D-xylose is a monosaccharide, or simple sugar, that does not require enzymes for digestion prior to absorption. Its absorption requires an intact mucosa only. D-xylose absorption test is a medical test do diagnose conditions that present with malabsorption of the proximal small intestine due to defects in the integrity of the gastrointestinal mucosa. A decreased urinary excretion of D-xylose, as seen in SIBO is treated with antibiotics. Celiac disease could also be a reason for non-infectious malabsorption of D-xylose.