Bioavailability of flavonoids after intraduodenal administration and their effects on the carbohydrate metabolism of dairy cows, exemplified by quercetin and its glucorhamnoside rutin

Flavonoids are secondary plant metabolites which have been shown to possess many health-promoting properties and are therefore already in use as feeding supplements for cattle, although scientific evidence for their efficacy in vivo is limited. Quercetin acts antioxidative and hepatoprotective and affects glucose and lipid metabolism in monogastrics in vitro and in vivo. If this is also true for ruminants quercetin could be beneficial in periparturient high-yielding dairy cows through amelioration of negative effects of free radical formation and reduction of the severity of liver lipidosis and ketosis. Recently it has been shown that bioavailability of quercetin is low after ruminal administration in cows, due to degradation by the rumen microbiota. The basic requirement for any positive effect is the absorption of quercetin from the gastro-intestinal tract. But currently it is not known whether quercetin could be absorbed from the small intestine in ruminants if degradation is prevented. The aim of this study was to investigate the bioavailability of quercetin after duodenal administration in dairy cows and to survey the effects of a long-term quercetin treatment. To evaluate bioavailability of quercetin, six German Holstein cows were equipped with duodenal cannulas and after recovery each cow received equivalent doses of quercetin (9, 18, 27 mg quercetin equivalents (QE)/kg BW) either as aglycone (QA) or as its glucorhamnoside rutin (RU). In addition, two control studies with duodenal administration of NaCl solution (0.9%) were conducted per cow in order to examine concentrations of flavonoids in plasma during regular feeding. Before and after administration of the test compounds blood samples were taken in defined time intervals over a period of 24 h. In between the runs a wash-out period of 2 d was applied to avoid possible carry-over effects. Concentrations of plasma quercetin aglycone and its metabolites isorhamnetin, tamarixetin and kaempferol were measured after treatment with glucuronidase/sulfatase by HPLC with fluorescence detection. After duodenal administration of QA plasma concentration of QA and its methylated metabolites clearly increased above baseline (Cmax of total flavonols 406.9 – 635.8 nmol/L; basal 3.9 nmol/L). Cmax of total flavonols increased in a dose-dependent manner but showed a high interindividual variability. However, peak time did not differ. Pre-administration baseline values of total flavonols were reached again 3 to 4 h after QA administration. In contrast, RU administration did not increase plasma levels of quercetin above baseline, irrespective of dose administered. Bioavailablitiy of quercetin and its metabolites as measured by the area under the curve (AUC) was affected by the quercetin source applied, whereby quercetin from RU was unavailable. Taken together, duodenal administration enhanced bioavailability of QA nearly to values which were previously reported in pigs after oral administration of QA. In contrast to findings in monogastrics or after oral administration in cows, quercetin from RU seems to be unavailable when administered duodenally. In the second experiment, long-term quercetin effects were investigated. Twice daily 18 mg quercetin (Q)/ kg BW were administered intraduodenally to 5 late lactation dairy cows (approximately 215 DIM) over a period of 28 days. Before and during administration, we took frequent blood samples for determination of plasma concentrations of flavonols and metabolites. Liver biopsies, in which the glycogen and fat content as well as mRNA expression of selected genes were measured, were taken prior to and at the end of the first and fourth week of Q administration. Furthermore, euglycemic, hyperinsulinemic and hyperglycemic clamp studies were conducted before and after 2 weeks of Q administration. DMI and most of the other zootechnical data remained unchanged during the long-term administration. The milk protein content increased about 5% in week 2 and 4 of Q administration compared to basal values while fat and lactose content of milk remained unchanged. Plasma non-esterified fatty acid, gamma-glutamyl transferase, cholesterol, glutamate dehydrogenase, triglyceride and albumin concentrations as well as liver fat and glycogen concentrations were not affected by Q supplementation. Plasma glucose and BHBA concentrations in plasma decreased and increased, respectively, under the influence of quercetin. During hyperglycemic clamp conditions, the relative increase of plasma insulin was higher after 2 weeks of Q administration and a tendency for an increased rQUICKI was observed. The relative mRNA expression levels of selected genes related to glucose metabolism, fat metabolism and antioxidative status were not altered after 1 and after 4 weeks of Q supplementation. In conclusion, the effects on insulin release and sensitivity support the assumption that administration of Q could be beneficial in regard to metabolic adaptation to early lactation of high-yielding cows.