Polyphenols are well known as the largest group of and most widely distributed phytochemicals among plants.1 These secondary plant metabolites, which are produced in response to environmental hazards that contribute to free-radical synthesis,2 are represented by more than 8,000 naturally occurring compounds. This family of widely divergent substances has gained increasing attention in recent years as polyphenols have been found – in vegetables, fruits, herbs, grains, tea, coffee beans, honey, and red wine – to be the most abundant sources of antioxidants in the human diet and are known to exert antioxidant, anti-inflammatory, and antimicrobial benefits to human health.3-7 The most prevalent and studied polyphenols are known as flavonoids, but nonflavonoid polyphenols are increasingly well investigated. This column will address the basic chemistry of these compounds. Subsequent columns will discuss the latest research on the cutaneous benefits of selected flavonoid and nonflavonoid polyphenols.
Plants synthesize polyphenols in response to environmental hazards that induce enhanced free-radical production.2 Polyphenol biosynthesis begins when phenylalanine ammonia lyase is induced by exposure to UV light, gamma irradiation, ozone, low temperatures, organic toxins, and/or heavy metals. Phenylalanine is catalytically deaminated to cinnamic acid and then cinnamic acid is converted to various polyphenols, which share a definitive structural component: a phenol or an aromatic ring with at least one hydroxyl group. Polyphenols are an exceedingly important source of antioxidants and are found in a vast spectrum of vegetables, fruits, herbs, grains, tea, coffee beans, honey, and red wine.
Chemistry and sources
Polyphenols share a common structural component: a phenol or an aromatic ring, usually two, with at least one hydroxyl, methyl, or acetyl group linked via a three-carbon bond to form a six-unit heterocyclic ring.8,9 When the “parent polyphenol” known as cinnamic acid is further catalytically transformed, scores of polyphenolic compounds result. These substances are divided into classes: glycosylated phenylpropanoids, flavonoids, isoflavonoids, stilbenoids, coumarins, curcuminoids, as well as phenolic polymers such as tannins, proanthocyanidins, suberin, lignins, and lignans. The flavonoids, which are the largest and most varied phenolic substances in plants, can be further divided into several categories: flavones (based on the 2-phenylchromen-4-one skeleton, such as apigenin and luteolin); flavonols (based on the 3-hydroxy-2-phenylchromen-4-one skeleton and functional group, such as quercetin, kaempferol, myricetin, and fisetin); flavanones (based on the 2,3-dihydro-2-phenylchromen-4-one skeleton and functional group, such as naringenin, hesperidin, and eriodictyol); isoflavones (based on the 3-phenylchromen-4-one skeleton, such as genistein and daidzein); flavanols – also known as flavan-3-ols or catechins – (based on the 2-phenyl-3,4-dihydro-2H-chromen-3-ol skeleton and functional groups, such as epicatechin, epicatechin 3-gallate, epigallocatechin, epigallocatechin 3-gallate (EGCG), catechin, and gallocatechin); and anthocyanins (based on the 2-phenylchromenylium ion skeleton, e.g., cyanidin and pelargonidin).5,10
The broader category of nonflavonoid polyphenols is rich and diverse, but is particularly noted for comprising the tannins, phenolic polymers of high molecular weight, which are divided into three classes, hydrolyzable tannins (such as ellagic acid, found in pomegranate, raspberries, strawberries, cranberries, and walnuts), derived tannins (created during food handling and processing and present in, for example, black and oolong teas), and condensed tannins (or proanthocyanidins, which are polymer chains of flavanols, such as catechins, and include pycnogenol, leukocyanidin, and leucoanthocyanin).1,4,5,8,10 There are a plethora of other nonflavonoid polyphenols, many of which confer health benefits, including stilbenes (such as resveratrol, found in red wine), lignans (such as enterodiol, found in flaxseed and flaxseed oil), lignins (found in green beans, carrots, peas, and Brazil nuts), and phenolic acids, such as hydroxybenzoic and hydroxycinnamic acids, among which caffeic and ferulic acids are often present in foods. In fact, hydroxycinnamic acids, which are the most common phenolic acids present in plant tissues, are present in numerous foods, such as apples, pears, plums, cherries, apricots, peaches, black currant, blueberries, potatoes, spinach, lettuce, cabbage, broccoli, asparagus, wine, and coffee.9
Some specific flavonoids can be found in the following food sources: flavonols in apples with skin, broccoli, olives, onions, and tea (green, black); flavones in celery and parsley; flavonones in grapefruit, oranges, and their juices; and catechins (flavanols) in apples (with or without skin), dark chocolate and cocoa, red wine, and tea (green, black).10
Broad health benefits have been associated with hundreds of polyphenolic substances. Notably, some of the best-known research results on polyphenols have reported on the success of various topical applications of green tea catechins, ferulic acid, and resveratrol, and other related compounds. Antioxidant, anti-inflammatory, and antimicrobial activities are the most common biologic properties associated with polyphenols, and antiaging activity has been widely reported.10
Conclusion
While the classification system for the 8,000 polyphenolic compounds may seem intimidating, the same essential activity is conferred by these abundant substances. Further, it is important to note the significant health benefits potentially derived from the oral consumption as well as topical application of polyphenols. The next two columns will delve into the research findings of flavonoid and nonflavonoid polyphenols.
Dr. Baumann is a private practice dermatologist, researcher, author and entrepreneur who practices in Miami. She founded the Cosmetic Dermatology Center at the University of Miami in 1997. Dr. Baumann wrote two textbooks: “Cosmetic Dermatology: Principles and Practice” (New York: McGraw-Hill, 2002), and “Cosmeceuticals and Cosmetic Ingredients,” (New York: McGraw-Hill, 2014), and a New York Times Best Sellers book for consumers, “The Skin Type Solution” (New York: Bantam Dell, 2006). Dr. Baumann has received funding for advisory boards and/or clinical research trials from Allergan, Evolus, Galderma, and Revance. She is the founder and CEO of Skin Type Solutions Franchise Systems LLC.
References
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5. Annu Rev Nutr. 2002;22:19-34 .
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8. J Nutr. 2003 Oct;133(10):3248S-3254S .
