Flavonoid

Flavonoids (or bioflavonoids; from the Latin word flavus, meaning yellow, their color in nature) are a class of polyphenolic secondary metabolites found in plants. Blackberry, black currant, chokeberry, and red cabbage are examples of plants with rich contents of flavonoids. In plant biology, flavonoids fulfill diverse functions, including attraction of pollinating insects, antioxidant protection against ultraviolet light, deterrence of environmental stresses and pathogens, and regulation of cell growth.[1][2]

Although commonly consumed in human and animal plant foods and in dietary supplements, flavonoids are not considered to be nutrients or biological antioxidants essential to body functions, and have no established effects on human health or prevention of diseases.[1][2][3]

Chemically, flavonoids have the general structure of a 15-carbon skeleton, which consists of two phenyl rings (A and B) and a heterocyclic ring (C, the ring containing the embedded oxygen).[1][4] This carbon structure can be abbreviated C6-C3-C6. According to the IUPAC nomenclature, they can be classified into flavonoids or bioflavonoids, isoflavonoids, derived from 3-phenylchromen-4-one (3-phenyl-1,4-benzopyrone) structure, and neoflavonoids, derived from 4-phenylcoumarin (4-phenyl-1,2-benzopyrone) structure.[5]

As ketone-containing compounds, the three flavonoid classes are grouped as anthoxanthins (flavones and flavonols).[1] This class was the first to be termed bioflavonoids. The terms flavonoid and bioflavonoid have also been more loosely used to describe non-ketone polyhydroxy polyphenol compounds, which are more specifically termed flavanoids.[4]

## Contents

* 1 History
* 2 Biosynthesis
* 3 Functions in plants
* 4 Subgroups
* 4.1 Anthocyanidins
* 4.2 Anthoxanthins
* 4.3 Flavanones
* 4.4 Flavanonols
* 4.5 Flavans
* 4.6 Isoflavonoids
* 5 Dietary sources
* 6 Dietary intake
* 7 Non-nutrient status in humans
* 7.1 Metabolism and excretion
* 7.2 Safety
* 8 Regulatory status
* 9 Research
* 9.1 Antioxidant research
* 9.2 Clinical research
* 9.3 Inflammation
* 9.4 Cancer
* 9.5 Cardiovascular diseases
* 10 Synthesis, detection, quantification, and semi-synthetic alterations
* 10.1 Color spectrum
* 10.2 Availability through microorganisms
* 10.3 Tests for detection
* 10.3.1 Shinoda test
* 10.3.2 Sodium hydroxide test
* 10.3.3 p-Dimethylaminocinnamaldehyde test
* 10.4 Quantification
* 10.5 Semi-synthetic alterations
* 11 See also
* 12 References
* 13 Further reading
* 13.1 Databases

## History

In the 1930s, Albert Szent-Györgyi and other scientists discovered that vitamin C alone was not as effective at preventing scurvy as the crude yellow extract from oranges, lemons or paprika. They attributed the increased activity of this extract to the other substances in this mixture, which they referred to as "citrin" (referring to citrus) or "vitamin P" (a reference to its effect on reducing the permeability of capillaries). The substances in question (hesperidin, eriodictyol, hesperidin methyl chalcone and neohesperidin) were later shown not to fulfil the criteria of a vitamin,[6] so that the term "vitamin P" is now obsolete.[7]

* Molecular structure of the flavone backbone (2-phenyl-1,4-benzopyrone)

* Isoflavan structure

* Neoflavonoids structure

## Biosynthesis

Main article: Biology:Flavonoid biosynthesis

Flavonoids are secondary metabolites synthesized mainly by plants. The general structure of flavonoids is a fifteen-carbon skeleton, containing two benzene rings connected by a three-carbon linking chain.[1] Therefore, they are depicted as C6-C3-C6 compounds. Depending on the chemical structure, degree of oxidation, and unsaturation of the linking chain (C3), flavonoids can be classified into different groups, such as anthocyanidins, flavonols, flavanones, flavan-3-ols, flavanonols, flavones, and isoflavones.[1] Chalcones, also called chalconoids, although lacking the heterocyclic ring, are also classified as flavonoids. Furthermore, flavonoids can be found in plants in glycoside-bound and free aglycone forms. The glycoside-bound form is the most common flavone and flavonol form consumed in the diet.[1]

A biochemical diagram showing the class of flavonoids and their source in nature through various inter-related plant species.

## Functions in plants

Numbering some 5,000 individual compounds, flavonoids are widely distributed in plants, fulfilling numerous functions, including attraction of pollinating insects, deterrence of environmental stresses, and regulation of cell growth.[1] They are the most important plant pigments for flower coloration, producing yellow, red or blue pigmentation in petals evolved to attract pollinators.[1]

In higher plants, they are involved in antioxidant roles in plant cells, filtration of ultraviolet light, symbiotic nitrogen fixation, and defense against pathogens and pests. They also act as plant chemical messengers, physiological regulators, and cell cycle inhibitors.[1][2] Flavonoids secreted by the root of their host plant help Rhizobia in the infection stage of their symbiotic relationship with legumes like peas, beans, clover, and soy. Rhizobia living in soil are able to sense the flavonoids and this triggers the secretion of Nod factors, which in turn are recognized by the host plant and can lead to root hair deformation and several cellular responses such as ion fluxes and the formation of a root nodule. In addition, some flavonoids have inhibitory activity against organisms that cause plant diseases, e.g. Fusarium oxysporum.[8]

## Subgroups

Flavonoids have been classified according to their chemical structure, and are usually subdivided into the following subgroups:[1][9]

### Anthocyanidins

Flavylium skeleton of anthocyanidins

Anthocyanidins are the aglycones of anthocyanins; they use the flavylium (2-phenylchromenylium) ion skeleton.[1]

Examples: cyanidin, delphinidin, malvidin, pelargonidin, peonidin, petunidin

### Anthoxanthins

Anthoxanthins are divided into two groups:[10]

Group  | Skeleton  | Examples

|
|

Description  | Functional groups  | Structural formula
3-hydroxyl  | 2,3-dihydro
Flavone | 2-phenylchromen-4-one | ✗  | ✗  | Image:Flavone skeleton colored.svg  | Luteolin, Apigenin, Tangeritin
Flavonol

3-hydroxyflavone | 3-hydroxy-2-phenylchromen-4-one | ✓  | ✗  | Image:Flavonol skeleton colored.svg  | Quercetin, Kaempferol, Myricetin, Fisetin, Galangin, Isorhamnetin, Pachypodol, Rhamnazin, Pyranoflavonols, Furanoflavonols,

### Flavanones

Flavanones

Group  | Skeleton  | Examples

|
|

Description  | Functional groups  | Structural formula
3-hydroxyl  | 2,3-dihydro
Flavanone | 2,3-dihydro-2-phenylchromen-4-one | ✗  | ✓  | Image:Flavanone skeleton colored.svg  | Hesperetin, Naringenin, Eriodictyol, Homoeriodictyol

### Flavanonols

Flavanonols

Group  | Skeleton  | Examples

|
|

Description  | Functional groups  | Structural formula
3-hydroxyl  | 2,3-dihydro
Flavanonol

3-Hydroxyflavanone

2,3-dihydroflavonol | 3-hydroxy-2,3-dihydro-2-phenylchromen-4-one | ✓  | ✓  | Image:Flavanonol skeleton colored.svg  | Taxifolin (or Dihydroquercetin), Dihydrokaempferol

### Flavans

Flavan structure

Include flavan-3-ols (flavanols), flavan-4-ols, and flavan-3,4-diols.

Skeleton  | Name

|

| Flavan-3-ol (flavanol)
| Flavan-4-ol
| Flavan-3,4-diol (leucoanthocyanidin)

* Flavan-3-ols (flavanols)
* Flavan-3-ols use the 2-phenyl-3,4-dihydro-2H-chromen-3-ol skeleton

Examples: catechin (C), gallocatechin (GC), catechin 3-gallate (Cg), gallocatechin 3-gallate (GCg), epicatechins (EC), epigallocatechin (EGC), epicatechin 3-gallate (ECg), epigallocatechin 3-gallate (EGCg)
* Theaflavin

Examples: theaflavin-3-gallate, theaflavin-3'-gallate, theaflavin-3,3'-digallate
* Thearubigin
* Proanthocyanidins are dimers, trimers, oligomers, or polymers of the flavanols

### Isoflavonoids

* Isoflavonoids
* Isoflavones use the 3-phenylchromen-4-one skeleton (with no hydroxyl group substitution on carbon at position 2)

Examples: genistein, daidzein, glycitein
* Isoflavanes
* Isoflavandiols
* Isoflavenes
* Coumestans
* Pterocarpans

## Dietary sources

Parsley is a source of flavones Blueberries are a source of dietary anthocyanins Flavonoids are found in citrus fruits, including red grapefruit

Flavonoids (specifically flavanoids such as the catechins) are "the most common group of polyphenolic compounds in the human diet and are found ubiquitously in plants".[1][2][11] Flavonols, the original bioflavonoids such as quercetin, are also found ubiquitously, but in lesser quantities. The widespread distribution of flavonoids, their variety and their relatively low toxicity compared to other active plant compounds (for instance alkaloids) mean that many animals, including humans, ingest significant quantities in their diet.[1][2][3]

Foods with a high flavonoid content include blackberries, black currants, parsley, onions, blueberries and strawberries, red cabbage, black tea, dark chocolate, and citrus fruits.[1][2][12] One study found high flavonoid content in buckwheat.[13]

Citrus flavonoids include hesperidin (a glycoside of the flavanone hesperetin), quercitrin, rutin (two glycosides of quercetin, and the flavone tangeritin.[1] The flavonoids are less concentrated in the pulp than in the peels (for example, 165 versus 1156 mg/100 g in pulp versus peel of satsuma mandarin, and 164 vis-à-vis 804 mg/100 g in pulp versus peel of clementine).[14]

Peanut (red) skin contains significant polyphenol content, including flavonoids.[15][16]

## Dietary intake

Adult flavonoid intake (mg per day) in Europe; pie charts indicate the relative consumption of different flavonoid compounds[17]

Food composition data for flavonoids were provided by the USDA database on flavonoids.[12] In the United States NHANES survey, mean flavonoid intake was 190 mg per day in adults, with flavan-3-ols as the main contributor.[18] In the European Union, based on data from the European Food Safety Authority (EFSA), mean flavonoid intake was 140 mg/d, although there were considerable differences among individual countries.[17] The main type of flavonoids consumed in the EU and USA were flavan-3-ols (80% for USA adults), mainly from tea or cocoa in chocolate, while intake of other flavonoids was considerably lower.[1][17][18]

Data are based on mean flavonoid intake of all countries included in the 2011 EFSA Comprehensive European Food Consumption Database.[17]

## Non-nutrient status in humans

Flavonoids are not considered as nutrients because there is no evidence for a cause-and-effect on specific cells or organs in vivo.[1][2][3] The European Food Safety Authority determined that dietary flavonoids do not have the characteristics of nutrients, as they do not reduce disease risk, affect physiological or behavioral functions, improve satiety, contribute calories, or influence the growth and development of children.[3] The bioavailability of flavonoids is low because they are extensively metabolized in the stomach, small intestine and liver, and are rapidly excreted.[1][2]

In the United States, flavonoids and other polyphenols are not included on the FDA list of nutrients.[19]

### Metabolism and excretion

Flavonoids are poorly absorbed in the human body (less than 5%), then are quickly metabolized into smaller fragments with unknown properties, and rapidly excreted.[1][2][20][21][22] Flavonoids have negligible antioxidant activity in the body, and the increase in antioxidant capacity of blood seen after consumption of flavonoid-rich foods is not caused directly by flavonoids, but by production of uric acid resulting from flavonoid depolymerization and excretion.[1][2][3] Microbial metabolism is a major contributor to the overall metabolism of dietary flavonoids.[1][2][23]

### Safety

Likely due to the low bioavailability and rapid metabolism and excretion of flavonoids, there are no safety concerns and no adverse effects associated with high dietary intakes of flavonoids from plant foods.[1]

## Regulatory status

Due to the absence of proof for flavonoid health effects in clinical research, neither the United States FDA nor the European Food Safety Authority has approved any flavonoids as prescription drugs.[1][20][24][25]

The FDA has warned numerous dietary supplement and food manufacturers, including Unilever, producer of Lipton tea in the U.S., about illegal advertising and misleading health claims regarding flavonoids, such as that they lower cholesterol or relieve pain.[26][27]

From 2020 to 2023, the FDA issued 11 warning letters to American manufacturers of flavonoid dietary supplements for false advertising of health claims and illegal misbranding of products.[28]

## Research

### Antioxidant research

Although flavonoids inhibit free radical activity in vitro, high dietary intakes in humans would be 100 to 1,000 times less than circulating concentrations of dietary and endogenous antioxidants, such as vitamin C, glutathione, and uric acid.[1][2] Further, after digestion and metabolism in the body, flavonoid derivatives would have lower antioxidant activity than the parent flavonoid, rendering the smaller flavonoid metabolite with negligible antioxidant function.[1][2][3]

### Clinical research

Although numerous preliminary clinical studies have been conducted to assess the potential for dietary flavonoid intake to affect disease risk, research has been inconclusive due to limitations of experimental design and absence of cause-and-effect evidence.[1][2][3]

### Inflammation

Inflammation has been implicated as a possible origin of numerous local and systemic diseases, such as cancer,[29] cardiovascular disorders,[30] diabetes mellitus,[31] and celiac disease.[32] There is no clinical evidence that dietary flavonoids affect any of these diseases.[1]

### Cancer

Clinical studies investigating the relationship between flavonoid consumption and cancer prevention or development are conflicting for most types of cancer, probably because most human studies have weak designs, such as a small sample size.[1][33] There is little evidence to indicate that dietary flavonoids affect human cancer risk in general.[1]

### Cardiovascular diseases

Although no significant association has been found between flavan-3-ol intake and cardiovascular disease mortality, clinical trials have shown improved endothelial function and reduced blood pressure (with a few studies showing inconsistent results).[1] Reviews of cohort studies in 2013 found that the studies had too many limitations to determine a possible relationship between increased flavonoid intake and decreased risk of cardiovascular disease, although a trend for an inverse relationship existed.[1][34]

In 2013, the EFSA decided to permit health claims that 200 mg/day of cocoa flavanols "help[s] maintain the elasticity of blood vessels."[35][36] The FDA followed suit in 2023, stating that there is "supportive, but not conclusive" evidence that 200 mg per day of cocoa flavanols can reduce the risk of cardiovascular disease. This is greater than the levels found in typical chocolate bars, which can also contribute to weight gain, potentially harming cardiovascular health.[37][38]

## Synthesis, detection, quantification, and semi-synthetic alterations

### Color spectrum

Flavonoid synthesis in plants is induced by light color spectrums at both high and low energy radiations. Low energy radiations are accepted by phytochrome, while high energy radiations are accepted by carotenoids, flavins, cryptochromes in addition to phytochromes. The photomorphogenic process of phytochrome-mediated flavonoid biosynthesis has been observed in Amaranthus, barley, maize, Sorghum and turnip. Red light promotes flavonoid synthesis.[39]

### Availability through microorganisms

Research has shown production of flavonoid molecules from genetically engineered microorganisms.[40][41]

### Tests for detection

#### Shinoda test

Four pieces of magnesium filings are added to the ethanolic extract followed by few drops of concentrated hydrochloric acid. A pink or red colour indicates the presence of flavonoid.[42] Colours varying from orange to red indicated flavones, red to crimson indicated flavonoids, crimson to magenta indicated flavonones.

#### Sodium hydroxide test

About 5 mg of the compound is dissolved in water, warmed, and filtered. 10% aqueous sodium hydroxide is added to 2 ml of this solution. This produces a yellow coloration. A change in color from yellow to colorless on addition of dilute hydrochloric acid is an indication for the presence of flavonoids.[43]

#### p-Dimethylaminocinnamaldehyde test

A colorimetric assay based upon the reaction of A-rings with the chromogen p-dimethylaminocinnamaldehyde (DMACA) has been developed for flavanoids in beer that can be compared with the vanillin procedure.[44]

### Quantification

Lamaison and Carnet have designed a test for the determination of the total flavonoid content of a sample (AlCI3 method). After proper mixing of the sample and the reagent, the mixture is incubated for ten minutes at ambient temperature and the absorbance of the solution is read at 440 nm. Flavonoid content is expressed in mg/g of quercetin.[45][46]

### Semi-synthetic alterations

Immobilized Candida antarctica lipase can be used to catalyze the regioselective acylation of flavonoids.[47]

## See also

* Phytochemical
* List of antioxidants in food
* List of phytochemicals in food
* Phytochemistry
* Secondary metabolites
* Homoisoflavonoids, related chemicals with a 16 carbons skeleton

## References

1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 1.24 1.25 1.26 1.27 1.28 1.29 1.30 1.31 "Flavonoids". Micronutrient Information Center, Linus Pauling Institute, Oregon State University, Corvallis, Oregon. 2025. https://lpi.oregonstate.edu/mic/dietary-factors/phytochemicals/flavonoids.
2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 "The biological relevance of direct antioxidant effects of polyphenols for cardiovascular health in humans is not established". The Journal of Nutrition 141 (5): 989S–1009S. May 2011. doi:10.3945/jn.110.131490. PMID 21451125. https://www.sciencedirect.com/science/article/pii/S0022316622025688.
3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Panel on Dietetic Products, Nutrition and Allergies, European Food Safety Authority (8 April 2011). "Scientific Opinion on the substantiation of health claims related to: flavonoids and ascorbic acid in fruit juices, including berry juices (ID 1186); flavonoids from citrus (ID 1471); flavonoids from Citrus paradisi Macfad. (ID 3324, 3325); flavonoids (ID 1470, 1693, 1920); flavonoids in cranberry juice (ID 1804); carotenoids (ID 1496, 1621, 1622, 1796); polyphenols (ID 1636, 1637, 1640, 1641, 1642, 1643); rye bread (ID 1179); protein hydrolysate (ID 1646); carbohydrates with a low/reduced glycaemic load (ID 476, 477, 478, 479, 602) and carbohydrates which induce a low/reduced glycaemic response (ID 727, 1122, 1171); alfalfa (ID 1361, 2585, 2722, 2793); caffeinated carbohydrate‐containing energy drinks (ID 1272); and soups (ID 1132, 1133) pursuant to Article 13(1) of Regulation (EC) No 1924/2006". EFSA Journal 9 (4). doi:10.2903/j.efsa.2011.2082. https://efsa.onlinelibrary.wiley.com/doi/epdf/10.2903/j.efsa.2011.2082.
4. ↑ 4.0 4.1 "Analysis of Conformational, Structural, Magnetic, and Electronic Properties Related to Antioxidant Activity: Revisiting Flavan, Anthocyanidin, Flavanone, Flavonol, Isoflavone, Flavone, and Flavan-3-ol". ACS Omega 6 (13): 8908–8918. April 2021. doi:10.1021/acsomega.0c06156. PMID 33842761.
5. ↑ IUPAC Compendium of Chemical Terminology (2nd ed.), Oxford: Blackwell Scientific, 1997, doi:10.1351/goldbook.F02424, ISBN 978-0-9678550-9-7
6. ↑ (in en) Vitamins and Hormones, Volume 7. New York: Academic Press. 1949. https://books.google.com/books?id=hc1TyKSumOkC&pg=PA210.
7. ↑ (in en) Vitamin C: Volume I. CRC Press. January 10, 2018. ISBN 978-1-351-08601-1. https://books.google.com/books?id=1kgPEAAAQBAJ&pg=PA101.
8. ↑ "Flavonoids from carnation (Dianthus caryophyllus) and their antifungal activity". Phytochemistry Letters 1 (1): 44–48. 2008. doi:10.1016/j.phytol.2007.10.001. Bibcode: 2008PChL....1...44G.
9. ↑ "Biotechnology of flavonoids and other phenylpropanoid-derived natural products. Part I: Chemical diversity, impacts on plant biology and human health". Biotechnology Journal 2 (10): 1214–1234. October 2007. doi:10.1002/biot.200700084. PMID 17935117.
10. ↑ "Isolation of a UDP-glucose: Flavonoid 5-O-glucosyltransferase gene and expression analysis of anthocyanin biosynthetic genes in herbaceous peony (Paeonia lactiflora Pall.)". Electronic Journal of Biotechnology 15 (6). November 15, 2012. doi:10.2225/vol15-issue6-fulltext-7.
11. ↑ "Flavonoids: modulators of brain function?". The British Journal of Nutrition 99 (E Suppl 1): ES60–ES77. May 2008. doi:10.1017/S0007114508965776. PMID 18503736.
12. ↑ 12.0 12.1 "Sources of Flavonoids in the U.S. Diet Using USDA's Updated Database on the Flavonoid Content of Selected Foods". Agricultural Research Service, US Department of Agriculture. 2006. https://www.ars.usda.gov/ARSUserFiles/80400525/Articles/AICR06_flav.pdf.
13. ↑ "Flavonoids and Antioxidative Activities in Buckwheat". Journal of Agricultural and Food Chemistry 44 (7): 1746–1750. 1996. doi:10.1021/jf9508357.
14. ↑ "Determination of flavonoids in pulp and peel of mandarin fruits (table 1)". Agriculturae Conspectus Scientificus 74 (3): 223. 2009. http://www.agr.unizg.hr/smotra/pdf_74/acs74_38.pdf. Retrieved 2020-07-31.
15. ↑ "Gamma-irradiation induced changes in microbiological status, phenolic profile and antioxidant activity of peanut skin". Journal of Functional Foods 12: 129–143. 2015. doi:10.1016/j.jff.2014.10.034.
16. ↑ "Peanut skin color: a biomarker for total polyphenolic content and antioxidative capacities of peanut cultivars". International Journal of Molecular Sciences 10 (11): 4941–4952. November 2009. doi:10.3390/ijms10114941. PMID 20087468.
17. ↑ 17.0 17.1 17.2 17.3 "Flavonoid intake in European adults (18 to 64 years)". PLoS One 10 (5). 2015. doi:10.1371/journal.pone.0128132. PMID 26010916. Bibcode: 2015PLoSO..1028132V.
18. ↑ 18.0 18.1 "Estimated dietary flavonoid intake and major food sources of U.S. adults". The Journal of Nutrition 137 (5): 1244–1252. May 2007. doi:10.1093/jn/137.5.1244. PMID 17449588.
19. ↑ "Daily Value on the Nutrition and Supplement Facts Labels - Reference Guide: Daily Values for Nutrients". US Food and Drug Administration. 5 March 2024. https://www.fda.gov/food/nutrition-facts-label/daily-value-nutrition-and-supplement-facts-labels?here’s_what_to_expect=#referenceguide.
20. ↑ 20.0 20.1 EFSA Panel on Dietetic Products, Nutrition and Allergies (2010). "Scientific Opinion on the substantiation of health claims related to various food(s)/food constituent(s) and protection of cells from premature aging, antioxidant activity, antioxidant content and antioxidant properties, and protection of DNA, proteins and lipids from oxidative damage pursuant to Article 13(1) of Regulation (EC) No 1924/20061". EFSA Journal 8 (2): 1489. doi:10.2903/j.efsa.2010.1489.
21. ↑ "Consumption of flavonoid-rich foods and increased plasma antioxidant capacity in humans: cause, consequence, or epiphenomenon?". Free Radical Biology & Medicine 41 (12): 1727–1746. December 2006. doi:10.1016/j.freeradbiomed.2006.04.033. PMID 17157175.
22. ↑ "Flavonoids: antioxidants or signalling molecules?". Free Radical Biology & Medicine 36 (7): 838–849. April 2004. doi:10.1016/j.freeradbiomed.2004.01.001. PMID 15019969.
23. ↑ "Metabolism of anthocyanins by human gut microflora and their influence on gut bacterial growth". Journal of Agricultural and Food Chemistry 60 (15): 3882–3890. April 2012. doi:10.1021/jf3002153. PMID 22439618.
24. ↑ "FDA approved drug products". US Food and Drug Administration. http://www.accessdata.fda.gov/scripts/cder/drugsatfda/index.cfm.
25. ↑ "Health Claims Meeting Significant Scientific Agreement". US Food and Drug Administration. https://www.fda.gov/Food/IngredientsPackagingLabeling/LabelingNutrition/ucm2006876.htm#Approved_Health_Claims.
26. ↑ Hensley S (September 7, 2010). "FDA To Lipton: Tea Can't Do That". https://www.npr.org/sections/health-shots/2010/09/07/129701566/fda-to-lipton-tea-can-t-do-that.
27. ↑ "Cherry companies warned by FDA against making health claims". November 1, 2005. https://theproducenews.com/cherry-companies-warned-fda-against-making-health-claims.
28. ↑ "Warning letters to US supplement manufacturers (search "flavonoid")". US Food and Drug Administration. 28 August 2025. https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/compliance-actions-and-activities/warning-letters.
29. ↑ "Flavonoids as prospective compounds for anti-cancer therapy". The International Journal of Biochemistry & Cell Biology 45 (12): 2821–2831. December 2013. doi:10.1016/j.biocel.2013.10.004. PMID 24128857.
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31. ↑ "Recent advances in understanding the anti-diabetic actions of dietary flavonoids". The Journal of Nutritional Biochemistry 24 (11): 1777–1789. November 2013. doi:10.1016/j.jnutbio.2013.06.003. PMID 24029069.
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33. ↑ "Flavonoids and cancer prevention: a review of the evidence". Journal of Nutrition in Gerontology and Geriatrics 31 (3): 206–238. 2012. doi:10.1080/21551197.2012.702534. PMID 22888839.
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35. ↑ "Scientific Opinion on the substantiation of a health claim related to cocoa flavanols and maintenance of normal endothelium-dependent vasodilation pursuant to Article 13(5) of Regulation (EC) No 1924/2006". EFSA Journal 10 (7). June 27, 2012. doi:10.2903/j.efsa.2012.2809. https://www.efsa.europa.eu/pt/efsajournal/pub/2809. Retrieved June 17, 2023.
36. ↑ "Cocoa flavanol health claim becomes EU law". September 4, 2013. https://www.confectionerynews.com/Article/2013/09/04/Cocoa-flavanol-health-claim-becomes-EU-law.
37. ↑ Kavanaugh C (February 1, 2023). ﻿RE: Petition for a Qualified Health Claim – for Cocoa Flavanols and Reduced Risk of Cardiovascular Disease (Docket No. FDA-2019-Q-0806)﻿ (Report). FDA. https://www.fda.gov/media/165090/download.
38. ↑ Aubrey A (February 12, 2023). "Is chocolate good for your heart? Finally the FDA has an answer – kind of". https://www.npr.org/sections/health-shots/2023/02/12/1156044919/chocolate-heart-health-flavanols.
39. ↑ (in en) Modern Plant Physiology. CRC Press. January 2004. p. 457. ISBN 9780849317149. https://books.google.com/books?id=03S6VbTjCmUC.
40. ↑ "Metabolic engineering of the complete pathway leading to heterologous biosynthesis of various flavonoids and stilbenoids in Saccharomyces cerevisiae". Metabolic Engineering 11 (6): 355–366. November 2009. doi:10.1016/j.ymben.2009.07.004. PMID 19631278.
41. ↑ "Biotechnology of flavonoids and other phenylpropanoid-derived natural products. Part II: Reconstruction of multienzyme pathways in plants and microbes". Biotechnology Journal 2 (10): 1235–1249. October 2007. doi:10.1002/biot.200700184. PMID 17935118.
42. ↑ "Phytochemical Analysis and Antimicrobial Activity of Scoparia dulcis and Nymphaea lotus". Australian Journal of Basic and Applied Sciences 3 (4): 3975–3979. 2009. http://connection.ebscohost.com/c/articles/51366872/phytochemical-analysis-antimicrobial-activity-scoparia-dulcis-nymphaea-lotus.
43. ↑ "A bioactive flavonoid from Pavetta crassipes K. Schum.". Organic and Medicinal Chemistry Letters 1 (1): 14. October 2011. doi:10.1186/2191-2858-1-14. PMID 22373191.
44. ↑ "A New Colourimetric Assay for Flavanoids in Pilsner Beers". Journal of the Institute of Brewing 91: 37–40. 1985. doi:10.1002/j.2050-0416.1985.tb04303.x.
45. ↑ "Teneurs en principaux flavonoïdes des fleurs de Cratageus monogyna Jacq. et de Cratageus laevigata (Poiret D.C.) en fonction de la végétation". Plantes Medicinales: Phytotherapie 25: 12–16. 1991.
46. ↑ Khokhlova, Kateryna; Zdoryk, Oleksandr; Vyshnevska, Liliia (January 2020). "Chromatographic characterization on flavonoids and triterpenes of leaves and flowers of 15 crataegus L. species". Natural Product Research 34 (2): 317–322. doi:10.1080/14786419.2018.1528589. ISSN 1478-6427. PMID 30417671.
47. ↑ "Regioselective acylation of flavonoids catalyzed by immobilized Candida antarctica lipase under reduced pressure". Biotechnology Letters 26 (13): 1073–1076. July 2004. doi:10.1023/B:BILE.0000032967.23282.15. PMID 15218382.

## Further reading

* Flavonoids: Chemistry, Biochemistry and Applications. Boca Raton, FL: CRC Press, Taylor & Francis. 2006. ISBN 978-0-8493-2021-7.
* The science of flavonoids. New York: Springer. 2006. ISBN 978-0-387-74550-3.
* Comparative Biochemistry of the Flavonoids. 1967. https://books.google.com/books?id=CyTf2oObc7cC.
* "The systematic identification of flavonoids". Journal of Molecular Structure 10 (2): 320. 1971. doi:10.1016/0022-2860(71)87109-0.

### Databases

* USDA Database for the Flavonoid Content of Selected Foods, Release 3.1 (December 2013); data for 506 foods in the 5 subclasses of flavonoids provided in a separate PDF updated May 2014
* FlavoDB, Bioinformatics Centre, India, November 2019

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Types of flavonoids

Flavonoids|

| Anthoxanthins|

| Flavones|

* Apigenin, Chrysin, et.c.

|

Flavonols|

* Quercetin, Kaempferol, et.c.

Isoflavones|

* Daidzein, Genistein, Orobol et.c.

Neoflavonoids|

* Dalbergichromene

Flavans|

| Flavan|

* Luteoliflavan

|

Flavan-3-ols
(flavanols)|

* Catechin, Gallocatechol, et.c.

Flavan-4-ols
(flavanols)|

* Apiforol, Luteoforol, et.c.

Flavan-3,4-diols|

* Leucocyanidin, Leucodelphinidin, et.c.

Flavanones|

* Hesperidin
* Naringenin
* Eriodictyol

Flavanonols|

* Taxifolin
* Aromadendrin, et.c.

Anthocyanidins|

| 3-deoxyanthocyanidins|

* Cyanidin, Delphinidin, et.c.

|

3-hydroxyanthocyanidin|

* Apigeninidin, Guibourtinidin, et.c.

Aurones|

* Aureusidin
* Leptosidin

Chalcones|

| Chalcones|

* Butein, Isoliquiritigenin, et.c.

|

Dihydrochalcone|

* Phloretin

Miscellaneous|

* List of phytochemicals in food
* C-methylated flavonoids
* O-methylated flavonoids
* Furanoflavonoids
* Pyranoflavonoids
* Prenylflavonoids
* Methylenedioxy
* Castavinols

Flavonoid biosynthesis

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Types of phenylpropanoids

Classes of phenylpropanoids|

* Hydroxycinnamic acids
* Chromones (Furanochromones)
* Cinnamaldehydes
* Monolignols
* Coumarins
* Chalcones
* Flavonoids
* Phenylpropenes
* Stilbenoids
* Lignans
* Lignins
* Suberins

Examples|

* Rhododendrin

Template:Polyphenol

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