate between intestinal fructose intolerance and hereditary fructose intolerance.
Intestinal and hereditary fructose intolerance
Intestinal fructose intolerance is by far the most common form. It arises from a previously unknown cause. A high-sugar diet and continued consumption of sugar substitutes may play a role.
Normally, fructose from food is absorbed into the body through the small intestine. The so-called glucose transporters (GLUT) play a central role in this. In the case of intestinal fructose intolerance, the function of GLUT-5 is restricted or there are too few transport proteins. Another cause is damaged mucous membrane in the small intestine, for example during/after acute gastrointestinal inflammation, Crohn’s disease, celiac disease, or taking medication. As a result, the fructose can only be insufficiently absorbed from the food in the small intestine – it is therefore not available to the body – and reaches the large intestine unused. If this happens without symptoms, doctors speak of fructose malabsorption (FM). However, if symptoms such as diarrhea, nausea, abdominal pain, or flatulence are added, the diagnosis is called intestinal fructose intolerance (IFI). Both clinical pictures are diagnosed using a fructose stress test, the so-called fructose hydrogen breath test.
The very rare hereditary fructose intolerance (HFI) is innate and significantly more dangerous than hereditary fructose intolerance. It must absolutely be excluded before a fructose stress test, otherwise, there is a risk to life for the patient. Because of a genetic change, those affected lack the enzyme aldolase B, which is involved in the breakdown of fructose. As a result, the ingested fructose is not broken down and competes in different ways with the glucose in the blood, the so-called blood sugar. This then results in dangerously low blood sugar levels (hypoglycemia) up to life-threatening shock. Other indicative symptoms of hereditary fructose intolerance are vomiting and bleeding disorders with rapid and frequent bruising.
Even healthy people can only tolerate around 50 grams of fructose per meal. With acquired fructose intolerance, this amount continues to decrease. In this way, more and more undigested fructose reaches the large intestine. There the fructose is an ideal breeding ground for bacteria. The gases and fatty acids produced during bacterial fructose digestion cause the typical symptoms of intestinal fructose intolerance. These are above all abdominal pain, gas, diarrhea, or constipation as well as irregular, often mushy stool.
Treatment By Renouncing Fructose
The therapeutic focus is on the elimination of small intestine weakness or small intestine colonization and the restriction/avoidance of fructose. Often other carbohydrates must also be avoided, for example, oligofructose (especially in fruits or prebiotic products), stachyose and raffinose (for example in legumes, leeks, and onions), lactulose (laxative), or sugar alcohols such as sorbitol, mannitol, maltitol, and palatinitol ( in diet products).
The only way to treat hereditary fructose intolerance is to avoid fructose completely. In the first twelve months of life, you should consistently refrain from eating fruit and vegetables; substitution with vitamins is recommended to compensate for this.