Quercetanus (1607) was one of the first medieval scientists to recognise the existence
of the essential oil industry for lavender and juniper oils leading to the discovery of
another group - the terpenes. Quercetin has earned a controversial reputation, first as a
mutagen and then as one of the strongest anti-cancer agents known, according to Terence
Leighton, Ph.D., professor of biochemistry at the University of California at Berkeley.
Quercetin is widely distributed in the plant kingdom and is the most abundant of the
flavonoid molecules. It is found in many often-consumed foods, including apple, onion, tea, berries, and brassica vegetables, as well as many seeds, nuts, flowers, barks, and
leaves. It is also found in medicinal botanicals, including Ginkgo biloba, Hypericum
perforatum (St. John's Wort), Sambucus canadensis (Elder), and many others. It is often a
major component of the medicinal activity of the plant, and has been shown in experimental studies to have numerous effects on the body.
It occurs very predominantly in red onions and broccoli. However if you think you
should immediately take it off your menu because of its mutagenicity, bear in mind that in a study published by the National Cancer Institute and the Beijing Institute for Cancer
Research in the People's Republic of China, persons with diets high in onions and garlic,
which at dry weight have qercetin levels of 3% to 4%, have a 20-fold lower cancer risk
than persons who don't eat these vegetables.
Note also that particular vegetables such as broccoli are renowned for other
anti-cancer substances. Broccoli in particular has both mustard oils as allyl
isothiocyanates and indoles such as carbinol which have their own toxicity, balanced by a
pronounced anti-cancer potential.
Quercetin's anti-tumor action appears diverse and includes inhibition of inoculated
cancer cells, chemical and virally induced cancers leukaemia and ovarian cancer. Related
flavonoids also give protection against UV-B damage. Despite being a mutagen under test
conditions, quercetin does not appear to be a carcinogen in hamsters, or in rats.
Quercetin is a member of a group of naturally occurring compounds, the flavonoids,
which have a common flavone nucleus composed of two benzene rings linked through a
heterocyclicpyrone ring. Quercetin is found in various plants, food products, and dyes of
natural origin. The estimated average daily intake of quercetin by an individual in the
United States is 25 mg. The Food and Drug Administration nominated quercetin for toxicity
and carcinogenicity studies in the rat because it is a chemical that is widely distributed
in foods. Quercetin was administered to rats by dosed feed since human exposure is by
Information in the literature showed that quercetin administered in the diet to rats at
levels up to approximately 4% caused a minor body weight effect, whereas higher dose
levels produced greater than 10% reduction in body weight gains relative to controls.
Based on this information, the NTP 2-year studies were conducted by administering 0, 1,000,
10,000, or 40,000 ppm quercetin (>95% pure) in feed to groups of 50 male and female
rats for 104 weeks. Ten additional animals per dose group were evaluated at 6 and 15
Under the conditions of these 2-year feed studies there was some
evidence of carcinogenic activity of quercetin in male F344/N rats
based on an increased incidence of renal tubule cell adenomas. There
was no evidence of carcinogenic activity of quercetin in female F344/N
rats receiving 1,000, 10,000 or 40,000 ppm.
Studies in experimental models indicate significant influence
against diabetic complication, viral infection, inflammatory and
allergic conditions and cancer.
Quercetin's aldose reductase-inhibiting properties make it a useful
addition to diabetic nutritional supplementation, to prevent cataract
and neurovascular complications.
Quercetin's anti-inflammatory activity appears to be due to its
antioxidant and inhibitory effects on inflammation-producing enzymes
(cyclooxygenase, lipoxygenase) and the subsequent inhibition of
inflammatory mediators, including leukotrienes and prostaglandins.
Inhibition of histamine release by mast cells and basophils also
contributes to quercetin's anti-inflammatory activity.
Quercetin is a strong antioxidant and a major dietary flavonoid. Epidemiological
studies suggest that consumption of quercetin protects against cardiovascular disease, but
its absorption in man is controversial. We fed nine subjects a single large dose of onions,
which contain glucose conjugates of quercetin, apples, which contain both glucose and
non-glucose quercetin glycosides, or pure quercetin-3-rutinoside, the major quercetin
glycoside in tea. Plasma levels were then measured over 36 h. Bioavailability of quercetin
from apples and of pure quercetin rutinoside was both 30% relative to onions. Peak levels
were achieved less than 0.7 h after ingestion of onions, 2.5 h after apples and 9 h after
the rutinoside. Half-lives of elimination were 28 h for onions and 23 h for apples. We
conclude that conjugation with glucose enhances absorption from the small gut. Because of
the long half-lives of elimination, repeated consumption of quercetin-containing foods
will cause accumulation of quercetin in blood.
Red wine contains one of the strongest anti-cancer angents known to man, which
"directly blocks the proliferation of cancer cells", according to Terance Leighton, PhD,
professor of microbiology at the University of California in Berkeley. The agent quercetin
is not found in white or rose wine, brandies or in red or white grape juices in any significant levels.
Quercetin was initially discovered in red wine because of its trait as a mutagen (a
subtance that mutates cells). Subsequent studies found that the substance, even in large
quantities, did not promote the development of cancer. Instead, it emerged as a powerful
natural anti-cancer agent that "simply overwhelms its danger as a mutagen", Dr. Lieghton said.
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Dr. Leighton, speaking at the 1989 American Chemical Society National Meeting, said
that in a study of 250 red wines from throughout the world, quercetin levels varied from
very low (less than 0.1 mg/L) to very high (greater than 7.0 mg/L).
Dr. Leighton and his team could not link the presence of quercetin
to a specific region, grape variety, vintage, cask, aging method or
length of aging in the bottle. Instead, they found it was directly
related to the fining process. Young red wines, particularly those
exposed to high pressure pressing, contain high concentrations of
substances that impart bitter and astrigent flavours, and therefore are
removed by fining with such agents as gelatine, casein or
polyvinylpolypyrrolidone. At levels of 5 pounds per 1,000 gallons of
wine, 85-90% of the quercetin is removed. At levels of 20 pounds per
1,000 gallons of wine, all the quercetin was removed from wines with
even the highest levels. That explains why, within even a single
variety of grapes and even among closely localized wineries, you have
great variations in mutagenicity (presence of quercetin) from producer
to producer, Dr. Leighton said. If other fining methods are used, the
quercetin remains in the wine.
Resveratrol and quercetin are polyphenols which have been detected in significant
amounts in green vegetables, citrus fruits and red grape wines. Beneficial effects
attributed to these compounds include anti-inflammatory, antiviral and antitumor
properties. The effect of resveratrol and quercetin on growth of human oral cancer cells
is unknown... Combining 50 microM of resveratrol with 10, 25 and 50 microM of quercetin
resulted in a gradual and significant increase in the inhibitory effect of quercetin on
cell growth and DNA synthesis. We conclude that resveratrol or a combination of
resveratrol and quercetin, in concentrations equivalent to that present in red wines, are
effective inhibitors of oral squamous carcinoma cell (SCC-25) growth and proliferation,
and warrant further investigation as cancer chemopreventive agents.
Flavonoids, particularly quercetin, are inhibitors of allergic
(IgE-mediated) mediator release from mast cells and basophils. In
previous studies reported from our laboratories (Hope et al., 1983)
quercetin inhibited not only IgE-mediated allergic mediator release
from mast cells but also IgG-mediated histamine and SRS-A
(peptidoleukotriene) release from chopped lung fragments from actively
sensitized guinea pigs. Interestingly, quercetin was much more potent
as an inhibitor of the release of SRS-A than histamine, suggesting that
it might also inhibit the biosynthesis of SRS-A. Subsequently we
demonstrated that quercetin was an effective inhibitor of
Æ5-lipoxygenase. This property of the compound most likely accounts for
its effect on peptidoleukotriene biosynthesis. The studies reported
herein further expand on these observations by evaluating the
activities of a variety of standard flavonoids on Æ5-lipoxygenase and
other enzymes known to be involved in the metabolism of arachidonic
acid in cells. Compounds such as the flavonoid, quercetin, which
exhibit both allergic mediator release activity and selective
inhibition of the biosynthesis of proinflammatory arachidonic acid
metabolites may be interesting prototypes which will lead to the
discovery of very effective antiallergic and antiinflammatory agents.
See also: Flavonoids
- TR-409 Toxicology and Carcinogenesis Studies of Quercetin
(CAS No. 117-39-5) in F344 Rats (Feed Studies). National Institute of
Environmental Health Sciences
- Hollman PC; van Trijp JM. Et
al. Relative bioavailability of the antioxidant flavonoid quercetin
from various foods in man. FEBS Lett, 418(1-2):152-6 1997 Nov 24
ML, and Alcaraz MJ, Anti-inflammatory activity and inhibition of
arachidonic acid metabolism by flavonoids. Agents Action 32, 283-287,
- Stavric B, Quercetin in our diet: from potent mutagen to probable anticarcinogen. Clin Biochem 27, 245-248, 1994
TM; Virji AS. Modulating effect of resveratrol and quercetin on oral
cancer cell growth and proliferation. Anticancer Drugs 1999
- Gail Finlayson. Wines & Vines Inc. July 1991.
AF, Tobias LD, Fiedler-Nagy C, et al. Effect of flavonoids on
arachidonic acid metabolism. Prog Clin Biol Res 213:231-242; 1986.