Lead Review Article November 1998: 31 7-333 Polyphenols: Chemistry, Dietary Sources, Metabolism, and Nutritional Significance Laura Bravo, Ph.D. Polyphenols constitute one of the most numer- fore, have been studied for taxonomic purposes or to de- ous and ubiquitous groups of plant metabolites termine adulteration of food products. Polyphenols have and are an integral pat? of both human and animal several industrial applications, such as in the production diets. Ranging from simple phenolic molecules to of paints, paper, and cosmetics, as tanning agents, and in highly polymerized compounds with molecular the food industry as additives (as natural colorants and weights of greater than 30,000 Da, the occurrence preservatives). In addition, some phenolic compounds, of this complex group of substances in plant foods the flavonoids, have applications as antibiotics and an- is extremely variable. Polyphenols traditionally have tidiarrheal, antiulcer, and anti-inflammatory agents, as well been considered antinutrients by animal nutruon- as in the treatment of diseases such as hypertension, vas- ists, because of the adverse effect of tannins, one cular fragility, allergies, hypercholesterolemia, and 0th- type of polyphenol, on protein digestibility. How- ever, recent interest in food phenolics has in- Polyphenolic compounds are ubiquitous in all plant creased greatly, owing to their antioxidant capac- ity (free radical scavenging and metal chelating organs and are, therefore, an integral part of the human activities) and their possible beneficial implications diet. Until recgntly, most of the nutritional interest in in human health, such as in the treatment and pre- polyphenolic compounds was in the deleterious effects vention of cancer, cardiovascular disease, and caused by the ability of certain polyphenols to bind and other pathologies. Much of the literature refers to precipitate macromolecules, such as dietary protein, car- a single group of plant phenolics, the flavonoids. bohydrate, and digestive enzymes, thereby reducing food This review offers an overview of the nutritional digestibility. Recent interest, however, in food phenolics effects of the main groups of polyphenolic com- has increased greatly because of the antioxidant and free pounds, including their metabolism, effects on radical-scavenging abilities associated with some phe- nutrient bioavailabilityl and antioxidant activityl as nolics and their potential effects on human health. This well as a brief description of the chemistry of review offers an overview of the main nutritional effects polyphenols and their occurrence in plant foods. of polyphenolic compounds as well as a brief description of the chemistry of polyphenols and their occurrence in plant foods. Introduction In reviews of the abundant scientific literature on polyphenolic compounds, particularly those studies of For decades, plant polyphenols have interested scientists the physiologic effects of plant polyphenols, most experi- because they are essential to plant physiology for their ments are devoted to the study of a specific group of contribution to plant morphology (ie., pigmentation),they phenolic compounds-the flavonoids. This review pro- are involved in growth and reproduction and provide vides a general overview of the nutritional significance of plants with resistance to pathogens and predators (by flavonoids as well as other types of food polyphenols, acting as phytoalexins or by increasing food astringency, including simple phenols and tannins. thus making food unpalatable), they protect crops from plague and preharvest seed germination, and for other Chemistry of Phenolic Compounds reasons. The polyphenolic profiles of plants differ be- tween varieties of the same species. Polyphenols, there- Phenolic compounds or polyphenols constitute one of the most numerous and widely distributed groups of sub- Dr. Bravo is with the Departarnento de stances in the plant kingdom, with more than 8000 phe- Metabolism0 y Nutricion, lnstituto del Frio (CSIC), nolic structures currently Polyphenols are prod- Ciudad Universitaria s/n, Madrid 28040, Spain. ucts of the secondary metabolism of plants. They arise Nutrition Reviews, Vol. 56, No. 11 317 biogenetically from two main synthetic pathways: the acids and many others. Associations with other com- shikimate pathway and the acetate path~ay.~This is an pounds, such as carboxylic and organic acids, amines, extremely wide and complex group of plant substances. and lipids, and linkages with other phenols are also com- Natural polyphenols can range from simple molecules, mon. such as phenolic acids, to highly polymerized compounds, According to Harb~rne,~polyphenols can be divided such as tannins. They occur primarily in conjugated form, into at least 10 different classes depending on their basic with one or more sugar residues linked to hydroxyl groups, chemical structure. Table 1 illustrates the basic chemical although direct linkages of the sugar unit to an aromatic structure of the main polyphenolic compounds. Fla- carbon atom also exist. The associated sugars can be vonoids, which constitutethe most important single group, present as monosaccharides, disaccharides, or even as can be further subdivided into 13 classes, with more than oligosaccharides. Glucose is the most common sugar resi- 5000 compounds described by 19904 (Table 2).A brief de- due, although galactose, rhamnose, xylose, and arabinose scription of the main classes of food phenolics follows. are also found, as well as glucuronic and galacturonic For more comprehensive reviews, see references 4-1 0. Table 1. Main Classes of Polyphenolic Compounds Class Basic Skeleton Basic Structure Simple phenols '6 Benzoquinones '6 ' Phenolic acids Acetophenones '6-'2 Phenylacetic acids Hydroxycinnamic acids Phenylpropenes Coumarins, isocoumarins Chromones Naftoquinones '6-'4 Xanthones c,c,-c, Stilbenes Anthraquinones Flavonoids c6-c3-c6 Lignans, neolignans (',5-'& Lignins ('6-'3)" 318 Nutrition Reviews, Vol. 56, No. 11 Table 2. Classification of Food Flavonoids as phenolic acids (e.g., gallic, vanillic, syringic, p- Flavonoid Basic Structure hydroxybenzoic) and aldehydes (e.g., vanillin, syringaldehyde,p-hydroxybenzaldehyde), also are fairly Chalcones common in higher plants and ferns. Phenylacetic acids and acetophenones (C,-C,) are, however, less frequently described in the literature. All of these compounds can be Dihydrochalcones Q)p found free, although their corresponding methyl and ethyl 0 esters and glycosides occur very commonly in free andor Aurones bound f~ms.~ Phenylpropanoid derivatives (C,-C,) also are an im- portant group of low-molecular-weight phenolics. Chromones are less known than coumarins, with the latter Flavones occurring naturally as glycosides (e.g., umbilliferone,aes- culetin, scopoletin). The most important phenylpropanoids are the hydroxycinnamic acids (p-coumaric, caffeic, feru- Flavonols lic, sinapic) and derivatives. Cinnamyl alcohols (coniferyl alcohol or guaiacyl, sinapyl alcohol or syringyl, and p- coumaryl alcohol orp-hydroxyphenyl) form the basic con- Dihydroflavonol stituent of lignins, and thus represent one of the major groups of plant phenolics. Phenylpropanoids and more Flavanones simple phenols (benzoic acid and benzaldehyde deriva- tives) are usually covalently linked to cell wall polysac- charides (predominantly ester-linked to arabinose units Flavanol of hemicellulose) or to the so-called core lignin.11*'2 Flavonoids (Table 2) represent the most common and widely distributed group of plant phenolics. Their com- Flavandiol or mon structure is that of diphenylpropanes ((2,-C,-C,) and leucoanthocyanidin consists of two aromatic rings linked through three car- bons that usually form an oxygenated heterocycle. Figure Anthocyanidin 1 represents the basic structure and the system used for carbon numbering of the flavonoid nucleus. Biogeneti- cally, the A ring usually comes from a molecule of resorci- Isoflavonoids no1 or phloroglucinol synthesized in the acetate pathway, whereas ring B is derived from the shikimate pathway.I3 Flavonoids occasionally occur in plants as aglycones, al- Biflavonoids though they are most commonly found as glycoside de- rivatives. Among the flavonoids, flavones (e.g., apigenin, luteolin, diosmetin), flavonols (e.g., quercetin, myricetin, kaempferol), and their glycosides are the most common Proanthocyanidins or condensed tannins 3' Simple Phenols and Flavonoids Among the most common and important low-molecular- weight phenolic compounds are simple phenolic deriva- tives and flavonoids. Simple phenols (C,), such as phenol itself, cresol, thymol, resorcinol, orcinol, etc., are wide- spread among different plant species, including hydro- 5 4 quinone and derivatives (e.g., arbutine, sesamol) and phlo- roglucinol. Phenolics with a C,-C, structure (Table l), such Figure 1. Basic structure and numbering system of flavonoids. Nutrition Reviews, Vol. 56, No. 11 319 compounds. They are widespread in the plant kingdom, third group of tannins, the phlorotannins, are found only with the exception of algae and fungi. Flavonols occur as in marine brown algae and are not commonly consumed 0-glycosides, but flavone 0-glycosides and C-glycosides by humans.17 are very common,14 with the latter characterized for pos- Hydrolyzable tannins. Hydrolyzable tannins consist sessing a carbon-carbon linkage between the anomeric of gallic acid and its dimeric condensation product, carbon of a sugar molecule and the C-6 or C-8 carbon of hexahydroxydiphenicacid, esterified