Quercetin plasma level

Animal studies In the study by Pietta et al. (39), a single dose of ginkgo leaf extract (EGb 761) was administered orally to rats. Metabolites found in the urine represented less than 40% of the flavonoids administered. The presence of phenylalkyl acids in the rat urine but not in the human urine (33,39) indicates that the flavonols were more extensively metabolized in humans than in rats. In the study by Watanabe et al. (40), mice received a diet containing ginkgo leaf extract (EGb761 36mg/kg daily) or a standard diet without the extract for four weeks. Afterwards, plasma levels of quercetin (12.0ng/mL vs. 4.8ng/mL), kaempferol (7.0ng/mL vs. 3.2 ng/ mL), and isorhamnetin (49.6ng/mL vs. Ong/mL) in both treatment groups were determined. The study indicates that these compounds can be absorbed intact into the blood stream. 

Meng X, Maliakal P, Lu H, Lee MJ, Yang CS. 2004. Urinary and plasma levels of resveratrol and quercetin in humans, mice, and rats after ingestion of pure compounds and grape juice. J Agric Food Chem 52 35-942. 

Another important field of application concerns food and beverages, especially wine, juices, and tea (A2, A11, A17, B4, K12, V7, Yl). The antioxidant components of food include vitamin E (a-tocopherol), vitamin A (retinoids), vitamin C (ascorbic acid), and also fi-carotene (provitamin A), other carotenoids (of which more than 600 compounds have been identified), flavonoids, simple phenols, and glucobrasicins (H3). Unfortunately, the TAC value of a food is not informative on the bioavailability of its antioxidants. It has been estimated that polyphenols are normally present in blood plasma at concentrations of 0.2-2 //M (PI). However, it has been demonstrated that feeding rats a quercetin-augmented diet can increase their plasma levels of quercetin and its metabolites up to 10-100 //M (M27), and transient increases in the concentration of plant-derived phenolic compounds can take place after ingestion of food and beverages, which may affect blood plasma TAC (see later). 

Another group of compounds which may nutritionally modify TAC of blood plasma is polyphenols. The possible contribution of polyphenolic components of food and beverages to the TAC of body fluids is a subject of controversy. It has been estimated that polyphenols are present in blood plasma at concentrations of 0.2-2 /xM (PI). However, feeding rats a quercetin-augmented diet can increase the plasma levels of quercetin and its metabolites up to 10-100 /xM (M27). 

As a result of the low concentration of flavanols in blood fractions such as plasma when food level doses are consumed, there are only a few techniques that have sufficient sensitivity. The first of these is gas chromatography coupled to a mass spectrometer for detection [88]. This technique is quite sensitive and provides the assurance of accurate identification of the analyte when the molecular and major fragmentation ions are monitored. The drawbacks are that it is necessary to derivatize the flavanols to achieve sufficient volatility and the technique cannot be adapted for analysis of glucuronide and sulfate conjugates. This method has been used to quantify the plasma levels of catechin and its methylated derivatives after wine consumption, and a modified version is able to detect catechin, quercetin, and resveratrol simultaneously [89]. 

Although the acute vasodilator effects, as shown in in vitro studies (see above), may participate in the antihypertensive effects, the reduced blood pressure persisted even 42-48 h after the last administration of quercetin, when the plasma quercetin concentration and its metabolites fell bellow 25% of the peak post-administration levels [43]. Furthermore, the antihypertensive effects of quercetin did not appear to be related to its antioxidant properties since quercetin did not lower the urinary isoprostane F20 excretion, a prostaglandin-like compound produced in a non enzymatic reaction of arachidonic acid in membrane lipids and superoxide, which is currently used as a reliable marker of oxidative stress. The mechanisms involved in the antihypertensive effects and protection from organ damage... 

In human plasma, quercetin metabolites transiently increased at 1 pM after the intake of a flavonol-rich diet [Hollman et al., 1995], after which concentration gradually decreased to nearly zero in <8 h [Manach et al., 1997]. Continuous ingestion of flavonol-rich foods may maintain a low level of quercetin metabolites in plasma, but this baseline level may be greatly influenced by individual differences [Moon et al., 2000]. 

Das demonstrated that oral administration of labeled 3-0-methylcatechin to three volunteers was followed by plasma peak levels within 2 hours after administration [69]. Hollman et al. studied the absorption of different glycosides of quercetin and the quercetin aglycone in nine healthy ileostomy subjects to avoid losses caused by colonic bacteria [30-31]. The absorption after oral administration decreased in the following order 52% for quercetin glucosides from fried onions > 24% for quercetin aglycone > 17% for quercetin nitinoside [31]. Consequently, humans absorb considerable amounts of quercetin, but the absorption depends on the glycosidic nature of the flavonoid [31]. 

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