Fructose is normally resorbed well, in other words it is readily absorbed into the bloodstream through the intestinal wall. Special transporters in the intestinal mucosa make this possible. When fructose malabsorption (intestinal fructose intolerance) occurs, transport of the fructose in the intestines is impaired.
Less than 25 g of fructose per hour can be absorbed by the body. This means that the glucose is not or is only incompletely resorbed.
As a result, it reaches deeper parts of the intestines where bacteria break it down into short-chain fatty acids, hydrogen and carbon dioxide. This causes pain, the level of which can vary depending on the amount absorbed.
The consequences of fructose breakdown in the large intestine can be symptoms such as bloating, cramps, diarrhoea, vomiting, nausea and digestive noises.
In the long term it is also possible that there will be changes to the intestinal flora, which among other things could have a negative effect on the production of folic acid there.
Other long-term effects can be fatigue, depression and irritability because the absorption of certain nutrients is limited.
If a zinc deficiency occurs as a result of fructose intolerance, this can result in an increased susceptibility to infections and poor wound healing.
The type and severity of symptoms that occur differ from case to case depending on the amount of fructose consumed and the extent of the intolerance. This means that it is difficult to make a reliable diagnosis based on the fructose intolerance symptoms.
A distinction must be made between intestinal fructose intolerance and hereditary fructose intolerance. This is a very rare congenital form of fructose intolerance caused by a missing enzyme. Patients must completely forego fructose in their diet for life. In contrast, many people who suffer from intestinal fructose intolerance can tolerate small amounts of fructose – depending on the extent of the intolerance. By consuming glucose at the same time, it is also possible to aid fructose absorption because glucose boosts the activity of the transporter protein. As a result, ordinary sugar (saccharose) is tolerated better by many than fructose, because saccharose molecules are comprised of one molecule of glucose and one molecule of fructose. Also fruit types that contain relatively more glucose than fructose, such as apricots, bananas or mandarins, cause relatively few problems in most cases.
Fructose malabsorption (intestinal fructose intolerance) is diagnosed by a H2 breath test. This requires a fructose solution to be drunk. The patient then exhales into a breath test device at regular intervals over a period of 3 hours.
If they are fructose intolerant, the hydrogen content in the breath rises. This is formed by the bacteria in the large intestine while the fructose is being digested, whereupon it then enters the bloodstream and is exhaled via the lungs. Measurement of the hydrogen content in the exhaled air then shows whether fructose has been resorbed in the intestines or whether there is a transport impairment. An accompanying diary of dietary problems should be kept to further improve the accuracy of the diagnosis.
If there is fructose malabsorption (intestinal fructose intolerance), the possibility of sorbitol intolerance (sorbitol = sugar substitute / sugar alcohol) should be looked into because both use the same transporter in the intestine.
In some cases, glucose intolerance occurs together with a lactose intolerance. The specific recommendations must be followed here.