John Myers
Stomach rumbling, flatulence, diarrhea: an intolerance to fructose can be behind these symptoms. The so-called intestinal (i.e. originating in the intestine) fructose intolerance is quite widespread.
This fructose intolerance is acquired at some point in life – in contrast to the extremely rare congenital form (hereditary fructose intolerance – HFI). An estimated 30 to 40 percent of the Central European population cannot absorb fructose properly through the small intestine, this is called fructose malabsorption. If there is also a functional disorder of the intestine, intolerance reactions occur.
A metabolic disorder is the cause of fructose intolerance
In contrast to a food allergy, an intolerance is not caused by a defense reaction of the immune system, but by a metabolic disorder. In fructose malabsorption, the absorption of fructose from the intestine into the bloodstream is disrupted. The wall of the small intestine contains transport proteins that absorb the nutrients from the chyme and transport them through the wall. The different transporters are responsible for different nutrients, the fructose transporter, for example, is called GLUT-5. If there are not enough of them or if they are defective, fructose remains in the food pulp and moves on to the large intestine, which is colonized by bacteria. They are happy about the sweet meal and use it to produce gases such as hydrogen or methane as well as short-chain fatty acids, which increase the osmotic pressure. This causes symptoms such as flatulence, a feeling of fullness, and diarrhea.
Fructose absorption can only be temporarily impaired, for example as a result of a disturbed intestinal flora after a gastrointestinal infection or the use of antibiotics. But it can also be permanent. In contrast to the rare congenital HFI, those affected with acquired fructose intolerance can still tolerate residual amounts of fructose. Because some of the fructose can “ride-along” on the glucose transporter GLUT-2. With HFI, on the other hand, the body absorbs all of the fructose but cannot metabolize it any further. HFI sufferers must therefore adhere to a strictly low-fructose diet for life.
Nausea, bloating, and cramps can be symptoms
Typical symptoms occur after eating foods containing fructose: nausea, a feeling of fullness, abdominal noises and flatulence, abdominal cramps, and diarrhea.
Fatigue, poor concentration, and psychological changes such as depression are often associated with fructose malabsorption. Because if GLUT-5 does not work, the essential amino acid tryptophan will not be properly absorbed. But it is a precursor for the body’s own mood enhancer serotonin, which gives us a feeling of serenity, inner peace, and contentment.
Fructose intolerance can progress to irritable bowel syndrome. Relatively often, those affected also have low folic acid and zinc levels, which can lead to deficiency symptoms and susceptibility to infections.
Hydrogen breath test for diagnosis
The so-called hydrogen breath test usually measures whether consumed fructose reaches the large intestine – contrary to what is intended. Under medical supervision, those affected take a glass of fructose dissolved in water on an empty stomach. The hydrogen content in the breath is then measured at intervals of two hours over a period of two hours. Based on the resulting typical curve, the doctor can conclude that there is fructose malabsorption. However, since methane gas can be formed from the hydrogen by certain bacteria in the intestine, it makes sense to also check the amount of methane in the breathing air. This means that the diagnosis is almost 100 percent reliable.
Fructose in fruits, but also in many processed foods
Fructose is not only found in fruit, but also in some vegetables and, for example, in honey, household sugar, in invert sugar (syrup), corn syrup, and inulin, as a sugar substitute in various confectionery, pastries, instant soups, and other finished products. In the case of fructose malabsorption, sorbitol is also not tolerated. Sorbitol (E 420) is a so-called sugar alcohol, which is particularly found in pome fruit such as pears, apples, plums, peaches, and apricots. It can also be found in numerous industrially produced foods as a sugar substitute or humectant and serves as a carrier for medicines. It is therefore urgently necessary for those affected to always read the lists of ingredients when shopping.
