Use of kinetic methods to evaluate D-xylose malabsorption in patients

D-Xylose kinetics were studied after administering 25 gm oral and 10 gm intravenous doses to six normal subjects and to 12 patients who were being evaluated for the presence of intestinal malabsorption. D-Xylose absorption was characterized by an absorption rate constant (k(a)) and a rate constant reflecting nonabsorptive removal of D-xylose from the small bowel (k(o)). In normal subjects, mean k(a) was 0.915 ± 0.228/hr (± SD), and the extent of oral D-xylose absorption averaged 81.0% ± 11.6%. In seven of the 12 patients, D-xylose absorption was less than 60% complete. In four of them, k(a) was below the normal limit of 0.367/hr and was consistent with a primary defect in intestinal D-xylose absorption. Two patients with low k(a) values and the remaining three patients with less than 60% D-xylose absorption had k(o) values exceeding 0.650/hr, suggesting that excessive nonabsorptive loss contributed to low D-xylose bioavailability. We found that standard tests may identify some patients as having primary defects in D-xylose absorption rate or nonabsorptive loss and propose that breath H₂ concentration measurements may also help distinguish between bacterial overgrowth and rapid intestinal transit as causes of excessive nonabsorptive loss.