Keywords: quercetin, flavonol, allergy, allergies, antioxidant, arthritis, asthma, cancer, cholesterol, diabetes, ergogenic, Helicobaaer, H, pylori, hypertension, infection, inflammation, injury, interstitial cystitis, lipids, pain, metabolic syndrome, prostatitis, sports
Description and Chemical Composition
Quercetin is categorized as a flavonol, one of the six subclasses of flavonoid compounds (Table 1). Flavonoids are a family of plant compounds that share a similar flavone backbone (a three-ringed molecule with hydroxyl [OH] groups attached). A multitude of other substitutions can occur, giving rise to the subclasses of flavonoids and the different compounds found within these subclasses. Flavonoids also occur as either glycosides (with attached sugars [glycosyl groups]) or as aglycones (without attached sugars). (1)
Flavonols are present in a wide variety of fruits and vegetables. In Western populations, estimated daily intake of flavonols is in the range of 20-50 mg/day. (2) Most of the dietary intake is as flavonol glycosides of quercetin, kaempferol, and myricetin rather than their aglycone forms (Table 2). Of this, about 13.82 rag/day is in the form of quercetin-type flavonols. (2)
The variety of dietary flavonols is created by the differential placement of phenolic-OH groups and attached sugars. All flavonols, including quercetin, have in common a 3-hydroxyflavone backbone (Figure 1). The determination of whether a flavonol is considered to be of the quercetin type as opposed to a kaempferol or myricetin type, for example, is based on the location of phenolic-OH groups. Figure 1 shows the possible attachment positions for hydroxyl and glycosyl groups.
The International Union of Pure and Applied Chemistry (IUPAC)) nomenclature for quercetin is (3,3',4',5,7-pentahydroxyflavanone (or its synonym 3,3',4',5,7-pentahydroxy-2-phenylchromen-4-one). This means that quercetin has an OH group attached at positions 3, 5, 7, 3', and 4' (Figure 2). The difference between quercetin and kaempferol is that the latter lacks the OH group at position 3'. The difference between quercetin and myricetin is that the latter has an extra OH group at position 5'.
By definition quercetin is an aglycone, lacking an attached sugar. It is a brilliant citron yellow color and is entirely insoluble in cold water, poorly soluble in hot water, but quite soluble in alcohol and lipids. A quercetin glycoside is formed by attaching a glycosyl group (a sugar such as glucose, rhamnose, or rutinose) as a replacement for one of the OH groups (commonly at position 3). The attached glycosyl group can change the solubility, absorption, and in vivo effects. As a general rule of thumb, the presence of a glycosyl group (quercetin glycoside) results in increased water solubility compared to quercetin aglycone. (1,3)
The feature that distinguishes one quercetin glycoside from another is the type of glycosyl group attached. Hyperoside (found in St. John's wort) has a 3-O-galactoside group (an oxygen bonded to a galactoside group) at position 3 rather than an OH group. Isoquercitin (found in mangoes) has a 3-O-glucoside. Rutin (found in high amounts in buckwheat, citrus fruits, and Ruta graveolens) has an attached rutinose sugar at position 4. All of these are glycoside forms of quercetin (quercetin glycosides).
Technically, the term quercetin...