Chemistry:Proanthocyanidin

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Short description: Class of polyphenols found in many plants

Proanthocyanidins are a class of polyphenols found in many plants, such as cranberry, blueberry, and grape seeds. Chemically, they are oligomeric flavonoids. Many are oligomers of catechin and epicatechin and their gallic acid esters. More complex polyphenols, having the same polymeric building block, form the group of tannins.

Proanthocyanidins were discovered in 1947 by Jacques Masquelier, who developed and patented techniques for the extraction of oligomeric proanthocyanidins from pine bark and grape seeds.[1] Proanthocyanidins are under preliminary research for the potential to reduce the risk of urinary tract infections (UTIs) by consuming cranberries, grape seeds or red wine.[2][3]

Distribution in plants

Proanthocyanidins, including the lesser bioactive and bioavailable polymers (four or more catechins), represent a group of condensed flavan-3-ols, such as procyanidins, prodelphinidins and propelargonidins. They can be found in many plants, most notably apples, maritime pine bark and that of most other pine species, cinnamon,[4] aronia fruit, cocoa beans, grape seed, grape skin (procyanidins and prodelphinidins),[5] and red wines of Vitis vinifera (the European wine grape). However, bilberry, cranberry, black currant, green tea, black tea, and other plants also contain these flavonoids. Cocoa beans contain the highest concentrations.[6] Proanthocyanidins also may be isolated from Quercus petraea and Q. robur heartwood (wine barrel oaks).[7] Açaí oil, obtained from the fruit of the açaí palm (Euterpe oleracea), is rich in numerous procyanidin oligomers.[8]

Apples contain on average per serving about eight times the amount of proanthocyanidin found in wine, with some of the highest amounts found in the Red Delicious and Granny Smith varieties.[9]

An extract of maritime pine bark called Pycnogenol bears 65–75 percent proanthocyanidins (procyanidins).[10] Thus a 100 mg serving would contain 65 to 75 mg of proanthocyanidins (procyanidins).

Proanthocyanidin glycosides can be isolated from cocoa liquor.[11]

The seed testas of field beans (Vicia faba) contain proanthocyanidins[12] that affect the digestibility in piglets[13] and could have an inhibitory activity on enzymes.[14] Cistus salviifolius also contains oligomeric proanthocyanidins.[15]

Dietary source[16] Proanthocyanidin

(mg/100g)

Fruits
Grape seeds 3532
Blueberries 332
Apples 70–141
Pears 32–42
Nuts
Hazelnuts 501
Other
Cinnamon bark 8108
Sorghum grains 3965
Baking chocolate 1636
Red wine 313

Analysis

Condensed tannins may be characterised by a number of techniques including depolymerisation, asymmetric flow field flow fractionation or small-angle X-ray scattering.

DMACA is a dye that is particularly useful for localization of proanthocyanidin compounds in plant histology. The use of the reagent results in blue staining.[17] It can also be used to titrate proanthocyanidins.

Proanthocyanidins from field beans (Vicia faba)[18] or barley[19] have been estimated using the vanillin-HCl method, resulting in a red color of the test in the presence of catechins or proanthocyanidins.

Proanthocyanidins can be titrated using the Procyanidolic Index (also called the Bates-Smith Assay). It is a testing method that measures the change in color when the product is mixed with certain chemicals. The greater the color changes, the higher the PCOs content is. However, the Procyanidolic Index is a relative value that can measure well over 100. Unfortunately, a Procyanidolic Index of 95 was erroneously taken to mean 95% PCO by some and began appearing on the labels of finished products. All current methods of analysis suggest that the actual PCO content of these products is much lower than 95%.[20][unreliable medical source?]

Gel permeation chromatography (GPC) analysis allows separation of monomers from larger proanthocyanidin molecules.[21]

Monomers of proanthocyanidins can be characterized by analysis with HPLC and mass spectrometry.[22] Condensed tannins can undergo acid-catalyzed cleavage in the presence of a nucleophile like phloroglucinol (reaction called phloroglucinolysis), thioglycolic acid (thioglycolysis), benzyl mercaptan or cysteamine (processes called thiolysis[23]) leading to the formation of oligomers that can be further analyzed.[24]

Tandem mass spectrometry can be used to sequence proanthocyanidins.[25]

Oligomeric proanthocyanidins

Oligomeric proanthocyanidins (OPC) strictly refer to dimer and trimer polymerizations of catechins. OPCs are found in most plants and thus are common in the human diet. Especially the skin, seeds, and seed coats of purple or red pigmented plants contain large amounts of OPCs.[6] They are dense in grape seeds and skin, and therefore in red wine and grape seed extract, cocoa, nuts and all Prunus fruits (most concentrated in the skin), and in the bark of Cinnamomum (cinnamon)[4] and Pinus pinaster (pine bark; formerly known as Pinus maritima), along with many other pine species. OPCs also can be found in blueberries, cranberries (notably procyanidin A2),[26] aronia,[27] hawthorn, rosehip, and sea buckthorn.[28]

Oligomeric proanthocyanidins can be extracted via Vaccinium pahalae from in vitro cell culture.[29] The US Department of Agriculture maintains a database of botanical and food sources of proanthocyanidins.[6]

Plant defense

In nature, proanthocyanidins serve among other chemical and induced defense mechanisms against plant pathogens and predators, such as occurs in strawberries.[30]

Bioavailability

Proanthocyanidin has low bioavailability, with 90% remaining unabsorbed from the intestines until metabolized by gut flora to the more bioavailable metabolites.[16]

Non-oxidative chemical depolymerisation

Condensed tannins can undergo acid-catalyzed cleavage in the presence of (or an excess of) a nucleophile[31] like phloroglucinol (reaction called phloroglucinolysis), benzyl mercaptan (reaction called thiolysis), thioglycolic acid (reaction called thioglycolysis) or cysteamine. Flavan-3-ol compounds used with methanol produce short-chain procyanidin dimers, trimers, or tetramers which are more absorbable.[32]

These techniques are generally called depolymerisation and give information such as average degree of polymerisation or percentage of galloylation. These are SN1 reactions, a type of substitution reaction in organic chemistry, involving a carbocation intermediate under strongly acidic conditions in polar protic solvents like methanol. The reaction leads to the formation of free and derived monomers that can be further analyzed or used to enhance procyanidin absorption and bioavailability.[32] The free monomers correspond to the terminal units of the condensed tannins chains.

In general, reactions are made in methanol, especially thiolysis, as benzyl mercaptan has a low solubility in water. They involve a moderate (50 to 90 °C) heating for a few minutes. Epimerisation may happen.

Phloroglucinolysis can be used for instance for proanthocyanidins characterisation in wine or in grape seeds and skin.[33]

Thioglycolysis can be used to study proanthocyanidins[34] or the oxidation of condensed tannins.[35] It is also used for lignin quantitation.[36] Reaction on condensed tannins from Douglas fir bark produces epicatechin and catechin thioglycolates.[37]

Condensed tannins from Lithocarpus glaber leaves have been analysed through acid-catalyzed degradation in the presence of cysteamine.[38]

Research

Urinary tract infections

Cranberries have A2-type proanthocyanidins (PACs) which may be important for the ability of PACs to bind to proteins, such as the adhesins present on E. coli fimbriae and were thought to inhibit bacterial infections, such as urinary tract infections (UTIs).[39] Clinical trials have produced mixed results when asking the question to confirm that PACs, particularly from cranberries, were an alternative to antibiotic prophylaxis for UTIs: 1) a 2014 scientific opinion by the European Food Safety Authority rejected physiological evidence that cranberry PACs have a role in inhibiting bacterial pathogens involved in UTIs;[2] 2) an updated 2023 Cochrane Collaboration review supported the use of cranberry products for the prevention of UTIs for certain groups.[3]

A 2017 systematic review concluded that cranberry products significantly reduced the incidence of UTIs, indicating that cranberry products may be effective particularly for individuals with recurrent infections.[40] In 2019, the American Urological Association released guidelines stating that a moderate level of evidence supports the use of cranberry products containing PACs for possible prevention from recurrent UTIs.[41]

Wine consumption

Proanthocyanidins are the principal polyphenols in red wine that are under research to assess risk of coronary heart disease and lower overall mortality.[42] With tannins, they also influence the aroma, flavor, mouth-feel and astringency of red wines.[43][44]

In red wines, total OPC content, including flavan-3-ols (catechins), was substantially higher (177  mg/L) than that in white wines (9  mg/L).[45]

Other

Proanthocyanidins found in the proprietary extract of maritime pine bark called Pycnogenol were not found (in 2012) to be effective as a treatment for any disease:

"Current evidence is insufficient to support Pycnogenol(®) use for the treatment of any chronic disorder. Well-designed, adequately powered trials are needed to establish the value of this treatment."[46]

Sources

Proanthocyanidins are present in fresh grapes, juice, red wine, and other darkly pigmented fruits such as cranberry, blackcurrant, elderberry, and aronia.[47] Although red wine may contain more proanthocyanidins by mass per unit of volume than does red grape juice, red grape juice contains more proanthocyanidins per average serving size. An eight US fluid ounces (240 ml) serving of grape juice averages 124 milligrams proanthocyanidins, whereas a five US fluid ounces (150 ml) serving of red wine averages 91 milligrams (i.e., 145.6 milligrams per 8 fl. oz. or 240 mL).[6] Many other foods and beverages may also contain proanthocyanidins, but few attain the levels found in red grape seeds and skins,[6] with a notable exception being aronia, which has the highest recorded level of proanthocyanidins among fruits assessed to date (664 milligrams per 100 g).[47]

See also

References

  1. ↑ Schwitters, Bert (1995). OPC in Practice. Publishing rights search incomplete. p. 15. ISBN 978-8886035132. 
  2. ↑ 2.0 2.1 "Scientific Opinion on the substantiation of a health claim related to CranMax® and reduction of the risk of urinary tract infection by inhibiting the adhesion of certain bacteria in the urinary tract pursuant to Article 14 of Regulation (EC) No 1924/2006". EFSA Journal 12 (5): 3657 (11 pgs). 2014. doi:10.2903/j.efsa.2014.3657. "A cause and effect relationship has not been established between the consumption of CranMax® and reduction of the risk of urinary tract infection by inhibiting the adhesion of certain bacteria in the urinary tract". 
  3. ↑ 3.0 3.1 Williams, Gabrielle; Stothart, Christopher I.; Hahn, Deirdre; Stephens, Jacqueline H.; Craig, Jonathan C.; Hodson, Elisabeth M. (2023-11-10). "Cranberries for preventing urinary tract infections". The Cochrane Database of Systematic Reviews 2023 (11): CD001321. doi:10.1002/14651858.CD001321.pub7. ISSN 1469-493X. PMID 37947276. 
  4. ↑ 4.0 4.1 "New identification of proanthocyanidins in cinnamon (Cinnamomum zeylanicum L.) using MALDI-TOF/TOF mass spectrometry". Analytical and Bioanalytical Chemistry 402 (3): 1327–1336. 2012. doi:10.1007/s00216-011-5557-3. PMID 22101466. 
  5. ↑ Souquet, J; Cheynier, Véronique; Brossaud, Franck; Moutounet, Michel (1996). "Polymeric proanthocyanidins from grape skins". Phytochemistry 43 (2): 509–512. doi:10.1016/0031-9422(96)00301-9. Bibcode: 1996PChem..43..509S. 
  6. ↑ 6.0 6.1 6.2 6.3 6.4 "USDA Database for the Proanthocyanidin Content of Selected Foods – 2004". USDA. 2004. http://www.ars.usda.gov/SP2UserFiles/Place/12354500/Data/PA/PA.pdf. 
  7. ↑ Vivas, N; Nonier, M; Pianet, I; Vivasdegaulejac, N; Fouquet, E (2006). "Proanthocyanidins from Quercus petraea and Q. robur heartwood: quantification and structures". Comptes Rendus Chimie 9: 120–126. doi:10.1016/j.crci.2005.09.001. https://comptes-rendus.academie-sciences.fr/chimie/articles/10.1016/j.crci.2005.09.001/. 
  8. ↑ "Chemical composition, antioxidant properties, and thermal stability of a phytochemical enriched oil from Acai (Euterpe oleracea Mart.)". J Agric Food Chem 56 (12): 4631–4636. Jun 2008. doi:10.1021/jf800161u. PMID 18522407. 
  9. ↑ Hammerstone, John F.; Lazarus, Sheryl A.; Schmitz, Harold H. (August 2000). "Procyanidin content and variation in some commonly consumed foods". The Journal of Nutrition 130 (8S Suppl): 2086S–2092S. doi:10.1093/jn/130.8.2086S. PMID 10917927. "Figure 5". 
  10. ↑ Rohdewald, P (2002). "A review of the French maritime pine bark extract (Pycnogenol), a herbal medication with a diverse clinical pharmacology". International Journal of Clinical Pharmacology and Therapeutics 40 (4): 158–168. doi:10.5414/cpp40158. PMID 11996210. 
  11. ↑ Hatano, T; Miyatake, H; Natsume, M; Osakabe, N; Takizawa, T; Ito, H; Yoshida, T (2002). "Proanthocyanidin glycosides and related polyphenols from cacao liquor and their antioxidant effects". Phytochemistry 59 (7): 749–758. doi:10.1016/S0031-9422(02)00051-1. PMID 11909632. Bibcode: 2002PChem..59..749H. 
  12. ↑ Merghem, R.; Jay, M.; Brun, N.; Voirin, B. (2004). "Qualitative analysis and HPLC isolation and identification of procyanidins fromvicia faba". Phytochemical Analysis 15 (2): 95–99. doi:10.1002/pca.731. PMID 15116939. Bibcode: 2004PChAn..15...95M. 
  13. ↑ Van Der Poel, A. F. B.; Dellaert, L. M. W.; Van Norel, A.; Helsper, J. P. F. G. (2007). "The digestibility in piglets of faba bean (Vicia faba L.) as affected by breeding towards the absence of condensed tannins". British Journal of Nutrition 68 (3): 793–800. doi:10.1079/BJN19920134. PMID 1493141. 
  14. ↑ Griffiths, D. W. (1981). "The polyphenolic content and enzyme inhibitory activity of testas from bean (Vicia faba) and pea (Pisum spp.) varieties". Journal of the Science of Food and Agriculture 32 (8): 797–804. doi:10.1002/jsfa.2740320808. Bibcode: 1981JSFA...32..797G. 
  15. ↑ Qa’Dan, F.; Petereit, F.; Mansoor, K.; Nahrstedt, A. (2006). "Antioxidant oligomeric proanthocyanidins fromCistus salvifolius". Natural Product Research 20 (13): 1216–1224. doi:10.1080/14786410600899225. PMID 17127512. 
  16. ↑ 16.0 16.1 "Bioactivity of dietary polyphenols: The role of metabolites". Critical Reviews in Food Science and Nutrition 60 (4): 626–659. 2020. doi:10.1080/10408398.2018.1546669. PMID 30614249. 
  17. ↑ Bogs, J.; Jaffe, F. W.; Takos, A. M.; Walker, A. R.; Robinson, S. P. (2007). "The Grapevine Transcription Factor VvMYBPA1 Regulates Proanthocyanidin Synthesis during Fruit Development". Plant Physiology 143 (3): 1347–1361. doi:10.1104/pp.106.093203. PMID 17208963. 
  18. ↑ Cabrera, A.; Martin, A. (2009). "Genetics of tannin content and its relationship with flower and testa colours in Vicia faba". The Journal of Agricultural Science 113: 93–98. doi:10.1017/S0021859600084665. 
  19. ↑ Kristensen, H.; Aastrup, S. (1986). "A non-destructive screening method for proanthocyanidin-free barley mutants". Carlsberg Research Communications 51 (7): 509–513. doi:10.1007/BF02906893. 
  20. ↑ Grape Seed Extract, White paper, The Grape Seed Method Evaluation Committee, Under the Auspices of NNFA ComPli
  21. ↑ Stringano, E; Gea, A; Salminen, J. P.; Mueller-Harvey, I (2011). "Simple solution for a complex problem: Proanthocyanidins, galloyl glucoses and ellagitannins fit on a single calibration curve in high performance-gel permeation chromatography". Journal of Chromatography A 1218 (43): 7804–7812. doi:10.1016/j.chroma.2011.08.082. PMID 21930278. 
  22. ↑ Engström, M. T.; Pälijärvi, M; Fryganas, C; Grabber, J. H.; Mueller-Harvey, I; Salminen, J. P. (2014). "Rapid Qualitative and Quantitative Analyses of Proanthocyanidin Oligomers and Polymers by UPLC-MS/MS". Journal of Agricultural and Food Chemistry 62 (15): 3390–3399. doi:10.1021/jf500745y. PMID 24665824. 
  23. ↑ Torres, J. L.; Lozano, C. (2001). "Chromatographic characterization of proanthocyanidins after thiolysis with cysteamine". Chromatographia 54 (7–8): 523–526. doi:10.1007/BF02491211. 
  24. ↑ Jorgensen, Emily M.; Marin, Anna B.; Kennedy, James A. (2004). "Analysis of the Oxidative Degradation of Proanthocyanidins under Basic Conditions". Journal of Agricultural and Food Chemistry 52 (8): 2292–2296. doi:10.1021/jf035311i. PMID 15080635. 
  25. ↑ Li, Hui-Jing; Deinzer, Max L (2007). "Tandem mass spectrometry for sequencing proanthocyanidins". Analytical Chemistry 79 (4): 1739–1748. doi:10.1021/ac061823v. INIST:18534021. PMID 17297981. 
  26. ↑ "Variation in proanthocyanidin content and composition among commonly grown North American cranberry cultivars (Vaccinium macrocarpon)". J Sci Food Agric 94 (13): 2738–2745. 2014. doi:10.1002/jsfa.6618. PMID 24532348. Bibcode: 2014JSFA...94.2738C. 
  27. ↑ Taheri, Rod; Connolly, Bryan A.; Brand, Mark H.; Bolling, Bradley W. (2013). "Underutilized Chokeberry (Aronia melanocarpa, Aronia arbutifolia, Aronia prunifolia) Accessions Are Rich Sources of Anthocyanins, Flavonoids, Hydroxycinnamic Acids, and Proanthocyanidins". Journal of Agricultural and Food Chemistry 61 (36): 8581–8588. doi:10.1021/jf402449q. PMID 23941506. 
  28. ↑ Rösch, Daniel R.; Mügge, Clemens; Fogliano, Vincenzo; Kroh, Lothar W. (2004). "Antioxidant Oligomeric Proanthocyanidins from Sea Buckthorn (Hippophaë rhamnoides) Pomace". Journal of Agricultural and Food Chemistry 52 (22): 6712–6718. doi:10.1021/jf040241g. PMID 15506806. 
  29. ↑ Kandil, F. E.; Song, L.; Pezzuto, J. M.; Marley, K.; Seigler, D. S.; Smith, M. A. L. (2000). "Isolation of oligomeric proanthocyanidins from flavonoid-producing cell cultures". In Vitro Cellular & Developmental Biology - Plant 36 (6): 492–500. doi:10.1007/s11627-000-0088-1. 
  30. ↑ Amil-Ruiz, F.; Blanco-Portales, R.; Munoz-Blanco, J.; Caballero, J. L. (2011). "The Strawberry Plant Defense Mechanism: A Molecular Review". Plant and Cell Physiology 52 (11): 1873–1903. doi:10.1093/pcp/pcr136. PMID 21984602. 
  31. ↑ Matthews, S.; Mila, I.; Scalbert, A.; Pollet, B.; Lapierre, C.; Hervé Du Penhoat, C. L. M.; Rolando, C.; Donnelly, D. M. X. (1997). "Method for Estimation of Proanthocyanidins Based on Their Acid Depolymerization in the Presence of Nucleophiles". Journal of Agricultural and Food Chemistry 45 (4): 1195–1201. doi:10.1021/jf9607573. 
  32. ↑ 32.0 32.1 Liu, H; Zou, T; Gao, J. M.; Gu, L (2013). "Depolymerization of cranberry procyanidins using (+)-catechin, (-)-epicatechin, and (-)-epigallocatechin gallate as chain breakers". Food Chemistry 141 (1): 488–494. doi:10.1016/j.foodchem.2013.03.003. PMID 23768384. 
  33. ↑ Kennedy, J. A.; Jones, G. P. (2001). "Analysis of Proanthocyanidin Cleavage Products Following Acid-Catalysis in the Presence of Excess Phloroglucinol". Journal of Agricultural and Food Chemistry 49 (4): 1740–1746. doi:10.1021/jf001030o. PMID 11308320. 
  34. ↑ Sears, K. D.; Casebier, R. L. (1968). "Cleavage of proanthocyanidins with thioglycollic acid". Chemical Communications (22): 1437. doi:10.1039/C19680001437. 
  35. ↑ Vernhet, A.; Dubascoux, S. P.; Cabane, B.; Fulcrand, H. L. N.; Dubreucq, E.; Poncet-Legrand, C. L. (2011). "Characterization of oxidized tannins: Comparison of depolymerization methods, asymmetric flow field-flow fractionation and small-angle X-ray scattering". Analytical and Bioanalytical Chemistry 401 (5): 1559–1569. doi:10.1007/s00216-011-5076-2. PMID 21573842. , Vernhet, A.; Dubascoux, S. P.; Cabane, B.; Fulcrand, H. L. N.; Dubreucq, E.; Poncet-Legrand, C. L. (2011). "Characterization of oxidized tannins: Comparison of depolymerization methods, asymmetric flow field-flow fractionation and small-angle X-ray scattering". Analytical and Bioanalytical Chemistry 401 (5): 1559–1569. doi:10.1007/s00216-011-5076-2. PMID 21573842. 
  36. ↑ Lange, B. M.; Lapierre, C.; Sandermann Jr, H. (1995). "Elicitor-Induced Spruce Stress Lignin (Structural Similarity to Early Developmental Lignins)". Plant Physiology 108 (3): 1277–1287. doi:10.1104/pp.108.3.1277. PMID 12228544. 
  37. ↑ Douglas-Fir Bark: Characterization of a Condensed Tannin Extract, by Hong-Keun Song, A thesis submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science, December 13, 1984
  38. ↑ Zhang, L. L.; Lin, Y. M. (2008). "HPLC, NMR and MALDI-TOF MS Analysis of Condensed Tannins from Lithocarpus glaber Leaves with Potent Free Radical Scavenging Activity". Molecules 13 (12): 2986–2997. doi:10.3390/molecules13122986. PMID 19052523. 
  39. ↑ Howell, AB (2007). "Bioactive compounds in cranberries and their role in prevention or urinary tract infections". Mol Nutr Food Res 51 (6): 732–737. doi:10.1002/mnfr.200700038. PMID 17487930. 
  40. ↑ Luís, Â; Domingues, F; Pereira, L (10 March 2017). "Can Cranberries Contribute to Reduce the Incidence of Urinary Tract Infections? A Systematic Review with Meta-Analysis and Trial Sequential Analysis of Clinical Trials.". Journal of Urology 198 (3): 614–621. doi:10.1016/j.juro.2017.03.078. PMID 28288837. 
  41. ↑ "Recurrent Uncomplicated Urinary Tract Infections in Women: AUA/CUA/SUFU Guideline (2019)". American Urological Association. 2019. https://www.auanet.org/guidelines/recurrent-uti. 
  42. ↑ Corder, R.; Mullen, W.; Khan, N. Q.; Marks, S. C.; Wood, E. G.; Carrier, M. J.; Crozier, A. (2006). "Oenology: Red wine procyanidins and vascular health". Nature 444 (7119): 566. doi:10.1038/444566a. PMID 17136085. Bibcode: 2006Natur.444..566C. 
  43. ↑ Absalon, C; Fabre, S; Tarascou, I; Fouquet, E; Pianet, I (2011). "New strategies to study the chemical nature of wine oligomeric procyanidins". Analytical and Bioanalytical Chemistry 401 (5): 1485–1495. doi:10.1007/s00216-011-4988-1. PMID 21573848. 
  44. ↑ Gonzalo-Diago, A; Dizy, M; Fernández-Zurbano, P (2013). "Taste and mouthfeel properties of red wines proanthocyanidins and their relation to the chemical composition". Journal of Agricultural and Food Chemistry 61 (37): 8861–8870. doi:10.1021/jf401041q. PMID 23889258. 
  45. ↑ Sánchez-Moreno, Concepción; Cao, Guohua; Ou, Boxin; Prior, Ronald L. (2003). "Anthocyanin and Proanthocyanidin Content in Selected White and Red Wines. Oxygen Radical Absorbance Capacity Comparison with Nontraditional Wines Obtained from Highbush Blueberry". Journal of Agricultural and Food Chemistry 51 (17): 4889–4896. doi:10.1021/jf030081t. PMID 12903941. 
  46. ↑ Robertson, Nina U.; Schoonees, Anel; Brand, Amanda; Visser, Janicke (29 September 2020). "Pine bark (Pinus spp.) extract for treating chronic disorders". The Cochrane Database of Systematic Reviews 2020 (9): CD008294. doi:10.1002/14651858.CD008294.pub5. ISSN 1469-493X. PMID 32990945. 
  47. ↑ 47.0 47.1 "Characterization of anthocyanins and proanthocyanidins in some cultivars of Ribes, Aronia, and Sambucus and their antioxidant capacity". Journal of Agricultural and Food Chemistry 52 (26): 7846–7856. 2004. doi:10.1021/jf0486850. PMID 15612766. 

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