Absorption and metabolism of quercetin and isoquercitrin (quercetin 3-O-glucose) were investigated in rats after in situ perfusion of jejunum plus ileum (15 nmol/min) for 30 min and compared with those of phloretin and phloridzin (phloretin 2'-O-glucose). After perfusion of the glucosides, the corresponding aglycone forms and conjugated derivatives appeared in the lumen. The conjugated metabolites were similar to those recovered after intestinal perfusion of the aglycone forms. Regardless of the aglycone or glucoside perfused, only conjugated forms were present in the mesenteric vein blood draining the perfused segment showing the importance of intestinal conjugation. The hydrolysis of glucosides was a prerequisite step before their conjugation by intestinal enzymes and their transport towards the mucosal and serosal sides. In contrast to phloridzin, lactase phloridzin hydrolase activity did not seem to be an essential pathway for isoquercitrin hydrolysis. The 3-O-glucosylation of quercetin improved the net absorption of the aglycone (P < 0.05), whereas phloretin absorption decreased when present as 2'-O-glucoside (P < 0.05). Whatever the perfused compound, the efficiency of the absorption seemed to be linked to the intestinal conjugation process and to the luminal secretion of metabolites.
