Flavan-3-ols (sometimes referred to asflavanols) are a subgroup offlavonoids. They are derivatives offlavans that possess a 2-phenyl-3,4-dihydro-2H-chromen-3-ol skeleton. Flavan-3-ols are structurally diverse and include a range of compounds, such ascatechin,epicatechin gallate,epigallocatechin,epigallocatechin gallate,proanthocyanidins,theaflavins,thearubigins. They play a part in plant defense and are present in the majority of plants.^[1]

The single-molecule (monomer) catechin, or isomer epicatechin (see diagram), adds four hydroxyls to flavan-3-ol, making building blocks for concatenated polymers (proanthocyanidins) and higher order polymers (anthocyanidins).^[2]

Flavan-3-ols possess two chiral carbons, meaning fourdiastereoisomers occur for each of them. They are distinguished from the yellow, ketone-containing flavonoids such asquercitin andrutin, which are calledflavonols. Early use of the termbioflavonoid was imprecisely applied to include theflavanols, which are distinguished by the absence of ketones. Catechin monomers, dimers, and trimers (oligomers) are colorless. Higher order polymers, anthocyanidins, exhibit deepening reds and becometannins.^[2]

Catechin andepicatechin areepimers, with (–)-epicatechin and (+)-catechin being the most common opticalisomers found in nature. Catechin was first isolated from the plant extractcatechu, from which it derives its name. Heating catechin past its point of decomposition releasespyrocatechol (also called catechol), which explains the common origin of the names of these compounds.

Epigallocatechin andgallocatechin contain an additional phenolichydroxyl group when compared toepicatechin andcatechin, respectively, similar to the difference inpyrogallol compared topyrocatechol.

Catechin gallates aregallic acidesters of the catechins; an example isepigallocatechin gallate, which is commonly the most abundant catechin in tea.Proanthocyanidins andthearubigins are oligomeric flavan-3-ols.

In contrast to many otherflavonoids, flavan-3-ols do not generally exist asglycosides in plants.^[3]

The flavonoids are products from acinnamoyl-CoA starter unit, with chain extension using three molecules ofmalonyl-CoA. Reactions are catalyzed by a type III PKS enzyme.^{[citation needed]} These enzymes usecoenzyme A esters, and have a single active site to perform the necessary series of reactions: chain extension, condensation, and cyclization. Chain extension of 4-hydroxycinnamoyl-CoA with three molecules of malonyl-CoA gives initially a polyketide (Figure 1), which can be folded. These allowClaisen-like reactions to occur, generatingaromatic rings.^[4][5]Fluorescence-lifetime imaging microscopy can be used to detect flavanols in plant cells.^[6]

Figure 1: Schematic overview of the flavan-3-ol (–)-epicatechin biosynthesis fromtyrosine (Tyr) orphenylalanine (Phe) in plants. Enzymes are indicated in blue, abbreviated as follows:

• E1:phenylalanine ammonia lyase
• E2:tyrosine ammonia lyase
• E3:cinnamate 4-hydroxylase
• E4:4-coumaroyl:CoA-ligase
• E5: chalcone synthase (naringenin-chalcone synthase)
• E6:chalcone isomerase
• E7:flavonoid 3′-hydroxylase
• E8:flavanone 3-hydroxylase
• E9:dihydroflavanol 4-reductase
• E10: anthocyanidin synthase (leucoanthocyanidin dioxygenase)
• E11:anthocyanidin reductase

Flavan-3-ols

Image	Name	Formula	Oligomers
	Catechin, C, (+)-Catechin	C15H14O6	Procyanidins
	Epicatechin, EC, (–)-Epicatechin (cis)	C15H14O6	Procyanidins
	Epigallocatechin, EGC	C15H14O7	Prodelphinidins
	Epicatechin gallate, ECG	C22H18O10
	Epigallocatechin gallate, EGCG, (–)-Epigallocatechin gallate	C22H18O11
	Epiafzelechin	C15H14O5
	Fisetinidol	C15H14O5
	Guibourtinidol	C15H14O4	Proguibourtinidins
	Mesquitol	C15H14O6
	Robinetinidol	C15H14O6	Prorobinetinidins

Flavan-3-ols are abundant inteas derived from the tea plantCamellia sinensis, as well as in somecocoas (made from the seeds ofTheobroma cacao), although the content is affected considerably by processing, especially inchocolate.^[8][9] Flavan-3-ols are also present in the human diet in fruits, in particularpome fruits,berries,vegetables, andwine.^[10] Their content in food is variable and affected by various factors, such ascultivar,processing, andpreparation.^[11]

Thebioavailability of flavan-3-ols depends on thefood matrix, type of compound and theirstereochemical configuration.^[3] While monomeric flavan-3-ols are readily taken up, oligomeric forms are not absorbed.^[3][12] Most data for human metabolism of flavan-3-ols are available for monomeric compounds, especiallyepiatechin. These compounds are taken up and metabolized upon uptake in thejejunum,^[13] mainly byO-methylation and glucuronidation,^[14] and then furthermetabolized by theliver. The colonicmicrobiome has also an important role in the metabolism of flavan-3-ols and they are catabolized to smaller compounds such as 5-(3′/4′-dihydroxyphenyl)-γ-valerolactones andhippuric acid.^[15][16] Only flavan-3-ols with an intact (epi)catechin moiety can be metabolized into 5-(3′/4′-dihydroxyphenyl)-γ-valerolactones (image in Gallery).^[17]

As catechins ingreen teaextract can behepatotoxic,Health Canada and EFSA have advised for caution,^[18] recommending intake should not exceed 800 mg per day.^[19]

Research has shown that flavan-3-ols may affectvascular function,blood pressure, andblood lipids, with only minor effects demonstrated, as of 2019.^[20][21] In 2015, theEuropean Commission approved ahealth claim forcocoa solids containing 200 mg of flavanols, stating that such intake "may contribute to maintenance of vascular elasticity and normal blood flow".^[22][23] As of 2022, food-based evidence indicates that intake of 400–600 mg per day of flavan-3-ols could have a small positive effect on cardiovascularbiomarkers.^[24]

• 
  Schematic representation of the flavan-3-ol (−)-epicatechin metabolism in humans as a function of time post-oral intake. SREM: structurally related (−)-epicatechin metabolites. 5C-RFM: 5-carbon ring fission metabolites. 3/1C-RFM: 3- and 1-carbon-side chain ring fission metabolites. The structures of the most abundant (−)-epicatechin metabolites present in the systemic circulation and in urine are depicted.^[16]
• 
  Flavan-3-ol precursors of the microbial metabolite 5-(3′/4′-dihydroxyphenyl)-γ-valerolactone (γVL). Only compounds with intact (epi)catechin moiety result in the formation of γVL by the intestinal microbiome. ECG, (−)-epicatechin-3-O-gallate; EGCG,Epigallocatechin gallate; EGC,Epigallocatechin.^[17]

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• Flavanols
• Nutrition
• Phytochemicals
• Polyphenols
• Flavonoids

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