Dr. Louay Labban
• Polyphenolic compounds that are ubiquitous
in nature and are categorized, according to
chemical structure, into flavonols, flavones,
flavanones, isoflavones, catechins,
anthocyanidins and chalcones
What are flavonols?
• Flavonols are phytochemical compounds
found in high concentrations in a variety of
plant-based foods and beverages. Based on
their structure, flavonols are classified as
flavonoids and include the following
compounds: quercitin, kaempferol, and
• Over 4,000 flavonoids have been identified,
many of which occur in fruits, vegetables and
beverages (tea, coffee, beer, wine and fruit
• The flavonoids have aroused considerable
interest recently because of their potential
beneficial effects on human health they have
been reported to have antiviral, antiallergic,
antiplatelet, antiinflammatory, antitumor and
• Antioxidants are compounds that protect cells
against the damaging effects of reactive
oxygen species, such as singlet oxygen,
superoxide, peroxyl radicals, hydroxyl radicals
• An imbalance between antioxidants and
reactive oxygen species results in oxidative
stress, leading to cellular damage. Oxidative
stress has been linked to cancer, aging,
atherosclerosis, ischemic injury, inflammation
and neurodegenerative diseases (Parkinson's
• Flavonoids may help provide protection
against these diseases by contributing, along
with antioxidant vitamins and enzymes, to the
total antioxidant defense system of the human
• Epidemiological studies have shown that
flavonoid intake is inversely related to
mortality from coronary heart disease and to
the incidence of heart attacks.
• The recognized dietary antioxidants are
vitamin C, vitamin E, selenium, and
carotenoids. However, recent studies have
demonstrated that flavonoids found in fruits
and vegetables may also act as antioxidants.
• Like alphatocopherol (vitamin E), flavonoids
contain chemical structural elements that may
be responsible for their antioxidant activities.
The contribution of flavonoids to the
antioxidant defense system may be substantial
considering that the total daily intake of
flavonoids can range from 50 to 800 mg.
• Flavonoid intake depends upon the
consumption of fruits, vegetables, and certain
beverages, such as red wine, tea, and beer.
The high consumption of tea and wine may be
most influential on total flavonoid intake in
certain groups of people.
• This intake is high compared to the average
daily intake of other dietary antioxidants like:
• vitamin C (70 mg)
• vitamin E (710 mg)
• or carotenoids (23 mg).
• Quercetin (a flavonol in vegetables, fruit skins,
• Xanthohumol (a prenylated chalcone in hops
• Isoxanthohumol (a prenylated flavanone in
hops and beer)
• Genistein (an isoflavone in soy) Prooxidant
• Chalconaringenin (a nonprenylated chalcone
in citrus fruits)
• Naringenin (a nonprenylated flavanone in
• The capacity of flavonoids to act as
antioxidants depends upon their molecular
structure. The position of hydroxyl groups and
other features in the chemical structure of
flavonoids are important for their antioxidant
and free radical scavenging activities.
• Quercetin, the most abundant dietary
flavonol, is a potent antioxidant because it has
all the right structural features for free radical
• The antioxidant properties of the
prenylflavonoids were compared to those of
quercetin (a flavonol), genistein (the major
isoflavone in soy), chalconaringenin (a
nonprenylated chalcone), naringenin (a
nonprenylated flavanone), and vitamin E.
• Xanthohumol, the major prenylchalcone in
hops and beer, is a more powerful antioxidant
than vitamin E or genistein. However,
xanthohumol was less potent than quercetin.
The potency of xanthohumol as an antioxidant
is markedly increased when combined with an
equivalent amount of vitamin E.
• A flavonoid chalcone (chalconaringenin) and a
flavanone (naringenin) with no prenyl groups
act as prooxidants, i.e. they promote rather
than limit the oxidation of LDL by copper.
However, adding a prenyl group to these
flavonoid molecules counteracted their
• The specific amounts of flavonols in foods are
affected by a range of factors including plant
type and growth, season, light, degree of
ripeness, food preparation, and processing.
• Despite these variables, high concentrations
of flavonols can be found in apples, apricots,
beans, broad beans, broccoli, cherry
tomatoes, chives, cranberries, kale, leeks,
pear, onions, red grapes, sweet cherries, and
white currants .
Beneficial effects associated with
consumption of flavonols
• Consumption of flavonols has been associated
with a variety of beneficial effects including:
• Increased activity of erythrocyte superoxide
dismutase (an antioxidant enzyme found in
red blood cells)
• A decrease in lymphocyte DNA damage, a
decrease in urinary 8-hydroxy-2’-
deoxyguanosine (a marker of oxidative
• An increase in plasma antioxidant capacity
(the ability to scavenge free radicals) .
• Apples: Studies have investigated the
relationship between consumption of apples
and susceptibility to chronic diseases such as:
• Diabetes .
• Broccoli: Cruciferous vegetables are part of
the plant family Brassicaceae, which includes:
• Brussels sprouts
• Consumption of this group of plant foods has
been associated with a reduction in risk of
several cancers including:
• Cranberries: Cranberries are commonly touted
as a remedy for treating urinary tract
infections. Current research has also
investigated the relationship between
consumption of cranberry products and
cancer and cardiovascular disease.