The bioavailability of quercetin has been intensively investigated in monogastric species, but knowledge about its bioavailability in ruminants does not exist. Thus, the aim of the present study was to determine the bioavailability of quercetin in nonlactating cows equipped with indwelling catheters placed in one jugular vein after intraruminal and additionally after i.v. application, respectively. Quercetin was administered intraruminally in equimolar amounts, either in the aglycone form or as its glucorhamnoside rutin, each at 2 dosages [10 and 50 mg of quercetin/kg of body weight (BW)]. In a second trial, 0.8 mg of quercetin aglycone/kg of BW was applied i.v. Blood samples were drawn 0.5, 1, 1.5, 2, 2.5, 3, 4, 6, 8, 12, and 24 h after intraruminal application and every 5 min (first hour), every 10 min(second hour), and at 3 and 6h after i.v. bolus application, respectively. Quercetin and quercetin metabolites with an intact flavonol structure (isorhamnetin, tamarixetin, and kaempferol) in plasma samples were analyzed by HPLC with fluorescence detection. After intraruminal application of quercetin and rutin, respectively, quercetin and its methylated (isorhamnetin, tamarixetin) and dehydroxylated (kaempferol) derivatives were present in plasma mainly as conjugated forms, whereas free quercetin and its derivatives were scarcely detected. For rutin, the relative bioavailability of total flavonols (sum of conjugated and nonconjugated quercetin and its conjugated and nonconjugated derivatives after intake of 50 mg/kg of BW) was 767.3% compared with quercetin aglycone (100%). Absolute bioavailability of total flavonols was only 0.1 and 0.5% after quercetin aglycone and rutin applications, respectively. Our data demonstrate that bioavailability of quercetin from rutin is substantially higher compared with that from quercetin aglycone in cows after intraruminal (or oral) application, unlike in monogastric species.

The aim of the present study was to investigate the ruminal degradation of the flavonol quercetin and to determine its potential antimicrobial effects on ruminal fermentation in cows. Ruminal degradation of quercetin (0 or 100μmol/L, respectively) as well as its influence on ruminal gas production (0, 50, or 100μmol of quercetin equivalents/L, respectively, either applied as aglycone or as its glucorhamnoside rutin) using concentrate, grass hay, and straw as substrates were investigated in vitro using the Hohenheim gas test. Additionally, the influence of quercetin on ruminal concentrations of volatile fatty acids and their molar ratio in rumen-fistulated, nonlactating cows (n=5) after intraruminal application of quercetin as aglycone or as rutin (0, 10, or 50mg of quercetin equivalents/kg of BW, respectively) was evaluated. Quercetin was rapidly and extensively degraded, whereby the disappearance of quercetin was accompanied by the simultaneous appearance of 2metabolites 3,4-dihydroxyphenylacetic acid and 4-methylcatechol. In vitro total gas and methane production were not reduced by the addition of quercetin aglycone or rutin, respectively, using concentrate, grass hay, and straw as substrates. As expected, however, effects of the substrates used were detected on total gas and methane production. Highest gas production was found with concentrate, whereas values obtained with grass hay and straw were lower. Relative methane production was highest with grass hay compared with concentrate and straw (27.1 vs. 25.0 and 25.5%). After intraruminal application of the quercetin aglycone or rutin, respectively, neither total concentration nor the molar ratio of volatile fatty acids in the rumen fluid were influenced. Results of the present study show that quercetin underlies rapid ruminal degradation, whereby 3,4-dihydroxyphenylacetic acid and 4-methylcatechol are the main metabolites, whereas the latter one most likely is formed by dehydroxylation from 3,4-dihydroxyphenylacetic acid. Regarding antimicrobial effects of quercetin, results obtained indicate that fermentation processes in the forestomachs are not substantially influenced by quercetin or rutin, respectively. With regard to potential health-promoting effects of quercetin, its application in cows, especially in the form of the better available rutin, might not be accompanied by negative effects on ruminal fermentation.

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