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Polyphenols (Template:IPAc-en) are a large family of naturally occurring phenols.<ref name="Quideau">Template:Cite journal</ref> They are abundant in plants and structurally diverse.<ref name="Quideau" /><ref name="lpi">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=Nonaka>Template:Cite journal</ref> Polyphenols include phenolic acids, flavonoids, tannic acid, and ellagitannin, some of which have been used historically as dyes and for tanning garments. Template:Toclimit
EtymologyEdit
The name derives from the Ancient Greek word {{#invoke:Lang|lang}} (Template:Transliteration, meaning "many, much") and the word ‘phenol’ which refers to a chemical structure formed by attachment of an aromatic benzenoid (phenyl) ring to a hydroxyl (-OH) group (hence the -ol suffix). The term "polyphenol" has been in use at least since 1894.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
DefinitionEdit
Polyphenols are natural products with "one or several hydroxyl groups on aromatic rings", including four principal classes: phenolic acids, flavonoids, stilbenes, and lignans.<ref name=lpi/><ref name="manach">Template:Cite journal</ref> Flavonoids can be grouped as flavones, flavonols, flavanols, flavanones, isoflavones, proanthocyanidins, and anthocyanins.<ref name=lpi/> Particularly abundant flavanoids in foods are catechin (tea, fruits), hesperetin (citrus fruits), cyanidin (red fruits and berries), daidzein (soybean), proanthocyanidins (apple, grape, cocoa), and quercetin (onion, tea, apples).<ref name="lpi" /> Polyphenols also include phenolic acids, such as caffeic acid, and lignans, which are derived from phenylalanine present in flax seed and other cereals.<ref name=lpi/>
WBSSH definitionEdit
The White–Bate-Smith–Swain–Haslam (WBSSH) definition<ref name="Haslam_and_Cai">Template:Cite journal</ref> characterized structural characteristics common to plant phenolics used in tanning (i.e., the tannins).<ref>Practical Polyphenolics, Edwin Haslam, 1998, Template:ISBN</ref>
In terms of properties, the WBSSH describes the polyphenols as follows:
- generally moderately water-soluble compounds
- with molecular weight of 500–4000 Da
- with >12 phenolic hydroxyl groups
- with 5–7 aromatic rings per 1000 Da
In terms of structures, the WBSSH recognizes two structural family that have these properties:
- proanthocyanidins and its derivatives
- galloyl and hexahydroxydiphenoyl esters and their derivatives
Quideau definitionEdit
According to Stéphane Quideau, the term "polyphenol" refers to compounds derived from the shikimate/phenylpropanoid and/or the polyketide pathway, featuring more than one phenolic unit and deprived of nitrogen-based functions.Template:Citation needed
Ellagic acid, a molecule at the core of naturally occurring phenolic compounds of varying sizes, is itself not a polyphenol by the WBSSH definition, but is by the Quideau definition. The raspberry ellagitannin,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> on the other hand, with its 14 gallic acid moieties (most in ellagic acid-type components), and more than 40 phenolic hydroxyl groups, meets the criteria of both definitions of a polyphenol. Other examples of compounds that fall under both the WBSSH and Quideau definitions include the black tea theaflavin-3-gallate shown below, and the hydrolyzable tannin, tannic acid.Template:Citation needed
ChemistryEdit
Polyphenols are reactive species toward oxidation, hence their description as antioxidants in vitro.<ref>Template:Cite journal</ref>
StructureEdit
Polyphenols, such as lignin, are larger molecules (macromolecules). Their upper molecular weight limit is about 800 daltons, which allows for the possibility to rapidly diffuse across cell membranes so that they can reach intracellular sites of action or remain as pigments once the cell senesces. Hence, many larger polyphenols are biosynthesized in situ from smaller polyphenols to non-hydrolyzable tannins and remain undiscovered in the plant matrix. Most polyphenols contain repeating phenolic moieties of pyrocatechol, resorcinol, pyrogallol, and phloroglucinol connected by esters (hydrolyzable tannins) or more stable C-C bonds (nonhydrolyzable condensed tannins). Proanthocyanidins are mostly polymeric units of catechin and epicatechin.
Polyphenols often have functional groups beyond hydroxyl groups. Ether ester linkages are common, as are carboxylic acids.
Analytical chemistryEdit
The analysis techniques are those of phytochemistry: extraction, isolation, structural elucidation,<ref>Template:Cite journal</ref> then quantification.Template:Citation needed
ReactivityEdit
Polyphenols readily react with metal ions to form coordination complexes, some of which form Metal-phenolic Networks.<ref>Template:Cite journal</ref>
ExtractionEdit
Extraction of polyphenols<ref>Template:Cite book</ref> can be performed using a solvent like water, hot water, methanol, methanol/formic acid, methanol/water/acetic or formic acid. Liquid–liquid extraction can be also performed or countercurrent chromatography. Solid phase extraction can also be made on C18 sorbent cartridges. Other techniques are ultrasonic extraction, heat reflux extraction, microwave-assisted extraction,<ref>Template:Cite journal</ref> critical carbon dioxide,<ref name=":0">Template:Cite journal</ref><ref>Template:Cite journal</ref> high-pressure liquid extraction<ref>Template:Cite journal</ref> or use of ethanol in an immersion extractor.<ref>Template:Cite journal</ref> The extraction conditions (temperature, extraction time, ratio of solvent to raw material, particle size of the sample, solvent type, and solvent concentrations) for different raw materials and extraction methods have to be optimized.<ref name=papoutsis2018a>Template:Cite journal</ref><ref name=papoutsis2018b>Template:Cite journal</ref>
Mainly found in the fruit skins and seeds, high levels of polyphenols may reflect only the measured extractable polyphenol (EPP) content of a fruit which may also contain non-extractable polyphenols. Black tea contains high amounts of polyphenol and makes up for 20% of its weight.<ref>Template:Cite journal</ref>
Concentration can be made by ultrafiltration.<ref>Template:Cite journal</ref> Purification can be achieved by preparative chromatography.
Analysis techniquesEdit
Phosphomolybdic acid is used as a reagent for staining phenolics in thin layer chromatography. Polyphenols can be studied by spectroscopy, especially in the ultraviolet domain, by fractionation or paper chromatography. They can also be analysed by chemical characterisation.
Instrumental chemistry analyses include separation by high performance liquid chromatography (HPLC), and especially by reversed-phase liquid chromatography (RPLC), can be coupled to mass spectrometry.<ref name=":0" />
Microscopy analysisEdit
The DMACA reagent is an histological dye specific to polyphenols used in microscopy analyses. The autofluorescence of polyphenols can also be used, especially for localisation of lignin and suberin. Where fluorescence of the molecules themselves is insufficient for visualization by light microscopy, DPBA (diphenylboric acid 2-aminoethyl ester, also referred to as Naturstoff reagent A) has traditionally been used, at least in plant science, to enhance the fluorescence signal.<ref>Template:Cite journalTemplate:Open access</ref>
QuantificationEdit
Polyphenolic content in vitro can be quantified by volumetric titration. An oxidizing agent, permanganate, is used to oxidize known concentrations of a standard tannin solution, producing a standard curve. The tannin content of the unknown is then expressed as equivalents of the appropriate hydrolyzable or condensed tannin.<ref>Template:Cite journal</ref>
Some methods for quantification of total polyphenol content in vitro are based on colorimetric measurements. Some tests are relatively specific to polyphenols (for instance the Porter's assay). Total phenols (or antioxidant effect) can be measured using the Folin–Ciocalteu reaction.<ref name=":0" /> Results are typically expressed as gallic acid equivalents. Polyphenols are seldom evaluated by antibody technologies.<ref>Template:Cite journal</ref>
Other tests measure the antioxidant capacity of a fraction. Some make use of the ABTS radical cation which is reactive towards most antioxidants including phenolics, thiols and vitamin C.<ref>Template:Cite journal</ref> During this reaction, the blue ABTS radical cation is converted back to its colorless neutral form. The reaction may be monitored spectrophotometrically. This assay is often referred to as the Trolox equivalent antioxidant capacity (TEAC) assay. The reactivity of the various antioxidants tested are compared to that of Trolox, which is a vitamin E analog.
Other antioxidant capacity assays which use Trolox as a standard include the diphenylpicrylhydrazyl (DPPH), oxygen radical absorbance capacity (ORAC),<ref>Template:Cite journal</ref> ferric reducing ability of plasma (FRAP)<ref>Template:Cite journal</ref> assays or inhibition of copper-catalyzed in vitro human low-density lipoprotein oxidation.<ref>Template:Cite journal</ref>
New methods including the use of biosensors can help monitor the content of polyphenols in food.<ref>Template:Cite journal</ref>
Quantitation results produced by the mean of diode array detector–coupled HPLC are generally given as relative rather than absolute values as there is a lack of commercially available standards for all polyphenolic molecules.Template:Citation needed
ApplicationsEdit
Some polyphenols are traditionally used as dyes in leather tanning. For instance, in the Indian subcontinent, pomegranate peel, high in tannins and other polyphenols, or its juice, is employed in the dyeing of non-synthetic fabrics.<ref name="jindal2004">Template:Cite book</ref>
Of some interest in the era of silver-based photography, pyrogallol and pyrocatechin are among the oldest photographic developers.<ref>Template:Cite book</ref><ref name="Gernsheim & Gernsheim">Template:Cite book</ref>
Aspirational use as green chemicalsEdit
Natural polyphenols have long been proposed as renewable precursors to produce plastics or resins by polymerization with formaldehyde,<ref>Template:Cite journal</ref> as well as adhesives for particleboards.<ref>Template:Cite journal</ref> The aims are generally to make use of plant residues from grape, olive (called pomaces), or pecan shells left after processing.<ref name=":0" />Template:Better source needed
OccurrenceEdit
The most abundant polyphenols are the condensed tannins, found in virtually all families of plants. Larger polyphenols are often concentrated in leaf tissue, the epidermis, bark layers, flowers and fruits but also play important roles in the decomposition of forest litter, and nutrient cycles in forest ecology. Absolute concentrations of total phenols in plant tissues differ widely depending on the literature source, type of polyphenols and assay; they are in the range of 1–25% total natural phenols and polyphenols, calculated with reference to the dry green leaf mass.<ref>Template:Cite journal</ref>
Polyphenols are also found in animals. In arthropods, such as insects,<ref>Template:Cite journal</ref> and crustaceans<ref>Template:Cite journal</ref> polyphenols play a role in epicuticle hardening (sclerotization). The hardening of the cuticle is due to the presence of a polyphenol oxidase.<ref>Template:Cite journal</ref> In crustaceans, there is a second oxidase activity leading to cuticle pigmentation.<ref>Template:Cite journal</ref> There is apparently no polyphenol tanning occurring in arachnids cuticle.<ref>Template:Cite journal</ref>
BiochemistryEdit
Polyphenols are thought to play diverse roles in the ecology of plants. These functions include:<ref>V. Lattanzio et al. (2006). "Role of phenolics in the resistance mechanisms of plants against fungal pathogens and insects" (and references therein). Phytochemistry: Advances in Research, 23–67. Template:ISBN.</ref>
- Release and suppression of growth hormones such as auxin.
- UV screens to protect against ionizing radiation and to provide coloration (plant pigments).<ref name="manach" />
- Deterrence of herbivores (sensory properties).
- Prevention of microbial infections (phytoalexins).<ref name="manach" /><ref>Template:Cite journal</ref>
- Signaling molecules in ripening and other growth processes.
- In some woods can explain their natural preservation against rot.<ref>Template:Cite journal</ref>
Flax and Myriophyllum spicatum (a submerged aquatic plant) secrete polyphenols that are involved in allelopathic interactions.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Biosynthesis and metabolismEdit
Polyphenols incorporate smaller parts and building blocks from simpler natural phenols, which originate from the phenylpropanoid pathway for the phenolic acids or the shikimic acid pathway for gallotannins and analogs. Flavonoids and caffeic acid derivatives are biosynthesized from phenylalanine and malonyl-CoA. Complex gallotannins develop through the in vitro oxidation of 1,2,3,4,6-pentagalloylglucose or dimerization processes resulting in hydrolyzable tannins. For anthocyanidins, precursors of the condensed tannin biosynthesis, dihydroflavonol reductase and leucoanthocyanidin reductase (LAR) are crucial enzymes with subsequent addition of catechin and epicatechin moieties for larger, non-hydrolyzable tannins.<ref>Template:Cite journal</ref>
The glycosylated form develops from glucosyltransferase activity and increases the solubility of polyphenols.<ref>Template:Cite journal</ref>
Polyphenol oxidase (PPO) is an enzyme that catalyses the oxidation of o-diphenols to produce o-quinones. It is the rapid polymerisation of o-quinones to produce black, brown or red polyphenolic pigments that causes fruit browning. In insects, PPO is involved in cuticle hardening.<ref>Template:Cite journal</ref>
Occurrence in foodEdit
Template:See also {{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}
Polyphenols comprise up to 0.2–0.3% fresh weight for many fruits. Consuming common servings of wine, chocolate, legumes or tea may also contribute to about one gram of intake per day.<ref name="lpi" /><ref>Template:Cite journal</ref> According to a 2005 review on polyphenols:
The most important food sources are commodities widely consumed in large quantities such as fruit and vegetables, green tea, black tea, red wine, coffee, chocolate, olives, and extra virgin olive oil. Herbs and spices, nuts and algae are also potentially significant for supplying certain polyphenols. Some polyphenols are specific to particular food (flavanones in citrus fruit, isoflavones in soya, phloridzin in apples); whereas others, such as quercetin, are found in all plant products such as fruit, vegetables, cereals, leguminous plants, tea, and wine.<ref name="BioavailRev2005">Template:Cite journal</ref>
Some polyphenols are considered antinutrients – compounds that interfere with the absorption of essential nutrients – especially iron and other metal ions, which may bind to digestive enzymes and other proteins, particularly in ruminants.<ref>Template:Cite journal</ref>
In a comparison of cooking methods, phenolic and carotenoid levels in vegetables were retained better by steaming compared to frying.<ref>Template:Cite journal</ref> Polyphenols in wine, beer and various nonalcoholic juice beverages can be removed using finings, substances that are usually added at or near the completion of the processing of brewing.Template:Citation needed
AstringencyEdit
With respect to food and beverages, the cause of astringency is not fully understood, but it is measured chemically as the ability of a substance to precipitate proteins.<ref>Staff, Sensory Society. Basic Tastes: Astringency Template:Webarchive</ref>
Astringency increases and bitterness decrease with the mean degree of polymerization. For water-soluble polyphenols, molecular weights between 500 and 3000 were reported to be required for protein precipitation. However, smaller molecules might still have astringent qualities likely due to the formation of unprecipitated complexes with proteins or cross-linking of proteins with simple phenols that have 1,2-dihydroxy or 1,2,3-trihydroxy groups.<ref>Template:Cite journal</ref> Flavonoid configurations can also cause significant differences in sensory properties, e.g., epicatechin, is more bitter and astringent than its chiral isomer catechin. In contrast, hydroxycinnamic acids do not have astringent qualities, but are bitter.<ref>Template:Cite journal</ref>
ResearchEdit
Polyphenols are a large, diverse group of compounds, making it difficult to determine their biological effects.<ref>Template:Cite journal</ref> They are not considered nutrients, as they are not used for growth, survival or reproduction, nor do they provide dietary energy. Therefore, they do not have recommended daily intake levels, as exist for vitamins, minerals, and fiber.<ref name="efsa-nut">Template:Cite journal</ref><ref name="nhs">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="usda">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In the United States, the Food and Drug Administration issued guidance to manufacturers that polyphenols cannot be mentioned on food labels as antioxidant nutrients unless physiological evidence exists to verify such a qualification and a Dietary Reference Intake value has been established Template:Ndash characteristics which have not been determined for polyphenols.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="nutra">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
In the European Union, two health claims were authorized between 2012 and 2015: 1) flavanols in cocoa solids at doses exceeding 200 mg per day may contribute to maintenance of vascular elasticity and normal blood flow;<ref name="EC-cocoa">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=":02">Template:Cite journal</ref> 2) olive oil polyphenols (5 mg of hydroxytyrosol and its derivatives (e.g. oleuropein complex and tyrosol) may "contribute to the protection of blood lipids from oxidative damage", if consumed daily.<ref name="EC-olive">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=":1">Template:Cite journal</ref>
As of 2022, clinical trials that assessed the effect of polyphenols on health biomarkers are limited, with results difficult to interpret due to the wide variation of intake values for both individual polyphenols and total polyphenols.<ref>Template:Cite journal</ref>
Template:AnchorPolyphenols were once considered as antioxidants, but this concept is obsolete.<ref>Template:Cite journal</ref> Most polyphenols are metabolized by catechol-O-methyltransferase, and therefore do not have the chemical structure allowing antioxidant activity in vivo; they may exert biological activity as signaling molecules.<ref name=lpi/><ref name=nutra/><ref>Template:Cite journal</ref> Some polyphenols are considered to be bioactive compounds<ref>Template:Cite journal</ref> for which development of dietary recommendations was under consideration in 2017.<ref>Template:Cite journal</ref>
Cardiovascular diseasesEdit
In the 1930s, polyphenols (then called vitamin P) were considered as a factor in capillary permeability, followed by various studies through the 21st century of a possible effect on cardiovascular diseases. For most polyphenols, there is no evidence for an effect on cardiovascular regulation, although there are some reviews showing a minor effect of consuming polyphenols, such as chlorogenic acid or flavan-3-ols, on blood pressure.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
CancerEdit
Higher intakes of soy isoflavones may be associated with reduced risks of breast cancer in postmenopausal women and prostate cancer in men.<ref name=lpi/>
A 2019 systematic review found that intake of soy and soy isoflavones is associated with a lower risk of mortality from gastric, colorectal, breast and lung cancers.<ref name="nach">Template:Cite journal</ref> The study found that an increase in isoflavone consumption by 10 mg per day was associated with a 7% decrease in risk from all cancers, and an increase in consumption of soy protein by 5 grams per day produced a 12% reduction in breast cancer risk.<ref name=nach/>
Cognitive functionEdit
Polyphenols are under preliminary research for possible cognitive effects in healthy adults.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
PhytoestrogensEdit
Isoflavones, which are structurally related to 17β-estradiol, are classified as phytoestrogens.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> A risk assessment by the European Food Safety Authority found no cause for concern when isoflavones are consumed in a normal diet.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
PhlebotonicEdit
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Phlebotonics of heterogeneous composition, consisting partly of citrus peel extracts (flavonoids, such as hesperidin) and synthetic compounds, are used to treat chronic venous insufficiency and hemorrhoids.<ref name="drugs">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Some are non-prescription dietary supplements, such as diosmin,<ref name="drugs" /> while one other – Vasculera (Diosmiplex) – is a prescription medical food intended for treating venous disorders.<ref name="dailymed-diosmiplex">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Their mechanism of action is undefined,<ref name="drugs" /> and clinical evidence of benefit for using phlebotonics to treat venous diseases is limited.<ref name="drugs" />
Gut microbiomeEdit
Polyphenols are extensively metabolized by the gut microbiota and are investigated as a potential metabolic factor in function of the gut microbiota.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Toxicity and adverse effectsEdit
Adverse effects of polyphenol intake range from mild (e.g., gastrointestinal tract symptoms)<ref name=lpi/> to severe (e.g., hemolytic anemia or hepatotoxicity).<ref name="davies">Template:Cite book</ref> In 1988, hemolytic anemia following polyphenol consumption was documented, resulting in the withdrawal of a catechin-containing drug.<ref>Template:Cite journal</ref> Polyphenols, particularly in beverages that contain them in high concentrations (tea, coffee, etc), inhibit the absorption of non-haem iron when consumed together in a single meal.<ref name=lpi/><ref name="SACN">Template:Cite book</ref><ref name="WHO">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="CDC">Template:Cite journal</ref> Research is limited on the effect of this inhibition on iron status.<ref name="SACN quote">Template:Cite book</ref>
Metabolism of polyphenols can result in flavonoid-drug interactions, such as in grapefruit–drug interactions, which involves inhibition of the liver enzyme, CYP3A4, likely by grapefruit furanocoumarins, a class of polyphenol.<ref name=lpi/><ref name="davies" /> The European Food Safety Authority established upper limits for some polyphenol-containing supplements and additives, such as green tea extract or curcumin.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> For most polyphenols found in the diet, an adverse effect beyond nutrient-drug interactions is unlikely.<ref name=lpi/>
See alsoEdit
- List of phytochemicals in food
- Dictionary of natural phenols and polyphenols molecular formulas
- Phytochemistry
- Polyphenolic proteins
- Secondary metabolites
ReferencesEdit
External linksEdit
Template:Phytochemical Template:Secondary metabolites Template:Natural phenol Template:Authority control