1. Quasi-vitamins: Flavonoids
Domina Petric, MD

2. There are seven general classes of flavonoids, classified by their common ring substituents.
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3. Flavonols are the most abundant flavonoids in human diets.
These derivatives (R3 hydroxy, R4 keto) include quercetin, kaempferol, isorhamnetic and myricetin.
Flavonols are the most abundant flavonoids in human diets.
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4. Flavonols
Flavonols are found in a variety of fruits and vegetables, often as glycosides.
Relatively high amounts (15-40 mg/100 g) are found in broccoli, kale, leeks and onions.
Flavanols are also found in red wine, tea and fruit juices.
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5. Green tea and dark chocolate are rich in catechins (40-65 mg/100 g).
Flavanols
These derivatives (R3 hydroxy) include catechin, epicatechin, epigallocatechin and their gallate derivatives.
Flavanols are found in apples, apricots and red grapes (2-20 mg/100 g).
Green tea and dark chocolate are rich in catechins (40-65 mg/100 g).
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6. Flavones
This group of some 300 compounds retains the basic flavane nucleus structure.
Flavones include apigenin and luteolin in very high concentrations (>600 mg/100 g) in parsley, and in lower but significant amounts in cereal grains, celery and citrus rinds (which contain polymethoxylated forms).
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7. Proanthocyanidins
These derivatives are polymeric flavanols, also called condensed tannins.
Proanthocyanidins are present in all plants.
These polyphenols have strong antioxidant properties in vitro.
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8. Anthocyanins
These derivatives (R3 and R4 reduced) exist as glycosides, their aglycone chromophores being referred to as anthocyanidins.
The most common anthocyanins are cyanidin, delphinidin, malvinidin, pelargonidin, peonidin, petunidin and malvidin.
Most are red or blue pigments.
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9. Anthocyanins
The richest sources (600 mg/100 g) are raspberries, black berries and blue berries.
Cherries, radishes, red cabbage, red skinned potato, red onions, and red wine are also good sources ( mg/100 g).
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10. Anthocyanins have antioxidant properties.
Unlike other flavonoids, anthocyanins are relatively unstable to cooking and high-temperature food processing.
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11. Flavanones
These derivatives (R4 keto, C ring otherwise reduced) are found primarily in citrus fruit (15-50 mg/100 g) where they are also present as O- and C-glycosides and methoxylated derivatives.
Flavanones include eriocitrin, neoericitrin, hesperidin, neohesperidin, naringin, narirutin, didymin and poncirin.
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12. Isoflavones
These derivatives (B aromatic ring linked at R3) are contained only in legumes, mostly as glycosides.
Isoflavones include daidzein, genistein, and glycitein, which are also referred to as phytoestrogens due to the affinities of their 7- and 4′-hydroxyl groups to binding mammalian estrogen receptors.
Soy products can contain mg/100 g.
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13. Flavonoid utilization
The hydroxyl groups of these polyphenols enable them to form glycosidic linkages with sugars.
Most flavonoids occur naturally as glycosides.
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14. Flavonoid utilization
Flavonoid glycosides appear to be hydrolyzed by glycosidases in saliva, the brush border of the intestines and the intestinal microflora.
Hindgut bacteria can also degrade the flavanoid by cleaving the heterocyclic ring, leading to the formation of various phenolic acids and their lactones, some of which may be absorbed from the colon.
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15. Flavonoid utilization
Upon absorption, flavonoids are conjugated as glucuronides or sulfates in the liver and are degraded to a variety of phenolic compounds that are rapidly excreted.
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16. Health effects
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17. Antioxidant activity
Flavonoids are able to chelate divalent metal cations (Cu2+, Fe2+), thus serving antioxidant functions by removing those catalysts of lipid peroxidation reactions.
The flavonol quercetin (multiple phenolic hydroxyl groups, carbonyl group at C-4, free C-3 and C-5 hydroxyl groups) can scavenge superoxide radical ions, hydroxyl radicals and fatty acyl peroxyl radicals.
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18. Antioxidant activity
Flavonols and some proanthocyanins have been shown to inhibit macrophage-mediated LDL oxidation in vitro, probably by protecting LDL-α-tocopherol from oxidation or by reacting with the tocopheroxyl radical to regenerate α-tocopherol.
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19. Enzyme modulation
Flavonoids have been found to interact with all classes of enzymes and to selectively affect the activities of some.
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20. Enzyme modulation
Induction of phase II enzymes: epoxide reductase, UDP-glucoronosyltansferase, glutathione-S-transferase, NAD(P)H:quinone oxidoreductase, by affecting their transcription through binding to promoter regions of their respective genes.
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21. Enzyme modulation
Inhibition of other enzymes: aldose reductase, phosphodiesterase, O-methyltransferase, and several serine- and threoninekinases, by direct binding to the respective protein.
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22. Tea flavanols: Induce phase II enzymes.
Inhibit redox-sensitive transcription factors: NFkB, AP-1.
Inhibit prooxidative enzymes: lipooxygenases, cyclooxygenases, nitric oxide synthase, xanthine oxidase.
Induce phase II enzymes.
Induce antioxidant enzymes: glutathione S-transferases, superoxide dismutases.
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23. Enzyme modulation
Flavanones have been shown to induce phase II enzymes and to exert anti-inflammatory effects.
Naringin has been implicated in the effect of grapefruit juice in inhibiting cytrochrome P450-dependent drug metabolism.
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24. Chronic diseases
Some epidemiologic studies have demonstrated associations of diets high in flavonoids (mainly quercetin) with reduced risks of cardiovascular diseases (21-53% reductions in prevalence) and cancers of the lung and rectum (44-47% reductions in prevalence).
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25. Chronic diseases Cardioprotective effects have been attributed to:
vasodilatory effects and blood pressure reduction
antioxidant protection of LDLs against lipid peroxidation
inhibition of platelet aggregation
reduced inflammation
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26. Chronic diseases
Quercetin has been found to inhibit the activation of c-Jun N-terminal kinase in the modulation of angiotensin-induced hypertrophy of vascular smooth muscle cells.
Various proanthocyanins have been shown to inhibit platelet activation and expression of interleukin-2, to lower serum levels of glucose, triglyceride and cholesterol.
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27. Chronic diseases
Various flavones have been found to inhibit cell proliferation and angiogenesis in vitro and to inhibit phorbol ester-induced skin cancer in the mouse model.
These effects may involve inhibition of protein kinase C, stimulation of DNA repair mechanisms and altered carcinogen metabolism.
Some proanthocyanins have been shown to induce apoptosis.
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28. Antiestrogenic effects of soy isoflavones
Isoflavones bind to estrogen receptors α and ß, thus affecting the estrogen-synthetic activity of 17ß-steroid oxidoreductase, as well as estrogen-dependent signal transduction pathways.
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29. Antiestrogenic effects of soy isoflavones
The consumption of soy isoflavones has been associated with higher bone mineral density in a limited number of epidemiological studies.
Soy isoflavones might be useful in improving bone mineralization for the prevention of osteoporosis.
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30. Literature
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