Determination of quercetin concentrations in fish tissues after feeding quercetin-containing diets

Concentrations of quercetin in fish tissues were measured for the first time using HPLC-electrochemical detection method. Its identity was also ascertained with UV-photodiode array detection. Quercetin, in aglycone form, was at measurable concentrations in tilapia plasma, liver, and whole body homogenate when fed with diets containing 1% quercetin (aglycone) for 1 or 15 weeks. Hydrolysis with glucuronidase/sulfatase treatment for the purpose of cleaving conjugates did not increase quercetin levels, suggesting that glucoronide or sulfate conjugates are not the major metabolic forms in Nile tilapia (Oreochromis niloticus). No quercetin was detected in plasma of rainbow trout (Salmo gairdneri) or white sturgeon (Acipenser transmontanus) fed commercial diets. The results suggest that quercetin is absorbed in tilapia and that this flavonoid is deposited mainly in aglycone form in the body after absorption.