Absorption of Quercetin and Rutin in Rat Small Intestine

<i>Background/Aims:</i> Dietary antioxidant flavonoids, especially flavonols, are ubiquitous constituents of plant foods with potential health-promoting effects. However, the actual bioavailability of these compounds in vivo, especially in the prevalent glycosidic form, remains a controversial point in making an assessment of their biological importance. Thus, absorption of quercetin and rutin in the small intestine has been determined. <i>Methods:</i> The bioavailability of quercetin and rutin (quercetin-3-rutinoside) was assessed in vivo, with single-meal experiments with rats and by an in vitro method with ligated loops of rat small intestine. The amount of quercetin or rutin in the plasma of rats or in the lumen of each intestinal segment was assayed by maximum absorption in the UV/VIS optical spectrum as was the amount of compound that had crossed the gut wall into the incubation buffer. In addition, uptake of [¹⁴C]quercetin was tested in vitro. <i>Results:</i> Absorption of both quercetin and rutin from the small intestine of rat was evident. However, rutin appeared to be absorbed more slowly than quercetin. Experiments with [¹⁴C]quercetin showed that only 1.5% quercetin crossed the gut wall in vitro and more than half of the total quercetin was bound to the small intestinal tissue. <i>Conclusions:</i> Both quercetin and rutin can attach to and traverse the small intestine of the rat. Binding of flavonoids to the intestinal wall components may however greatly limit their absorption from the small intestine.