In Reply

We thank our correspondent for pointing out the association of fructose absorption and irritable bowel syndrome. Sugar consumption has latterly shown a downward trend in the USA as well as Germany. Precise data on fructose intake in Germany are lacking, but from the data of free sugar consumption it is possible to estimate a fructose consumption of 6–7% (equivalent to 30 g fructose in a total energy requirement of 2000 kcal) (1, 2).

Fructose malabsorption is caused by an overload of the intestinal transport capacity for fructose, which distinguishes it from the rare hereditary fructose intolerance (mutation of the aldolase-B gene) (2). The latter is not accompanied by diarrhea and requires a fructose-free diet (fructose intake is contraindicated). Data by Disse et al suggest that the phylogenetically restricted absorption capacity in fructose malabsorption may be a protective factor against developing overweight (3). Even healthy subjects cannot adequately metabolize larger amounts of fructose (80% positive breath test, 55% abdominal symptoms in 50 g fructose) (4).

The association of fructose (-malabsorption) and irritable bowel syndrome has been investigated in mostly non-blinded or non-randomized controlled trials. Positive effects were observed for a low-fructose diet (for example, the FODMAP diet) but whether these observations are really a consequence of reduced fructose intake or rather a placebo effect or caused by the reduction of fermentable oligo-/di- and monosaccharides or polyols cannot be stated with any certainty on the basis of the available data.

In patients with irritable bowel syndrome and fructose dependent symptoms (chronic abdominal pain, flatulence, osmotic diarrhea), fructose malabsorption should be diagnostically evaluated (abnormal H2 breath test and relevant clinical symptoms).