Fructose malabsorption syndrome

The fructose malabsorption syndrome, formerly known as oral fructose intolerance or fructose food intolerance (IAF), is a digestive disorder in which fructose absorption is impaired by the presence of defective fructose transporters in enterocytes of the small intestine. This causes an increase in fructose levels inside the entire intestine.

Fructose malabsorption is quite common, affecting up to 1 in 3 people. The occurrence in patients suffering from symptoms of irritable bowel syndrome is no greater than the occurrence in the normal population. However, due to the similarities between the symptoms of both conditions, patients with fructose malabsorption syndrome often fit the profile of those with irritable bowel syndrome. In some cases, fructose malabsorption syndrome may be caused by various pathologies that can cause damage to the epithelium inside the intestine, such as celiac disease.

Fructose malabsorption syndrome should not be confused with hereditary fructose intolerance, a potentially fatal condition in which there is a defect in a liver enzyme that breaks down fructose.

Symptoms

Fructose malabsorption can cause gastrointestinal symptoms such as pain, abdominal bloating, flatulence or diarrhea.

Pathophysiology

Fructose is absorbed in the small intestine without the aid of digestive enzymes. However, even healthy people cannot properly absorb more than 25–50 g of fructose with each intake. Simultaneous ingestion of fructose and sorbitol appears to increase fructose malabsorption. Fructose that is not adequately absorbed is fermented by the intestinal flora producing hydrogen, carbon dioxide, methane and short-chain fatty acids. This abnormal increase in hydrogen levels can be detected by means of a hydrogen breath test.

The physiological consequences of fructose malabsorption include increased osmotic load, rapid bacterial fermentation, intestinal motility disturbances, mucosal biofilm formation, and an altered intestinal flora profile. These effects are additive with those of other poorly absorbed short-chain carbohydrates such as sorbitol. The clinical significance of these events depends on the response of the intestine to such changes. Some effects of fructose malabsorption is that it decreases blood levels of tryptophan, folic acid, and zinc.

Dietary restriction of free fructose and/or fructans can provide symptomatic relief in a high proportion of patients with functional bowel disorders.

Diagnosis

The diagnostic assay, when used, is similar to that used in the diagnosis of lactose intolerance. It is called the expired hydrogen test and is the method currently used for clinical diagnosis. However, some authors argue that these assays are not an appropriate diagnostic tool because a negative result does not exclude a positive response to dietary fructose restriction.

Treatment

There is no known cure, but proper diet and the enzyme xylose isomerase may help. Simultaneous ingestion of glucose along with fructose improves fructose absorption and may prevent the development of symptoms. For example, patients can tolerate fruits such as grapes or bananas, which contain similar amounts of glucose and fructose, but apples are not tolerated because they contain very high levels of fructose and low levels of glucose.