The aim of the present study was to investigate the bioavailability of quercetin from onion bulb (OB) and onion skin (OS) extracts in ruminants. Three non-lactating cows equipped with a permanent rumen fistula intraruminally received equimolar amounts of quercetin as either aglycone, rutin, or OB or OS extract, respectively, at a dose of 50 mg of quercetin equivalents/kg of body weight. Blood samples were drawn before and frequently within the 24 h period after application of the respective substance. Quercetin and quercetin metabolites with an intact flavonol structure (kaempferol, isorhamnetin, and tamarixetin) were analyzed in plasma samples by high-performance liquid chromatography with fluorescence detection. All quercetin sources administered resulted in a fast increase of the plasma concentrations of quercetin and total flavonols (sum of quercetin and its metabolites), followed by a rapid decline, whereby significant higher concentrations occurred with OB extract and rutin compared to quercetin aglycone and OS extract, respectively. The results clearly demonstrate a higher systemic availability of quercetin from OB extract and rutin. Taken together, OB extract with a high content of quercetin glucosides is an interesting source for the application of quercetin to ruminants.
Keywords: bioavailability; cow; onion extracts; quercetin.