Introduction

Quercetin is a dietary flavonol found widely in fruits, vegetables and nuts in many different glycosidic forms. Major dietary sources are lettuce, chili pepper, cranberry, onion, black chokeberry, black elderberry, caper, tomato, broccoli and apple (Rothwell et al., 2013). In onions, quercetin is bound to 1 or 2 glucose molecules (quercetin-4´-glucoside and quercetin-3,4´-glucoside). Examples of dietary quercetin  are quercetin galactosides, which are found in apples, quercetin arabinosides, which are present in berries, quercetin-3-rutinoside (rutin), which are found in capers (Erlund, 2004).

Quercetin has been reported to exhibit antioxidative, anti-carcinogenic, anti-inflammatory, anti-microbial, anti-viral, antiaging,anti-thrombotic, anti-aggregatory and vasodilating effects (Hayek et al., 1997; Chopra et al., 2000; Verma et al., 1988; Deschner et al., 1991; Pereira et al., 1996, Ferry et al., 1996; Erlund et al., 2004). Although quercetin has lots of health benefits, its bioavailability is relatively poor and highly variable. This study compiles and discusses bioavailability studies of food-derived quercetin and quercetin in supplements.

Bioavailability of quercetin

Bioavailability is the fraction of an orally administered substance that is absorbed and available for physiologic activity or storage (Jackson et al., 1997). Bioavailability is classified as absolute or relative based on its pharmacokinetics assessment (Toutain et al., 2004). Absolute bioavailability is more accurate, and is calculated as the area under the plasma concentration-time curve of an ingested substance (AUC_oral) relative to its intravenous administration (AUC_i.v) (Levine et al., 1996).  Alternatively, relative bioavailability is simpler, but less accurate, and is calculated as AUC_oral.  However, studies examining relative bioavailability of quercetin are more common (Guo et al., 2015).

In contrast to the form in most supplements, most of the quercetin in foods is attached to a sugar molecule and this conjugate is known as  a glycoside. The onion plant tends to attach glucose to form quercetin-3-glucoside (isoquercetin) while apple trees and tea plants tend to attach rutinose, yielding rutin (Heim et al.,2002; Yoo et al., 2010). Differences in quercetin-conjugated glycosides affect its bioavailability (Lee et al., 2012; Hollman et al., 1997a; Hollmann et al., 1997b). The size and polarities of these compounds can cause difficulty crossing membranes in the gut. Oppositely, this is not the case for isoquercetin. A clinical research compared quercetin bioavailability from different foods and supplements (Hollman et al., 1997a; Hollman et al., 1997b). Absorption from isoquercetin-rich onions was 52%, compared to 24% from a standard quercetin supplement (Hollman et al., 1997b). Likewise, animal studies showed superior bioavailability of isoquercetin relative to quercetin and rutin (Manach et al., 1997).  Likewise, in another study, bioavailability of onion-derived quercetin, which is mainly quercetin glucoside, compared to apple-derived quercetin, which contains quercetin rhamnoside and quercetin galactoside (Yoo et al., 2010; Lee et al., 2012). In this study, AUC_0-24h and C_max of quercetin following the consumption of onions were 2 and 3 times greater, respectively, than those following the consumption of apples (Lee et al., 2012). Quercetin T_max did not differ between dietary sources, but t_1/2 of apple-derived quercetin was more than 4 times shorter after consuming onions (15 h). These findings support that onion-derived quercetin is more bioavailable due to its greater absorption (Lee et al., 2012).

Studies in pigs showed that quercetin glucoside, compared to quercetin aglycone, had greater bioavailability. Some hypothesis may clarify this phenomenon. First of all, quercetin glucoside (0.76±0.01) is more water soluble than quercetin aglycone (1.82±0.32) on the basis of its lower octanol-water partition coefficient (Rothwell et al., 2005). Another hypothesis is quercetin glucoside is absorbed through sodium-dependent glucose transporter 1 (SLGT1), but it is not in use for quercetin aglycone (Wolffram et al., 2002). SGLT1-mediated absorption provides greater intestinal uptake of quercetin glucoside (Guo et al., 2015). The site and manner in which quercetin absorbed depends upon its chemical structure (Guo et al., 2015). In vitro studies put a hypothesis that the glucose moiety was using a transporter that normally pumps glucose across membranes of the intestinal wall (Gee et al., 2000). The rapid absorption observed with isoquercetin is consistent with this active transport. For quercetin and rutin, which cannot use the transporter, peak levels are riched within 2-4 and 6-8 hours respectively (Olthof et al., 2000; Erlund et al., 2000). Inversely, isoquercetin can reach peak concentrations in less than 40 minutes (Olthof et al., 2000). Mechanisms explaining gastric absorption of quercetin aglycone is unclear, but studies in Caco-2 cell monolayers support that intestinal absorption of quercetin aglycone occurs primarily by passive diffusion and secondarily by organic anion transporting polypeptide (OATP) (Nait et al., 2009). Contrasting quercetin aglycone, glycosylated forms of quercetin (quercetin glucoside, quercetin rutinoside) are not absorbed in the stomach (Crespy et al., 2002). Quercetin glycosides such as quercetin glucoside, quercetin galactoside, quercetin arabinoside, are deglycosylated to quercetin aglycone prior to absorption by lactase phlorizin hydrolase (LPH), a β-glucosidase residing at the brush border (Arts et al., 2004;  Nemath et al., 2003). Afterwards, quercetin aglycone is passively absorbed (Day, A. J., 2003). Different from those, quercetin rutinoside is absorbed in the colon following deglycosylation, which appears to be mediated by gut microbiota-derived  β-glucosidase that generates quercetin aglycone and facilitates its colonic absorption (Day, A.J., 2003; Jaganath, I. B., 2006, Kim, D.H., 2008).

The extent to which quercetin is absorbed in clinical studies can be estimated by multiplying the plasma maximum concentration (C_max) of quercetin by plasma volume (estimated at 3 L for adults) and dividing by the administered dose (Davy et al., 1994; Retzlaff, J. A., 1969; Guo et al.,2015). Healthy participants ingesting grape juice containing 10 mg quercetin aglycone had a quercetin C_max of 16 µM, which represents approximately 1.4% of the ingested dose (Golderg et al., 2003).  Quercetin glucoside is also poorly absorbed as evidenced by an estimated absorption of 6.9 in healthy participants who ingested onion-derived quercetin glucoside at a dose corresponding to 100 mg quercetin aglycone (Graefe et al., 2001).

Bioavailability of quercetin is related to its bioaccessibility and thereby solubility in the vehicle used for its administration. Quercetin is relatively lipophilic with low water solubility. The poor solubility of quercetin and crstalline form at body temperatures limitnits bioaccessibility and its bioavailability. Absolute bioavailability of quercetin was 16% in rats and its  C_max was 2.01 µM following administration of quercetin suspended in aqueous solution. Administration of quercetin aglycone dissolved  in an ethanol and PEG 200 solution increased its absolute bioavailability to 27.5% and   C_max to 3.44 µM(Khaled et al., 2003; Pool et al., 2013). Octanol-water partition coefficient of quercetin (1.82±0.32) is nearly double that of quercetin-3-glucoside (0.76±0.01), but it is lower than that of kaempferol (3.11±0.54). Water solubility of quercetin is 1.53-12.5 mg/L at gastrointestinal pH levels (pH 2-7) (Pool et al., 2013).

Poor bioavailability of quercetin aglycone and glycosides is also related to their propensity, and that of their metabolites, to be effluxed back into the intestinal lumen following enterocyte uptake (Crespy et al., 1999; Crespy et al., 2001). Quercetin aglycone or glycosides are effluxed across the apical membrane of enterocytes, as indicated in Caco-2 cell monolayer studies showing that their permeability from the basolateral to apical side was more than 2 times greater than their apical to basolateral permeability (Nait et al., 2009; Walgren et al., 1998). Most of the absorbed quercetin aglycone is rapidly metabolized and secreted back into the intestinal lumen (Crespy et al., 2001).

Quercetin bioavailability is better when quercetin is consumed as a cereal bar ingredient instead of capsule  (Egert et al., 2012). Its greater absorption may be related with manufacturing process that yields a homogenous solid dispersion of quercetin with other cereal ingredients. Solid dispersions have greater surface area that promotes dissolution in the intestinal lumen, thereby promoting bioavailability (Guo et al., 2015). Dietary fat improved quercetin bioavailability in a study with pigs (Lesser et al., 2004). Quercetin ingestion with short chain fructooligosaccharide (FOS) improves quercetin bioavailability (Matsukawa et al., 2009). Quercetin bioavailability of vacuum impregnated apple chips (AUC_{0-1440 min} = 104±24 µmol.min.L^-1) as functional food was similar to the supplementation with apple peel extract capsules (AUC_{0-1440 min} = 87±24 µmol.min.L^-1) in humans (Petersen et al., 2016). More research are needed to prove that quercetin in food matrix provides greater bioavailability than capsule forms.

Quercetin bioavailability is characterized by high intersubject variability (Kaushik et al., 2012).  For instance, quercetin AUC_{0-24 h} was 8.9-89.1 µM.h following ingestion of onion-derived quercetin glucosides at a dose equivalent to 100 mg quercetin aglycone (Graefe et al., 2001). Quercetin C_max was 0.29-2.26 µM  in adults who ingested a beverage containing 500 mg quercetin aglycone (Kaushik et al., 2002). Intersubject variations for time to C_max (T_max) and elimination half-life (t_1/2) of quercetin in adults were 69% and 122% respectively, following ingestion of 100 mg apple-derived quercetin glycosides (Lee et al., 2012). Likewise, 50 mg quercetin supplementation in adults results in highly variable plasma concentrations (38-194 nM) (Egert et al., 2008). Differences in β-glucosidase activity, a determinant of intestinal uptake of quercetin glucosides, promote intersubject variation in quercetin glycoside absorption (Nemeth et al., 2003; Day et al., 2003; Guo et al., 2015). Additionally, intersubject variations in intestinal and hepatic phase II quercetin –metabolizing enzymes (UGT, specifically UGT1A family; SULT, specifially SULT1A family; COMT) are speculated to contribute to interindividual differences in quercetin metabolism (Egert et al., 2008).

There is no clear evidence demonstrating that gender and age affect quercetin bioavailability (Guo et al., 2015).  Exceptional finding was that quercetin from quercetin-3-rutinoside was more bioavailable in women compared with men (Erlund et al., 2000). A quercetin study in humans suggest that individual differences in plasma vitamin C status may contribute to intersubject variability in quercetin bioavailability (Guo et al., 2014).  Some in vitro studies also showed that vitamin C protects quercetin against oxidative degredation (Skaper et al., 1997; Takahama et al., 2003). More clinical studies are necessary to define if vitamin C status regulates quercetin bioavailability.

In conclusion, quercetin has several health effects and thereby, its bioavailability is really significant and unfortunately, is poor. Many factors such as glucose moieties, solubility, human factor, vitamin C status and food matrix can affect bioavailability. More research is warranted to evaluate and improve bioavailability of quercetin.

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