Oral bioavailability of quercetin from different quercetin glycosides in dogs

Abstract

Although the flavonol quercetin is used as a supplement in commercial dog food, data on quercetin bioavailability in dogs are not available. Thus, we investigated quercetin bioavailability (measured as area under the concentration -time curve) in nine adult beagle dogs at an oral dose of 10 mg/kg body weight (b.w.). The major fraction (. 80 %) of flavonols circulating in blood plasma were conjugated metabolites of quercetin. The absolute bioavailability of quercetin (i.e. the fraction that reaches the systemic circulation) was only about 4 %. We also compared the oral bioavailability between the aglycone quercetin and its more often used glucorhamnoside (rutin) and 3-O-glucoside (isoquercitrin) at an equimolar dose of 30 mmol/kg b.w. (corresponding to 10 mg quercetin/kg). Quercetin and isoquercitrin were mainly absorbed in the small intestine with isoquercitrin being one and a half times more bioavailable than quercetin. Maximal plasma concentration after isoquercitrin treatment was 0·89 (SEM 0·07) mmol/l. Although quercetin absorption from rutin was delayed, relative bioavailability was not lower than from the aglycone itself. The latter observation is in clear contrast to findings in human subjects, pigs or rats and might indicate that rutin is a better source of quercetin in dogs than in other species. However, potential in vivo quercetin effects beyond the gastrointestinal tract are limited by the intensive metabolism as well as by the rather low bioavailability of this flavonol.Quercetin glycosides: Quercetin: Dogs: Bioavailability: Flavonoids Plant flavonoids including the flavonol quercetin have become increasingly popular as dietary supplements in recent years. These compounds have been suggested to have various beneficial effects like preventing cancer, suppressing inflammation or decreasing fat absorption (1) . Quercetin, an effective antioxidant in vitro, is added to certain commercial dog foods and marketed as a compound preventing or improving a multitude of age-related diseases that are associated with increased oxidative stress. In order to evaluate potential effects of such compounds in vivo, a sound knowledge of their bioavailability and metabolism is prerequisite. In rats and pigs, the absolute bioavailability (i.e. the fraction of an ingested compound that reaches the systemic circulation) of unchanged quercetin was only 5 % and , 1 %, respectively (2,3) . However, the major fraction of plasma metabolites with an intact flavonol structure consists of methylated, glucuronidated and sulphated derivatives of quercetin (4 -6) . Taking also these metabolites into consideration, the absolute bioavailability of quercetin was noticeably higher with 59 and 17 % for rats and pigs, respectively (2,3) . Peak plasma concentrations after oral quercetin doses of 10 or 50 mg/kg body weight (b.w.) were in the low mmol/l range in these studies. This was also true for human subjects that ingested up to 200 mg of the flavonol in a single dose (7) .In plants, flavonols like quercetin occur mainly as O-glycosides ...

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“…This compound has been suggested to have various beneficial effects like preventing cancer, suppressing inflammation or decreasing fat absorption and agerelated diseases that are associated with increased oxidative stress (Reinboth et al, (2010).…”

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