The name derives from the Ancient Greek word πολύς (polus, 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 as is found in alcohols (hence the -ol suffix). The term "polyphenol" has been in use at least since 1894.
The term polyphenol is not well-defined, but is generally agreed that they are natural products "having a polyphenol structure (i.e., several hydroxyl groups on aromatic rings)" including four principal classes: "phenolic acids, flavonoids, stilbenes, and lignans".
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 teatheaflavin-3-gallate shown below, and the hydrolyzable tannin, tannic acid.
Polyphenols are often 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.
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.
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.
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.
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. 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, thiols 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.
Some polyphenols are traditionally used as dyes. For instance, in the Indian subcontinent, the pomegranatepeel, high in tannins and other polyphenols, or its juice, is employed in the dyeing of non-synthetic fabrics.
Polyphenols, especially tannins, were used traditionally for tanning leather and today also as precursors in green chemistry notably to produce plastics or resins by polymerisation with or without the use of formaldehyde or adhesives for particleboards. The aims are generally to make use of plant residues from grape, olive (called pomaces) or pecan shells left after processing.
Signaling molecules in ripening and other growth processes.
Occurrence in nature
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.
High levels of polyphenols in some woods can explain their natural preservation against rot.
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. 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.
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.
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.
A review published in 2005 found that astringency increases and bitterness decreases 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 chiralisomercatechin. In contrast, hydroxycinnamic acids do not have astringent qualities, but are bitter.
Polyphenols are a large, diverse group of compounds, making it difficult to determine their biological effects. 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. 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 – characteristics which have not been determined for polyphenols.
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; 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.
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.
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.
Higher intakes of soy isoflavones may be associated with reduced risks of breast cancer in postmenopausal women and prostate cancer in men.
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. 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.
Isoflavones, which are structurally related to 17β-estradiol, are classified as phytoestrogens. A risk assessment by the European Food Safety Authority found no cause for concern when isoflavones are consumed in a normal diet.
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. 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 affect beyond nutrient-drug interactions is unlikely.
^Martín Santos MA, Bonilla Venceslada JL, Martín Martín A, García García I (2005). "Estimating the selectivity of ozone in the removal of polyphenols from vinasse". Journal of Chemical Technology and Biotechnology. 80 (4): 433–438. doi:10.1002/jctb.1222. INIST:16622840.
^Owen RW, Haubner R, Hull WE, Erben G, Spiegelhalder B, Bartsch H, Haber B (December 2003). "Isolation and structure elucidation of the major individual polyphenols in carob fibre". Food and Chemical Toxicology. 41 (12): 1727–1738. doi:10.1016/S0278-6915(03)00200-X. PMID14563398.
^Pan X (2003). "Microwave-assisted extraction of tea polyphenols and tea caffeine from green tea leaves". Chemical Engineering and Processing. 42 (2): 129–133. doi:10.1016/S0255-2701(02)00037-5.
^ abcdAizpurua-Olaizola O, Ormazabal M, Vallejo A, Olivares M, Navarro P, Etxebarria N, Usobiaga A (January 2015). "Optimization of supercritical fluid consecutive extractions of fatty acids and polyphenols from Vitis vinifera grape wastes". Journal of Food Science. 80 (1): E101–E107. doi:10.1111/1750-3841.12715. PMID25471637.
^Alonso-Salces RM, Korta E, Barranco A, Berrueta LA, Gallo B, Vicente F (November 2001). "Pressurized liquid extraction for the determination of polyphenols in apple". Journal of Chromatography A. 933 (1–2): 37–43. doi:10.1016/S0021-9673(01)01212-2. PMID11758745.
^Sineiro J, Domínguez H, Núñez MJ, Lema JM (1996). "Ethanol extraction of polyphenols in an immersion extractor. Effect of pulsing flow". Journal of the American Oil Chemists' Society. 73 (9): 1121–1125. doi:10.1007/BF02523372. S2CID96009875.
^Arranz S, Saura-Calixto F, Shaha S, Kroon PA (August 2009). "High contents of nonextractable polyphenols in fruits suggest that polyphenol contents of plant foods have been underestimated". Journal of Agricultural and Food Chemistry. 57 (16): 7298–7303. doi:10.1021/jf9016652. hdl:10261/82508. PMID19637929.
^Nawaz H, Shi J, Mittal GS, Kakuda Y (2006). "Extraction of polyphenols from grape seeds and concentration by ultrafiltration". Separation and Purification Technology. 48 (2): 176–181. doi:10.1016/j.seppur.2005.07.006.
^Gani M, Mcguinness BJ, Da Vies AP (1998). "Monoclonal antibodies against tea polyphenols: A novel immunoassay to detect polyphenols in biological fluids". Food and Agricultural Immunology. 10: 13–22. doi:10.1080/09540109809354964.
^Walker RB, Everette JD (February 2009). "Comparative reaction rates of various antioxidants with ABTS radical cation". Journal of Agricultural and Food Chemistry. 57 (4): 1156–1161. doi:10.1021/jf8026765. PMID19199590.
^Roy MK, Koide M, Rao TP, Okubo T, Ogasawara Y, Juneja LR (March 2010). "ORAC and DPPH assay comparison to assess antioxidant capacity of tea infusions: relationship between total polyphenol and individual catechin content". International Journal of Food Sciences and Nutrition. 61 (2): 109–124. doi:10.3109/09637480903292601. PMID20109129. S2CID1929167.
^Pulido R, Bravo L, Saura-Calixto F (August 2000). "Antioxidant activity of dietary polyphenols as determined by a modified ferric reducing/antioxidant power assay". Journal of Agricultural and Food Chemistry. 48 (8): 3396–3402. doi:10.1021/jf9913458. hdl:10261/112476. PMID10956123.
^Meyer AS, Yi OS, Pearson DA, Waterhouse AL, Frankel EN (1997). "Inhibition of Human Low-Density Lipoprotein Oxidation in Relation to Composition of Phenolic Antioxidants in Grapes (Vitis vinifera)". Journal of Agricultural and Food Chemistry. 45 (5): 1638–1643. doi:10.1021/jf960721a.
^Mello LD, Sotomayor MD, Kubota LT (2003). "HRP-based amperometric biosensor for the polyphenols determination in vegetables extract". Sensors and Actuators B: Chemical. 96 (3): 636–645. doi:10.1016/j.snb.2003.07.008.
^Pizzi A, Valenezuela J, Westermeyer C (1994). "Low formaldehyde emission, fast pressing, pine and pecan tannin adhesives for exterior particleboard". Holz Als Roh- und Werkstoff. 52 (5): 311–315. doi:10.1007/BF02621421. S2CID36500389.
^Krasnow MN, Murphy TM (June 2004). "Polyphenol glucosylating activity in cell suspensions of grape (Vitis vinifera)". Journal of Agricultural and Food Chemistry. 52 (11): 3467–3472. doi:10.1021/jf035234r. PMID15161217.
^Malek SR (1961). "Polyphenols and their quinone derivatives in the cuticle of the desert locust, Schistocerca gregaria (Forskål)". Comparative Biochemistry and Physiology. 2: 35–77. doi:10.1016/0010-406X(61)90071-8.
^Hufnagel JC, Hofmann T (February 2008). "Orosensory-directed identification of astringent mouthfeel and bitter-tasting compounds in red wine". Journal of Agricultural and Food Chemistry. 56 (4): 1376–1386. doi:10.1021/jf073031n. PMID18193832.
^ abNachvak, Seyed Mostafa; Moradi, Shima; Anjom-Shoae, Javad; et al. (September 2019). "Soy, Soy Isoflavones, and Protein Intake in Relation to Mortality from All Causes, Cancers, and Cardiovascular Diseases: A Systematic Review and Dose-Response Meta-Analysis of Prospective Cohort Studies". Journal of the Academy of Nutrition and Dietetics. 119 (9): 1483–1500.e17. doi:10.1016/j.jand.2019.04.011. ISSN2212-2672. PMID31278047.