Similar to lactose intolerance, there are also disorders of fructose absorption in the small intestine. In this intolerance, known as fructose malabsorption, there is a limited transport capacity of the transport protein in the mucous membrane of the small intestine, as a result of which the fructose is insufficiently absorbed in the small intestine.

The fructose enters the large intestine and is decomposed by bacteria. The resulting metabolic products can lead to symptoms such as flatulence, cramping abdominal pain and diarrhoea.
When the fructose is broken down by bacteria in the large intestine, hydrogen is produced, among other things, which reaches the lungs via the bloodstream and can be detected in the exhaled air.

In the H₂ breath test, a defined amount of fructose is dissolved in water, the solution is drunk and the amount of hydrogen in the exhaled air is measured. Before the test, a so-called “zero value” is determined so that the results can be interpreted.
In the case of fructose malabsorption, a reduction in the amount of fructose and sorbitol in the diet is necessary to relieve the symptoms. However, with a completely fructose-free diet, there is a further decrease in fructose transport protein. Increasing the amount of fat and protein in the diet leads to an increased expression of the fructose transport protein and thus to an improved absorption of fructose in the small intestine.

Basically, in the case of fructose intolerance, it must be considered that many diabetic products, food supplements and calorie-reduced foods contain sorbitol. Since fructose is formed during the breakdown of sorbitol, the intake of such products increases the symptoms.