Unlike glucose and galactose, fructose is absorbed passively. Fructose absorption by the apical transporter GLUT5 requires a concentration gradient, which is maintained both by the active absorption of nutrients and water and by fructose transport out of the enterocytes by the basolateral transporter GLUT2. After excretion into the portal blood stream, fructose is rapidly metabolized in the liver. Whether a given amount of fructose is absorbed adequately is a matter of local conditions. Fructose is better absorbed as a part of a meal or in a mixture with actively absorbed molecules such as glucose and amino acids.
The breath hydrogen test (BHT) has given us a tremendous amount of insight into the fate of ingested carbohydrates, including fructose. Many, if not most, healthy children are not able to completely absorb a test dose of fructose. Consequently, fruit juices containing fructose in excess of glucose, such as apple juice, might especially lead to symptoms of carbohydrate intolerance and toddlers’ diarrhea. Obviously, this “condition” is essentially different from the inborn error of metabolism, hereditary fructose intolerance.
The case report by Wenzel et al. addresses 2 important points. First, the authors rightly point out that the fructose BHT is not innocuous under all circumstances. It should be preceded by careful evaluation of the patient’s history with reference to the possibility of hereditary fructose intolerance. Second, the clinical usefulness of the fructose BHT is limited. Because healthy children also “malabsorb” fructose, the diagnostic sensitivity of the test is low. When the child’s history suggests a role for fructose, the clinician, instead of performing a BHT, should provide the parents with dietary advice aimed at normalizing the child’s feeding pattern and excluding fruit juices from the diet. It is seldom, if ever, necessary to eliminate other sources of fructose. The effect of the diet on the symptoms is sufficient proof.
Author Contributions: All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article.
Authors’ Disclosures of Potential Conflicts of Interest: No authors declared any potential conflicts of interest.
Role of Sponsor: The funding organizations played no role in the design of study, choice of enrolled patients, review and interpretation of data, or preparation or approval of manuscript.
- © 2009 The American Association for Clinical Chemistry