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Polyphenols () are a large family of naturally occurring phenols. They are abundant in plants and structurally diverse. Polyphenols include , , tannic acid, and ellagitannin, some of which have been used historically as and for tanning garments.
In terms of properties, the WBSSH describes the polyphenols as follows:
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, 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.
Polyphenols often have functional groups beyond . Ether ester linkages are common, as are .
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.
Concentration can be made by ultrafiltration. Purification can be achieved by preparative chromatography.
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.
Some methods for quantification of total polyphenol content in vitro are based on colorimetry 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. Results are typically expressed as gallic acid equivalents. Polyphenols are seldom evaluated by antibody technologies.
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, and vitamin C. 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), ferric reducing ability of plasma (FRAP) assays or inhibition of copper-catalyzed in vitro human low-density lipoprotein oxidation.
New methods including the use of can help monitor the content of polyphenols in food.
Quantitation results produced by the mean of diode array detector–coupled HPLC are generally given as relative rather than as there is a lack of commercially available standards for all polyphenolic molecules.
Of some interest in the era of silver-based photography, pyrogallol and pyrocatechin are among the oldest photographic developers. (1998). 9780240802770 ISBN 9780240802770
Polyphenols are also found in animals. In , such as insects, and polyphenols play a role in epicuticle hardening (sclerotization). The hardening of the cuticle is due to the presence of a polyphenol oxidase. In crustaceans, there is a second oxidase activity leading to cuticle pigmentation. There is apparently no polyphenol tanning occurring in cuticle.
Flax and Myriophyllum spicatum (a submerged aquatic plant) secrete polyphenols that are involved in allelopathy interactions.
The glycosylated form develops from glucosyltransferase activity and increases the solubility of polyphenols.
Polyphenol oxidase (PPO) is an enzyme that catalyses the oxidation of to produce . 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.
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.
Some polyphenols are considered – compounds that interfere with the absorption of essential nutrients – especially iron and other metal ions, which may bind to and other proteins, particularly in .
In a comparison of cooking methods, phenolic and carotenoid levels in vegetables were retained better by steaming compared to frying. 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.
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. 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.
In the European Union, two were authorized between 2012 and 2015: 1) flavanols in at doses exceeding 200 mg per day may contribute to maintenance of vascular elasticity and normal blood flow; and 2) olive oil polyphenols (5 mg of hydroxytyrosol and its derivatives such as oleuropein complex and tyrosol) may "contribute to the protection of blood lipids from oxidative damage", if consumed daily.
As of 2022, that assessed the effect of polyphenols on health are limited, with results difficult to interpret due to the wide variation of intake values for both individual polyphenols and total polyphenols.
Polyphenols were once considered as antioxidants, but this concept is obsolete. 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 Cell signaling. Some polyphenols are considered to be bioactive compounds for which development of dietary recommendations was under consideration in 2017.
A 2019 systematic review found that intake of soy and soy isoflavones is associated with a lower risk of Mortality rate from gastric, colorectal, breast and lung cancers. 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.
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 , a class of polyphenol. The European Food Safety Authority established upper limits for some polyphenol-containing supplements and additives, such as green tea extract or curcumin. For most polyphenols found in the diet, an adverse effect beyond nutrient-drug interactions is unlikely.
Cardiovascular diseases
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.
Cancer
Higher intakes of soy isoflavones may be associated with reduced risks of breast cancer in postmenopausal women and prostate cancer in men.
Cognitive function
Polyphenols are under preliminary research for possible Cognition in healthy adults.
Phytoestrogens
, which are structurally related to 17β-estradiol, are classified as . A risk assessment by the European Food Safety Authority found no cause for concern when isoflavones are consumed in a normal diet.
Phlebotonic
Phlebotonics of heterogeneous composition, consisting partly of citrus peel (, such as hesperidin) and synthetic compounds, are used to treat chronic venous insufficiency and . Some are non-prescription dietary supplements, such as diosmin, while one other – Vasculera (Diosmiplex) – is a prescription medical food intended for treating venous disorders. Their mechanism of action is undefined, and clinical evidence of benefit for using phlebotonics to treat venous diseases is limited.
Gut microbiome
Polyphenols are extensively metabolized by the gut microbiota and are investigated as a potential metabolic factor in function of the gut microbiota.
Toxicity and adverse effects
of polyphenol intake range from mild (e.g., gastrointestinal tract symptoms) to severe (e.g., hemolytic anemia or hepatotoxicity). (2025). 9781118354407, Wiley. ISBN 9781118354407 In 1988, hemolytic anemia following polyphenol consumption was documented, resulting in the withdrawal of a catechin-containing drug. Polyphenols, particularly in beverages that contain them in high concentrations (tea, coffee, etc), inhibit the absorption of heme when consumed together in a single meal. Research is limited on the effect of this inhibition on Iron tests.
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