Contents
What are polyphenols?
Polyphenols are naturally occurring compounds found largely in the fruits, vegetables, cereals and beverages. Fruits like grapes, apple, pear, cherries and berries contains up to 200–300 mg polyphenols per 100 grams fresh weight 1). The products manufactured from these fruits, also contain polyphenols in significant amounts. Cereals, dry legumes, chocolate and beverages, such as tea, coffee, or wine also contribute to the polyphenolic intake 2), 3). Typically a glass of red wine or a cup of tea or coffee contains about 100 mg polyphenols.
Polyphenols are secondary metabolites of plants and are generally involved in defense against ultraviolet radiation or aggression by pathogens and may also contribute to the bitterness, astringency of the food. Researchers have explored that these molecules are very good antioxidants and may neutralize the destructive reactivity of undesired reactive oxygen/nitrogen species produced as byproduct during metabolic processes in the body. Epidemiological studies have revealed that polyphenols provide a significant protection against development of several chronic diseases such as cardiovascular diseases (CVDs), cancer, diabetes, infections, aging, asthma, etc 4).
Towards the end of 20th century, epidemiological studies and associated meta-analyses strongly suggested that long term consumption of diets rich in plant polyphenols offered some protection against development of cancers, cardiovascular diseases, diabetes, osteoporosis and neurodegenerative diseases 5), 6). Whether acting in the gastrointestinal tract or in the liver, the potent antioxidant effects of polyphenols are widely accepted as health promoting 7), 8). Antivira, antibacterial, anti-inflammatory, neuroprotective, and anticarcinogenic effects have also been attributed to polyphenols 9), 10), 11), 12). Polyphenols and other food phenolics are the subject of increasing scientific interest because of their possible beneficial effects on human health.
Polyphenols such as resveratrol, epigallocatechin gallate (EGCG), and curcumin have been acknowledged for having beneficial effects on cardiovascular health, while some have also been shown to be protective in aging.
Figure 1. Polyphenols Health Benefits
Structure and Classes of Polyphenols
More than 8,000 polyphenolic compounds have been identified in various plant species. All plant phenolic compounds arise from a common intermediate, phenylalanine, or a close precursor, shikimic acid. Primarily they occur in conjugated forms, with one or more sugar residues linked to hydroxyl groups, although direct linkages of the sugar (polysaccharide or monosaccharide) to an aromatic carbon also exist. Association with other compounds, like carboxylic and organic acids, amines, lipids and linkage with other phenol is also common 14).
Chemical structures of the different classes of polyphenols. Polyphenols are classified on the basis of the number of phenol rings that they contain and of the structural elements that bind these rings to one another 15). They are broadly dived in four classes; Phenolic acids, flavonoids, stilbenes and lignans 16). Phenolic acids are further divided into hydroxyl benzoic and hydroxyl cinnamic acids. Phenolic acids account for about a third of the polyphenolic compounds in our diet and are found in all plant material, but are particularly abundant in acidic-tasting fruits. Caffeic acid, gallic acid, ferulic acid are some common phenolic acids. Flavonoids are most abundant polyphenols in human diet and share a common basic structure consist of two aromatic rings, which are bound together by three carbon atoms that form an oxygenated heterocycle 17). Biogenetically, one ring usually arises from a molecule of resorcinol, and other ring is derived from the shikimate pathway. Stilbenes contain two phenyl moieties connected by a twocarbon methylene bridge. Most stilbenes in plants act as antifungal phytoalexins, compounds that are synthesized only in response to infection or injury. The most extensively studied stilbene is resveratrol. Lignans are diphenolic compounds that contain a 2,3-dibenzylbutane structure that is formed by the dimerization of two cinnamic acid residues. Figure 2 illustrates the different groups of polyphenols and their chemical structures.
Figure 2. Polyphenols Chemical Structure
Flavonoids
Flavonols are the most ubiquitous flavonoids in foods, and the main representatives are quercetin and kaempferol. Favonoids comprise the most studied group of polyphenols. More than 4,000 varieties of flavonoids have been identified, many of which are responsible for the attractive colours of the flowers, fruits and leaves. They are generally present at relatively low concentrations of ≈15–30 mg/kg fresh wt. The richest sources are onions (up to 1.2 g/kg fresh wt), curly kale, leeks, broccoli, and blueberries (Table 2). Red wine and tea also contain up to 45 mg flavonols/L. These compounds are present in glycosylated forms. The associated sugar moiety is very often glucose or rhamnose, but other sugars may also be involved (eg, galactose, arabinose, xylose, glucuronic acid). Fruit often contains between 5 and 10 different flavonol glycosides. These flavonols accumulate in the outer and aerial tissues (skin and leaves) because their biosynthesis is stimulated by light. Marked differences in concentration exist between pieces of fruit on the same tree and even between different sides of a single piece of fruit, depending on exposure to sunlight 19). Similarly, in leafy vegetables such as lettuce and cabbage, the glycoside concentration is ≥10 times as high in the green outer leaves as in the inner light-colored leaves 20). This phenomenon also accounts for the higher flavonol content of cherry tomatoes than of standard tomatoes, because they have different proportions of skin to whole fruit.
Flavones are much less common than flavonols in fruit and vegetables. Flavones consist chiefly of glycosides of luteolin and apigenin. The only important edible sources of flavones identified to date are parsley and celery (Table 2). Cereals such as millet and wheat contain C-glycosides of flavones. The skin of citrus fruit contains large quantities of polymethoxylated flavones: tangeretin, nobiletin, and sinensetin (up to 6.5 g/L of essential oil of mandarin). These polymethoxylated flavones are the most hydrophobic flavonoids.
In human foods, flavanones are found in tomatoes and certain aromatic plants such as mint, but they are present in high concentrations only in citrus fruit. The main aglycones are naringenin in grapefruit, hesperetin in oranges, and eriodictyol in lemons. Flavanones are generally glycosylated by a disaccharide at position 7: either a neohesperidose, which imparts a bitter taste (such as to naringin in grapefruit), or a rutinose, which is flavorless. Orange juice contains between 200 and 600 mg hesperidin/L and 15–85 mg narirutin/L, and a single glass of orange juice may contain between 40 and 140 mg flavanone glycosides. Because the solid parts of citrus fruit, particularly the albedo (the white spongy portion) and the membranes separating the segments, have a very high flavanone content, the whole fruit may contain up to 5 times as much as a glass of orange juice.
Isoflavones are flavonoids with structural similarities to estrogens. Although they are not steroids, they have hydroxyl groups in positions 7 and 4 in a configuration analogous to that of the hydroxyls in the estradiol molecule. This confers pseudohormonal properties on them, including the ability to bind to estrogen receptors, and they are consequently classified as phytoestrogens. Isoflavones are found almost exclusively in leguminous plants. Soya and its processed products are the main source of isoflavones in the human diet. They contain 3 main molecules: genistein, daidzein, and glycitein, generally in a concentration ratio of 1:1:0.2. They are sensitive to heat and are often hydrolyzed to glycosides during the course of industrial processing, as in the production of soya milk (36). The fermentation carried out during the manufacturing of certain foods, such as miso and tempeh, results in the hydrolysis of glycosides to aglycones. The aglycones are highly resistant to heat. The isoflavone content of soya and its manufactured products varies greatly as a function of geographic zone, growing conditions, and processing. Soybeans contain between 580 and 3800 mg isoflavones/kg fresh wt, and soymilk contains between 30 and 175 mg/L.
Flavanols exist in both the monomer form (catechins) and the polymer form (proanthocyanidins). Catechins are found in many types of fruit (apricots, which contain 250 mg/kg fresh wt, are the richest source; Table 2). They are also present in red wine (up to 300 mg/L), but green tea and chocolate are by far the richest sources. An infusion of green tea contains up to 200 mg catechins. Black tea contains fewer monomer flavanols, which are oxidized during “fermentation” (heating) of tea leaves to more complex condensed polyphenols known as theaflavins (dimers) and thearubigins (polymers). Catechin and epicatechin are the main flavanols in fruit, whereas gallocatechin, epigallocatechin, and epigallocatechin gallate are found in certain seeds of leguminous plants, in grapes, and more importantly in tea. In contrast to other classes of flavonoids, flavanols are not glycosylated in foods. The tea epicatechins are remarkably stable when exposed to heat as long as the pH is acidic: only ≈15% of these substances are degraded after 7 h in boiling water at pH 5.
Proanthocyanidins, which are also known as condensed tannins, are dimers, oligomers, and polymers of catechins that are bound together.Through the formation of complexes with salivary proteins, condensed tannins are responsible for the astringent character of fruit (grapes, peaches, kakis, apples, pears, berries, etc) and beverages (wine, cider, tea, beer, etc) and for the bitterness of chocolate. This astringency changes over the course of maturation and often disappears when the fruit reaches ripeness; this change has been well explained in the kaki fruit by polymerization reactions with acetaldehyde. Such polymerization of tannins probably accounts for the apparent reduction in tannin content that is commonly seen during the ripening of many types of fruit. It is difficult to estimate the proanthocyanidin content of foods because proanthocyanidins have a wide range of structures and molecular weights. The only available data concern dimers and trimers, which are as abundant as the catechins themselves.
Anthocyanins are pigments dissolved in the vacuolar sap of the epidermal tissues of flowers and fruit, to which they impart a pink, red, blue, or purple color. They exist in different chemical forms, both colored and uncolored, according to pH. Although they are highly unstable in the aglycone form (anthocyanidins), while they are in plants, they are resistant to light, pH, and oxidation conditions that are likely to degrade them. Degradation is prevented by glycosylation, generally with a glucose at position 3, and esterification with various organic acids (citric and malic acids) and phenolic acids. In addition, anthocyanins are stabilized by the formation of complexes with other flavonoids (copigmentation). In the human diet, anthocyanins are found in red wine, certain varieties of cereals, and certain leafy and root vegetables (aubergines, cabbage, beans, onions, radishes), but they are most abundant in fruit. Cyanidin is the most common anthocyanidin in foods. Food contents are generally proportional to color intensity and reach values up to 2–4 g/kg fresh wt in blackcurrants or blackberries (Table 2). These values increase as the fruit ripens. Anthocyanins are found mainly in the skin, except for certain types of red fruit, in which they also occur in the flesh (cherries and strawberries). Wine contains ≈200–350 mg anthocyanins/L, and these anthocyanins are transformed into various complex structures as the wine ages.
Phenolic Acids
Phenolic acids are found abundantly in foods and divided into two classes: derivatives of benzoic acid and derivatives of cinnamic acid. The hydroxybenzoic acid content of edible plants is generally low, with the exception of certain red fruits, black radish and onions, which can have concentrations of several tens of milligrams per kilogram fresh weight 21). The hydroxycinnamic acids are more common than hydroxybenzoic acids and consist chiefly of p-coumaric, caffeic, ferulic and sinapic acids.
Caffeic acid, both free and esterified, is generally the most abundant phenolic acid and represents between 75% and 100% of the total hydroxycinnamic acid content of most fruit. Hydroxycinnamic acids are found in all parts of fruit, although the highest concentrations are seen in the outer parts of ripe fruit. Concentrations generally decrease during the course of ripening, but total quantities increase as the fruit increases in size.
Ferulic acid is the most abundant phenolic acid found in cereal grains, which constitute its main dietary source. The ferulic acid content of wheat grain is ≈0.8–2 g/kg dry wt, which may represent up to 90% of total polyphenols. Ferulic acid is found chiefly in the outer parts of the grain. The aleurone layer and the pericarp of wheat grain contain 98% of the total ferulic acid. The ferulic acid content of different wheat flours is thus directly related to levels of sieving, and bran is the main source of polyphenols 22). Rice and oat flours contain approximately the same quantity of phenolic acids as wheat flour (63 mg/kg), although the content in maize flour is about 3 times as high. Ferulic acid is found chiefly in the trans form, which is esterified to arabinoxylans and hemicelluloses in the aleurone and pericarp. Only 10% of ferulic acid is found in soluble free form in wheat bran. Several dimers of ferulic acid are also found in cereals and form bridge structures between chains of hemicellulose.
Tea is an important source of gallic acid: tea leaves may contain up to 4.5 g/kg fresh wt 23). Furthermore, hydroxybenzoic acids are components of complex structures such as hydrolyzable tannins (gallotannins in mangoes and ellagitannins in red fruit such as strawberries, raspberries, and blackberries) 24). Because these hydroxybenzoic acids, both free and esterified, are found in only a few plants eaten by humans, they have not been extensively studied and are not currently considered to be of great nutritional interest.
The hydroxycinnamic acids are more common than are the hydroxybenzoic acids and consist chiefly of p-coumaric, caffeic, ferulic, and sinapic acids. These acids are rarely found in the free form, except in processed food that has undergone freezing, sterilization, or fermentation. The bound forms are glycosylated derivatives or esters of quinic acid, shikimic acid, and tartaric acid. Caffeic and quinic acid combine to form chlorogenic acid, which is found in many types of fruit and in high concentrations in coffee: a single cup may contain 70–350 mg chlorogenic acid 25). The types of fruit having the highest content (blueberries, kiwis, plums, cherries, apples) contain 0.5–2 g hydroxycinnamic acids/kg fresh wt (Table 2) 26).
Stilbenes
Stilbenes are found in only low quantities in the human diet. One of these, resveratrol, for which anticarcinogenic effects have been shown during screening of medicinal plants and which has been extensively studied, is found in low quantities in wine (0.3–7 mg aglycones/L and 15 mg glycosides/L in red wine). However, because resveratrol is found in such small quantities in the diet, any protective effect of this molecule is unlikely at normal nutritional intakes. Most stilbenes in plants act as antifungal phytoalexins, compounds that are synthesized only in response to infection or injury. One of the best studied, naturally occurring polyphenol stilbene is resveratrol (3,4′,5-trihydroxystilbene), found largely in grapes. A product of grapes, red wine also contains significant amount of resveratrol.
Lignans
The richest dietary source is linseed (flaxseed), which contains secoisolariciresinol (up to 3.7 g/kg dry weight) and low quantities of matairesinol 27). Other cereals, grains, fruit, and certain vegetables also contain traces of these same lignans, but concentrations in linseed are ≈1000 times as high as concentrations in these other food sources. Lignans are metabolized to enterodiol and enterolactone by the intestinal microflora. The low quantities of secoisolariciresinol and matairesinol that are ingested as part of our normal diet do not account for the concentrations of the metabolites enterodiol and enterolactone that are classically measured in plasma and urine. Thus, there are undoubtedly other lignans of plant origin that are precursors of enterodiol and enterolactone and that have not yet been identified. Thompson et al 28) confirmed that oleaginous seeds (linseed/flaxseed) are the richest source and identified algae, leguminous plants (lentils), cereals (triticale and wheat), vegetables (garlic, asparagus, carrots), and fruit (pears, prunes) as minor sources.
Foods high in Polyphenols
In general, the total intake of polyphenols is approximated at 1 g/individual/day and polyphenols are considered by many to be the major source of antioxidants in our diet 29), 30), 31). However, this estimate varies depending on the type of diet. For example, total polyphenol intake in the Finnish diet is 817–919 mg/individual/day 32). In the Vietnamese diet, it is 595 mg/individual/day 33) and in the Mediterranean diet, polyphenol intake ranges between 1800 and 3000 mg/individual/day 34). Still, and due to their low absorption, it has been suggested that their major sites of antioxidant activity are the stomach 35) and the intestine 36).
Fruit and beverages such as tea and red wine constitute the main sources of polyphenols. Certain polyphenols such as quercetin are found in all plant products (fruit, vegetables, cereals, leguminous plants, fruit juices, tea, wine, infusions, etc), whereas others are specific to particular foods (flavanones in citrus fruit, isoflavones in soya, phloridzin in apples). In most cases, foods contain complex mixtures of polyphenols, which are often poorly characterized. Apples, for example, contain flavanol monomers (epicatechin mainly) or oligomers (procyanidin B2 mainly), chlorogenic acid and small quantities of other hydroxycinnamic acids, 2 glycosides of phloretin, several quercetin glycosides, and anthocyanins such as cyanidin 3-galactoside in the skin of certain red varieties. Apples are one of the rare types of food for which fairly precise data on polyphenol composition are available. Differences in polyphenol composition between varieties of apples have notably been studied. The polyphenol profiles of all varieties of apples are practically identical, but concentrations may range from 0.1 to 5 g total polyphenols/kg fresh wt and may be as high as 10 g/kg in certain varieties of cider apples 37), 38).
Distribution of phenolics in plants at the tissue, cellular and sub cellular levels is not uniform. Insoluble phenolics are found in cell walls, while soluble phenolics are present within the plant cell vacuoles 39). Certain polyphenols like quercetin are found in all plant products; fruit, vegetables, cereals, fruit juices, tea, wine, infusions etc., whereas flavanones and isoflavones are specific to particular foods. In most cases, foods contain complex mixtures of polyphenols. The outer layers of plants contain higher levels of phenolics than those located in their inner parts 40).
Numerous factors affect the polyphenol content of plants, these include degree of ripeness at the time of harvest, environmental factors, processing and storage. Polyphenolic content of the foods are greatly affected by environmental factors as well as edaphic factors like soil type, sun exposure, rainfall etc. The degree of ripeness considerably affects the concentrations and proportions of various polyphenols 41). In general, it has been observed that phenolic acid content decreases during ripening, whereas anthocyanin concentrations increase. Many polyphenols, especially phenolic acids, are directly involved in the response of plants to different types of stress: they contribute to healing by lignifications of damaged areas possess antimicrobial properties, and their concentrations may increase after infection 42).
Table 1. Main factors affecting the bioavailability of dietary polyphenols in humans
| External factors | Environmental factors (i.e., sun exposure, degree of ripeness); food availability |
| Food processing related factors | Thermal treatments; homogenization; liophylization; cooking and methods of culinary preparation; storage |
| Food related factors | Food matrix; presence of positive or negative effectors of absorption (i.e., fat, fiber) |
| Interaction with other compounds | Bonds with proteins (i.e., albumin) or with polyphenols with similar mechanism of absorption |
| Polyphenols related factors | Chemical structure; concentration in food; amount introduced |
| Host related factors | Intestinal factors (i.e., enzyme activity; intestinal transit time; colonic microflora). Systemic factors (i.e., gender and age; disorders and/or pathologies; genetics; physiological condition) |
Methods of culinary preparation also have a marked effect on the polyphenol content of foods. For example, simple peeling of fruit and vegetables can eliminate a significant portion of polyphenols because these substances are often present in higher concentrations in the outer parts than in the inner parts. Cooking may also have a major effect. Onions and tomatoes lose between 75% and 80% of their initial quercetin content after boiling for 15 min, 65% after cooking in a microwave oven, and 30% after frying 44). Steam cooking of vegetables, which avoids leaching, is preferable. Potatoes contain up to 190 mg chlorogenic acid/kg, mainly in the skin 45). Extensive loss occurs during cooking, and no remaining phenolic acids were found in French fries or freeze-dried mashed potatoes 46).
Industrial food processing also affects polyphenol content. As with fruit peeling, dehulling of legume seeds and decortication and bolting of cereals can result in a loss of some polyphenols. Grinding of plant tissues may lead to oxidative degradation of polyphenols as a result of cellular decompartmentation and contact between cytoplasmic polyphenol oxidase and phenolic substrates present in the vacuoles. Polyphenols are then transformed into brown pigments that are polymerized to different degrees. This unwanted process can occur, for example, during the process of making jam or compote from fruit. Production of fruit juice often involves clarification or stabilization steps specifically aimed at removing certain flavonoids responsible for discoloration and haze formation. Manufactured fruit juices thus have low flavonoid content. The pectinolytic enzymes used during such processing also hydrolyze the esters of hydroxycinnamic acid 47). Conversely, maceration operations facilitate diffusion of polyphenols in juice, as occurs during vinification of red wine. This maceration accounts for the fact that the polyphenol content of red wines is 10 times as high as that of white wines 48) and is also higher than that of grape juice 49).
Another factor that directly affects the polyphenol content of the foods is storage. Studies have proved that polyphenolic content of the foods change on storage, the reason is easy oxidation of these polyphenols 50). Oxidation reactions result in the formation of more or less polymerized substances, which lead to changes in the quality of foods, particularly in color and organoleptic characteristics. Such changes may be beneficial, as is the case with black tea or harmful as in browning of fruit. Storage of wheat flour results in marked loss of phenolic acids 51). After six months of storage, flour contained the same phenolic acids in qualitative terms, but their concentrations were 70% lower compared with fresh. Cold storage, in contrast, has slight effect on the content of polyphenols in apples, pears or onions 52). Cooking also has a major effect on concentration of polyphenols. Onions and tomatoes lose between 75% and 80% of their initial quercetin content after boiling for 15 min, 65% after cooking in a microwave oven, and 30% after frying 53).
Because of the wide range of existing polyphenols and the considerable number of factors that can modify their concentration in foods, no reference food-composition tables (as they exist for other micronutrients such as vitamins) have yet been drawn up. Only partial data for certain polyphenols, such as flavonols and flavones, catechins, and isoflavones, have been published on the basis of direct food analysis or bibliographic compilations.
Since March 2003, a database in which the flavonoid contents of 225 selected foods were compiled from 97 bibliographic sources has been available on the US Department of Agriculture website at 54) https://www.ars.usda.gov/ARSUserFiles/80400525/Articles/JAFC54_9966-9977Flavonoid.pdf and here at https://search.usa.gov/search?query=flavonoid&op=Search&affiliate=fnic
Table 2. Polyphenols Rich Foods
| Source (serving size) | Polyphenol content | ||
|---|---|---|---|
| By wt or vol | By serving | ||
| mg/kg fresh wt (or mg/L) | mg/serving | ||
| Hydroxybenzoic acids | Blackberry (100 g) | 80–270 | 8–27 |
| Protocatechuic acid | Raspberry (100 g) | 60–100 | 6–10 |
| Gallic acid | Black currant (100 g) | 40–130 | 4–13 |
| p-Hydroxybenzoic acid | Strawberry (200 g) | 20–90 | 4–18 |
| Hydroxycinnamic acids | Blueberry (100 g) | 2000–2200 | 200–220 |
| Caffeic acid | Kiwi (100 g) | 600–1000 | 60–100 |
| Chlorogenic acid | Cherry (200 g) | 180–1150 | 36–230 |
| Coumaric acid | Plum (200 g) | 140–1150 | 28–230 |
| Ferulic acid | Aubergine (200 g) | 600–660 | 120–132 |
| Sinapic acid | Apple (200 g) | 50–600 | 10–120 |
| Pear (200 g) | 15–600 | 3–120 | |
| Chicory (200 g) | 200–500 | 40–100 | |
| Artichoke (100 g) | 450 | 45 | |
| Potato (200 g) | 100–190 | 20–38 | |
| Corn flour (75 g) | 310 | 23 | |
| Flour: wheat, rice, oat (75 g) | 70–90 | 5–7 | |
| Cider (200 mL) | 10–500 | 2–100 | |
| Coffee (200 mL) | 350–1750 | 70–350 | |
| Anthocyanins | Aubergine (200 g) | 7500 | 1500 |
| Cyanidin | Blackberry (100 g) | 1000–4000 | 100–400 |
| Pelargonidin | Black currant (100 g) | 1300–4000 | 130–400 |
| Peonidin | Blueberry (100 g) | 250–5000 | 25–500 |
| Delphinidin | Black grape (200 g) | 300–7500 | 60–1500 |
| Malvidin | Cherry (200 g) | 350–4500 | 70–900 |
| Rhubarb (100 g) | 2000 | 200 | |
| Strawberry (200 g) | 150–750 | 30–150 | |
| Red wine (100 mL) | 200–350 | 20–35 | |
| Plum (200 g) | 20–250 | 4–50 | |
| Red cabbage (200 g) | 250 | 50 | |
| Flavonols | Yellow onion (100 g) | 350–1200 | 35–120 |
| Quercetin | Curly kale (200 g) | 300–600 | 60–120 |
| Kaempferol | Leek (200 g) | 30–225 | 6–45 |
| Myricetin | Cherry tomato (200 g) | 15–200 | 3–40 |
| Broccoli (200 g) | 40–100 | 8–20 | |
| Blueberry (100 g) | 30–160 | 3–16 | |
| Black currant (100 g) | 30–70 | 3–7 | |
| Apricot (200 g) | 25–50 | 5–10 | |
| Apple (200 g) | 20–40 | 4–8 | |
| Beans, green or white (200 g) | 10–50 | 2–10 | |
| Black grape (200 g) | 15–40 | 3–8 | |
| Tomato (200 g) | 2–15 | 0.4–3.0 | |
| Black tea infusion (200 mL) | 30–45 | 6–9 | |
| Green tea infusion (200 mL) | 20–35 | 4–7 | |
| Red wine (100 mL) | 2–30 | 0.2–3 | |
| Flavones | Parsley (5 g) | 240–1850 | 1.2–9.2 |
| Apigenin | Celery (200 g) | 20–140 | 4–28 |
| Luteolin | Capsicum pepper (100 g) | 5–10 | 0.5–1 |
| Flavanones | Orange juice (200 mL) | 215–685 | 40–140 |
| Hesperetin | Grapefruit juice (200 mL) | 100–650 | 20–130 |
| Naringenin | Lemon juice (200 mL) | 50–300 | 10–60 |
| Eriodictyol | |||
| Isoflavones | Soy flour (75 g) | 800–1800 | 60–135 |
| Daidzein | Soybeans, boiled (200 g) | 200–900 | 40–180 |
| Genistein | Miso (100 g) | 250–900 | 25–90 |
| Glycitein | Tofu (100 g) | 80–700 | 8–70 |
| Tempeh (100 g) | 430–530 | 43–53 | |
| Soy milk (200 mL) | 30–175 | 6–35 | |
| Monomeric flavanols | Chocolate (50 g) | 460–610 | 23–30 |
| Catechin | Beans (200 g) | 350–550 | 70–110 |
| Epicatechin | Apricot (200 g) | 100–250 | 20–50 |
| Cherry (200 g) | 50–220 | 10–44 | |
| Grape (200 g) | 30–175 | 6–35 | |
| Peach (200 g) | 50–140 | 10–28 | |
| Blackberry (100 g) | 130 | 13 | |
| Apple (200 g) | 20–120 | 4–24 | |
| Green tea (200 mL) | 100–800 | 20–160 | |
| Black tea (200 mL) | 60–500 | 12–100 | |
| Red wine (100 mL) | 80–300 | 8–30 | |
| Cider (200 mL) | 40 | 8 | |
Table 3. Plant, fruit and vegetable-based Polyphenols separated according to differences in chemical structure
| Groups | Classes | Examples and derivatives | Excellent sourcesa |
|---|---|---|---|
| Phenolics | Cinnamic | Caffeic acid | Coffee beans |
| Benzoic | Gallic acid | Tea leaves | |
| Flavonoidsb | Flavonols | Quercetin; kaempferol Quercetin glucosidesc | Yellow onions, deep green vegetables, cherry tomatoes |
| Flavones | Intensely flavored vegetables and herbs (celery, parsley) | ||
| Isoflavones | Soy products | ||
| Anthocyanins | Dark blue, red and deep red fruit and vegetables; red wine | ||
| Flavanonesd | Citrus fruit and pith of peel, concentrated tomato (paste) | ||
| Flavanols | Naringin; hesperetin; eriodictyol; naringenin | ||
| Catechins and derivatives | Tannins, grape seeds, cocoa | ||
| Epichatechin | |||
| Gallocatechin | |||
| Epigallocatechin | |||
| Epigallocatechin gallate | |||
| Proanthocyanidin | |||
| Stilbenes | Resveratrol | Trans and cis-resveratrol | Wine, red grapes |
| Piceid | Trans and cis-piceid | Red grape juice | |
| Lignans | For example, enterolignans | Seeds, legumes, wholegrains, bulbs, certain fruit | |
| Allicin |
aGood sources: flavonols (broccoli, leeks, ‘oily pigment’ of citrus peel); anthocyanidins (aubergines, blackberries, black currants, black grapes); naringin flavanols (cocoa, green beans, apricots, cherries, green tea, grape seeds)
bSix of 13 classes
cBioavailability greater than extract
dNaringin (grapefruit) bioavailability greater than hesperitin (oranges); eriodictyol (lemons)
Bioavailability of Polyphenols
Bioavailability is the proportion of the nutrient that is digested, absorbed and metabolized through normal pathways. Bioavailability of each and every polyphenol differs however there is no relation between the quantity of polyphenols in food and their bioavailability in human body. Generally, aglycones can be absorbed from the small intestine; however most polyphenols are present in food in the form of esters, glycosides or polymers that cannot be absorbed in native form 57). Before absorption, these compounds must be hydrolyzed by intestinal enzymes or by colonic microflora. During the course of the absorption, polyphenols undergo extensive modification; in fact they are conjugated in the intestinal cells and later in the liver by methylation, sulfation and/or glucuronidation.18 As a consequence, the forms reaching the blood and tissues are different from those present in food and it is very difficult to identify all the metabolites and to evaluate their biological activity. Importantly it is the chemical structure of polyphenols and not its concentration that determines the rate and extent of absorption and the nature of the metabolites circulating in the plasma. The most common polyphenols in our diet are not necessarily those showing highest concentration of active metabolites in target tissues; consequently the biological properties of polyphenols greatly differ from one polyphenol to another. Evidence, although indirect, of their absorption through the gut barrier is given by the increase in the antioxidant capacity of the plasma after the consumption of polyphenols-rich foods.
Polyphenols also differs in their site of absorption in humans. Some of the polyphenols are well absorbed in the gastro-intestinal tract while others in intestine or other part of the digestive tract. In foods, all flavonoids except flavanols exist in glycosylated forms. The fate of glycosides in the stomach is not clear yet. Most of the glycosides probably resist acid hydrolysis in the stomach and thus arrive intact in the intestine where only aglycones and few glucosides can be absorbed. Experimental studies carried out in rats23 showed that the absorption at gastric level is possible for some flavonoids, such as quercetin, but not for their glycosides. Moreover it has been recently shown that, in rats and mice, anthocyanins are absorbed from the stomach.
Accumulation of polyphenols in the tissues is the most important phase of polyphenol metabolism because this is the concentration which is biologically active for exerting the effects of polyphenols. Studies have shown that the polyphenols are able to penetrate tissues, particularly those in which they are metabolized such as intestine and liver. Excretion of polyphenols with their derivatives occurs through urine and bile. It has been observed that the extensively conjugated metabolites are more likely to be eliminated in bile, whereas small conjugates, such as monosulfates, are preferentially excreted in urine. Amount of metabolites excreted in urine is roughly correlated with maximum plasma concentrations. Urinary excretion percentage is quite high for flavanones from citrus fruit and decreases from isoflavones to flavonols. Thus the health beneficial effects of the polyphenols depend upon both the intake and bioavailability 58).
Polyphenols Health Benefits
Epidemiological studies have repeatedly shown an inverse association between the risk of chronic human diseases and the consumption of polyphenolic rich diet 59), 60). It is well established that polyphenol-rich foods and beverages may increase plasma antioxidant capacity 61). This increase in the antioxidative capacity of plasma following the consumption of polyphenol-rich food may be explained either by the presence of reducing polyphenols and their metabolites in plasma, by their effects upon concentrations of other reducing agents (sparing effects of polyphenols on other endogenous antioxidants), or by their effect on the absorption of pro-oxidative food components, such as iron 62). Consumption of antioxidants has been associated with reduced levels of oxidative damage to lymphocytic DNA. Similar observations have been made with plyphenol-rich food and beverages indicating the protective effects of polyphenols 63). There are increasing evidences that as antioxidants, polyphenols may protect cell constituents against oxidative damage and, therefore, limit the risk of various degenerative diseases associated with oxidative stress 64), 65), 66).
Dietary polyphenols exert preventive effects in treatment of asthma 67), 68). In asthma the airways react by narrowing or obstructing when they become irritated. This makes it difficult for the air to move in and out. This narrowing or obstruction can cause one or a combination of symptoms such as wheezing, coughing, shortness of breath and chest tightness. Epidemiological evidence that polyphenols might protect against obstructive lung disease come from studies that have reported negative associations of apple intake with prevalence and incidence of asthma, and a positive association with lung function 69), 70). Increased consumption of the soy isoflavone, genistein, was associated with better lung function in asthmatic patients 71).
Intake of polyphenols is also reported as beneficial in osteoporosis. Supplementation of diet with genistein, daidzein or their glycosides for several weeks prevents the loss of bone mineral density and trabecular volume caused by the ovariectomy 72). Polyphenols also protect skin damages induced from sunlight. Study on animals provide evidence that polyphenols present in the tea, when applied orally or topically, ameliorate adverse skin reactions following UV exposure, including skin damage, erythema and lipid peroxidation 73).
Black tea polyphenols are reported to be helpful in mineral absorption in intestine as well as to possess antiviral activity. Theaflavins present in black tea were found to have anti HIV-1 activity. These polyphenols inhibited the entry of HIV-1 cells into the target cells. HIV-1 entry into the target cell involves fusion of glycoprotein (GP) and envelope of the virus with the cell membrane of the host cells. Haptad repeat units present at N and C terminals of GP41 (membrane protein) on the viral envelope, fuse to form the fusion active GP41 core, which is a six-helical bundle. Theaflavins were found to block the formation of this six-helix bundle required for entry of the virus into the host 74). Theaflavin 3 3′ digallate, and theaflavin 3′ gallate were found to inhibit Severe Acute Respiratory Syndrome (SARS) corona virus. This antiviral activity was due to inhibition of the chymotrypsin like protease (3CL Pro) which is involved in the proteolytic processing during viral multiplication.58
Polyphenols Cardio-Protective Effect
Number of studies has demonstrated that consumption of polyphenols limits the incidence of coronary heart diseases.39–41 Atherosclerosis is a chronic inflammatory disease that develops in lesion-prone regions of medium-sized arteries. Atherosclerotic lesions may be present and clinically silent for decades before becoming active and producing pathological conditions such as acute myocardial infarction, unstable angina or sudden cardiac death.42 Polyphenols are potent inhibitors of LDL oxidation and this type of oxidation is considered to be a key mechanism in development of atherosclerosis.43 Other mechanisms by which polyphenols may be protective against cardiovascular diseases are antioxidant, anti-platelet, anti-inflammatory effects as well as increasing HDL, and improving endothelial function.44 Polyphenols may also contribute to stabilization of the atheroma plaque.
Quercetin, the abundant polyphenol in onion has been shown to be inversely associated with mortality from coronary heart disease by inhibiting the expression of metalloproteinase 1 (MMP1), and the disruption of atherosclerotic plaques.44 Tea catechins have been shown to inhibit the invasion and proliferation of the smooth muscle cells in the arterial wall, a mechanism that may contribute to slow down the formation of the atheromatous lesion.45 Polyphenols may also exert antithrombotic effects by means of inhibiting platelet aggregation. Consumption of red wine or non-alcoholic wine reduces bleeding time and platelet aggregation. Thrombosis induced by stenosis of coronary artery is inhibited when red wine or grape juice is administrated.46 Polyphenols can improve endothelial dysfunction associated with different risk factors for atherosclerosis before the formation of plaque; its use as a prognostic tool for coronary heart diseases has also been proposed.47 It has been observed that consumption of black tea about 450 ml increases artery dilation 2 hours after intake and consumption of 240 mL red wine for 30 days countered the endothelial dysfunction induced by a high fat diet.48 Long term regular intake of black tea was found to lower blood pressure in a cross-sectional study of 218 women above 70 years of age. Excretion of 4-O-methylgallic acid (4OMGA, a biomarker for tea polyphenols in body) was monitored. A higher consumption of tea and therefore higher excretion of 4OMGA were associated with lower blood pressure (BP). Tea polyphenols may be the components responsible for the lowering of BP. The effect may be due to antioxidant activity as well as improvement of endothelial function or estrogen like activity 75).
Resveratrol, the wine polyphenol prevents the platelet aggregation via preferential inhibition of cyclooxygenase 1 (COX 1) activity, which synthesizes thromboxane A2, an inducer of the platelet aggregation and vasoconstrictor 76). In addition to this, resveratrol is capable of relaxing the isolated arteries and rat aortic rings. The ability to stimulate Ca++-activated K+ channels and to enhance nitric oxide signaling in the endothelium are other pathways by which resveratrol exerts vasorelaxant activity 77), 78). Direct relation between cardiovascular diseases (CVDs) and oxidation of LDL is now well established. Oxidation of LDL particles is strongly associated with the risk of coronary heart diseases and myocardial infarctions. Studies have shown that resveratrol potentially inhibits the oxidation of the LDL particles via chelating copper or by direct scavenging of the free radicals. Resveratrol is the active compound in red wine which is attributed for “French Paradox”, the low incidence of CVD despite the intake of high-fat diet and smoking among French 79), 80). Association between polyphenol intake or the consumption of polyphenol-rich foods and incident of cardiovascular diseases were also examined in several epidemiological studies and it was found that consumption of polyphenol rich diet have been associated to a lower risk of myocardial infarction in both case-control and cohort studies 81).
Polyphenols Anti-Cancer Effect
Effect of polyphenols on human cancer cell lines, is most often protective and induce a reduction of the number of tumors or of their growth 82). These effects have been observed at various sites, including mouth, stomach, duodenum, colon, liver, lung, mammary gland or skin. Many polyphenols, such as quercetin, catechins, isoflavones, lignans, flavanones, ellagic acid, red wine polyphenols, resveratrol and curcumin have been tested; all of them showed protective effects in some models although their mechanisms of action were found to be different 83).
Development of cancer or carcinogenesis is a multistage and microevolutionary process. Into the three major stages of carcinogenesis: initiation, promotion and progression. Initiation is a heritable aberration of a cell. Cells so initiated can undergo transformation to malignancy if promotion and progression follow. Promotion, on the other hand, is affected by factors that do not alter DNA sequences and involves the selection and clonal expansion of initiated cells.
Several mechanisms of action have been identified for chemoprevention effect of polyphenols, these include estrogenic/antiestrogenic activity, antiproliferation, induction of cell cycle arrest or apoptosis, prevention of oxidation, induction of detoxification enzymes, regulation of the host immune system, anti-inflammatory activity and changes in cellular signaling 84).
Polyphenols influence the metabolism of pro-carcinogens by modulating the expression of cytochrome P450 enzymes involved in their activation to carcinogens. They may also facilitate their excretion by increasing the expression of phase II conjugating enzymes. This induction of phase II enzymes may have its origin in the toxicity of polyphenols. Polyphenols can form potentially toxic quinones in the body that are, themselves, substrates of these enzymes. The intake of polyphenols could then activate these enzymes for their own detoxication and, thus, induce a general boosting of our defenses against toxic xenobiotics 85). It has been demonstrated that tea catechins in the form of capsules when given to men with high-grade prostate intraepithelial neoplasia (PIN) demonstrated cancer preventive activity by inhibiting the conversion of high grade PIN lesions to cancer 86).
Theaflavins and thearubigins, the abundant polyphenols in black tea have also been shown to possess strong anticancer property. Black tea polyphenols were found to inhibit proliferation and increase apoptosis in Du 145 prostate carcinoma cells. Higher level of insulin like growth factor-1 (IGF-1) was found to be associated with a higher risk of development of prostate cancer. IGF-1 binding to its receptor is a part of signal transduction pathway which causes cell proliferation 87). Black tea polyphenol addition was found to block IGF-1 induced progression of cells into S phase of cell cycle at a dose of 40 mg/ml in prostate carcinoma cells 88).
Quercetin has also been reported to possess anticancer property against benzo(a)pyrene induced lung carcinogenesis in mice, an effect attrtibuted to its free radical scavenging activity. Resveratrol prevents all stages of development of cancer and has been found to be effective in most types of cancer including lung, skin, breast, prostate, gastric and colorectal cancer. It has also been shown to suppress angiogenesis and metastasis. Extensive data in human cell cultures indicate that resveratrol can modulate multiple pathways involved in cell growth, apoptosis and inflammation. The anti-carcinogenic effects of resveratrol appears to be closely associated with its antioxidant activity, and it has been shown to inhibit cyclooxygenase, hydroperoxidase, protein kinase C, Bcl-2 phosphorylation, Akt, focal adhesion kinase, NFκB, matrix metalloprotease-9 and cell cycle regulators. These and other in vitro and in vivo studies provide a rationale in support of the use of dietary polyphenols in human cancer chemoprevention, in a combinatorial approach with either chemotherapeutic drugs or cytotoxic factors for efficient treatment of drug refractory tumor cells.
Polyphenols Anti-Diabetic Effect
Impairment in glucose metabolism leads to physiological imbalance with the onset of the hyperglycemia and subsequently diabetes mellitus. There are two main categories of diabetes; type-1 and type-2. Studies have shown that several physiological parameters of the body get altered in the diabetic conditions 89), 90). Long term effects of diabetes include progressive development of specific complements such as retinopathy, which affects eyes and lead to blindness; nephropathy in which the renal functions are altered or disturbed and neuropathy which is associated with the risks of amputations, foot ulcers and features of autonomic disturbance including sexual dysfunctions. Numerous studies report the antidiabetic effects of polyphenols. Tea catechins have been investigated for their anti-diabetic potential 91), 92). Polyphenols may affect glycemia through different mechanisms, including the inhibition of glucose absorption in the gut or of its uptake by peripheral tissues. The hypoglycemic effects of diacetylated anthocyanins at a 10 mg/kg diet dosage were observed with maltose as a glucose source, but not with sucrose or glucose 93). This suggests that these effects are due to an inhibition of α-glucosidase in the gut mucosa. Inhibition of α-amylase and sucrase in rats by catechin at a dose of about 50 mg/kg diet or higher was also observed.
The inhibition of intestinal glycosidases and glucose transporter by polyphenols has been studied. Individual polyphenols, such as (+)catechin, (−)epicatechin, (−)epigallocatechin, epicatechin gallate, isoflavones from soyabeans, tannic acid, glycyrrhizin from licorice root, chlorogenic acid and saponins also decrease S-Glut-1 mediated intestinal transport of glucose. Saponins additionally delay the transfer of glucose from stomach to the small intestine. Resveratrol has also been reported to act as an anti-diabetic agent. Many mechanisms have been proposed to explain the anti-diabetic action of this stilbene, modulation of SIRT1 is one of them which improves whole-body glucose homeostasis and insulin sensitivity in diabetic rats. It is reported that in cultured LLC-PK1 cells, high glucose induced cytotoxicity and oxidative stress was inhibited by grape seed polyphenols. Resveratrol inhibits diabetes-induced changes in the kidney (diabetic nephropathy) and significantly ameliorates renal dysfunction and oxidative stress in diabetic rats. Treatment with resveratrol also decreased insulin secretion and delayed the onset of insulin resistance. A possible mechanism was thought to be related to the inhibition of K + ATP and K + V channel in beta cells.
Onion polyphenols, especially quercetin is known to possess strong anti diabetic activity. A recent study shows that quercetin has ability to protect the alterations in diabetic patients during oxidative stress. Quercetin significantly protected the lipid peroxidation and inhibition antioxidant system in diabetics. Hibiscus sabdariffa extract contains polyphenolic acids, flavonoids, protocatechuic acid and anthocyanins. A study performed by Lee et al. 94) showed that polyphenols present in the extracts from Hibiscus sabdariffa attenuate diabetic nephropathy including pathology, serum lipid profile and oxidative markers in kidney. Ferulic acid (FA) is another polyphenol very abundant in vegetables and maize bran. Several lines of evidence have shown that ferulic acid acts as a potent anti-diabetic agent by acting at many levels. It was demonstrated that FA lowered blood glucose followed by a significantly increased plasma insulin and a negative correlation between blood glucose and plasma insulin 95), 96).
Polyphenols Anti-Aging Effect
Aging is the accumulation process of diverse detrimental changes in the cells and tissues with advancing age, resulting in an increase in the risks of disease and death. Among many theories purposed for the explaining the mechanism of aging, free radical/oxidative stress theory is one of the most accepted one 97). A certain amount of oxidative damage takes place even under normal conditions; however, the rate of this damage increases during the aging process as the efficiency of antioxidative and repair mechanisms decrease. Antioxidant capacity of the plasma is related to dietary intake of antioxidants; it has been found that the intake of antioxidant rich diet is effective in reducing the deleterious effects of aging and behavior. Several researches suggest that the combination of antioxidant/anti-inflammatory polyphenolic compounds found in fruits and vegetables may show efficacy as anti-aging compounds 98), 99). Subset of the flavonoids known as anthocyanins, are particularly abundant in brightly colored fruits such as berry fruits and concord grapes and grape seeds. Anthocyanins are responsible for the colors in fruits, and they have been shown to have potent antioxidant/anti-inflammatory activities, as well as to inhibit lipid peroxidation and the inflammatory mediators cyclo-oxygenase (COX)-1 and -2 100).
Fruit and vegetable extracts that have high levels of flavonoids also display high total antioxidant activity such as spinach, strawberries and blueberries. It is reported that the dietary supplementations (for 8 weeks) with spinach, strawberry or blueberry extracts in a control diet were also effective in reversing age-related deficits in brain and behavioral function in aged rats 101). A recent study demonstrates that the tea catechins carry strong anti-aging activity and consuming green-tea rich in these catechins, may delay the onset of aging 102).
Polyphenols are also beneficial in ameliorating the adverse effects of the aging on nervous system or brain. Paramount importance for the relevance of food polyphenols in the protection of the aging brain is the ability of these compounds to cross the blood-brain barrier (BBB), which tightly controls the influx in the brain of metabolites and nutrients as well as of drugs. Resveratrol has been found to consistently prolong the life span; its action is linked to an event called caloric restriction or partial food deprivation 103).
Grape polyphenol, resveratrol is very recent entry as an antiaging agent. It has been shown that the early target of the resveratrol is the sirtuin class of nicotinamide adenine dinucleotide (NAD)-dependent deacetylases. Seven sirtuins have been identified in mammals, of which SIRT-1 is believed to mediate the beneficial effects on health and longevity of both caloric restriction and resveratro 104)l. Resveratrol increased insulin sensitivity, decreased the expression of IGF-1 and increased AMP-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor-c coactivator 1a (PGC-1a) activity. When examined for the mechanism, it activated forkhead box O (FOXO), which regulates the expression of genes that contribute both to longevity and resistance to various stresses and insulin-like growth factorbinding protein 1 (IGFBP-1) 105). There are experimental evidences that resveratrol can extend lifespan in the yeast Saccharomyces cerevisiae, the fruit fly Drosophila melanogaster, the nematode worm C. elegans, and seasonal fish Nothobranchius furzeri. Recently quercetin has also been reported to exert preventive effect against aging 106).
Polyphenols Neuro-Protective Effects
Oxidative stress and damage to brain macromolecules is an important process in neurodegenerative diseases. Alzheimer’s disease is one of the most common occurring neurodisorder affecting up to 18 million people worldwide. Because polyphenols are highly antioxidative in nature, their consumption may provide protection in neurological diseases 107). It was observed that the people drinking three to four glasses of wine per day had 80% decreased incidence of dementia and Alzheimer’s disease compared to those who drank less or did not drink at all 108).
Resveratrol, abundantly present in wine scavenges O2− and OH· in vitro, as well as lipid hydroperoxyl free radicals, this efficient antioxidant activity is probably involved in the beneficial effect of the moderate consume of red wine against dementia in the elderly. Resveratrol inhibits nuclear factor κB signaling and thus gives protection against microglia-dependent β-amyloid toxicity in a model of Alzheimer’s disease and this activity is related with the activation of the SIRT-1. It was found that the consumption of fruit and vegetable juices containing high concentrations of polyphenols, at least three times per week, may play an important role in delaying the onset of Alzheimer’s disease 109). Polyphenols from fruits and vegetables seem to be invaluable potential agents in neuroprotection by virtue of their ability to influence and modulate several cellular processes such as signaling, proliferation, apoptosis, redox balance and differentiation 110).
Recently Aquilano et al. 111) reported that administration of polyphenols provide protective effects against Parkinson’s disease, a neurological disorder characterized by degeneration of dopaminergic neurons in the substantia nigra zona compacta. Nutritional studies have linked the consumption of green tea to the reduced risk of developing Parkinson’s disease. In animal models epigallocatechin gallate (EGCG) has been shown to exert a protective role against the neurotoxin MPTP (N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), an inducer of a Parkinson’s-like disease, either by competitively inhibiting the uptake of the drug, due to molecular similarity or by scavenging MPTP-mediated radical formation. EGCG may also protect neurons by activating several signaling pathways, involving MAP kinases which are fundamental for cell survival. The therapeutic role of catechins in Parkinson’s disease is also due to their ability to chelate iron. This property contributes to their antioxidant activity by preventing redox-active transition metal from catalyzing free radicals formation. Moreover, the antioxidant function is also related to the induction of the expression of antioxidant and detoxifying enzymes particularly in the brain, which is not sufficiently endowed of a well-organized antioxidant defense system.89 Maize bran polyphenol, ferulic acid is also reported to be beneficial in Alzheimer’s disease. This effect is due to its antioxidant and anti-inflammatory properties.
Summary
Fruits like grapes, apple, pear, cherries and berries contains up to 200–300 mg polyphenols per 100 grams fresh weight and beverages such as tea and red wine represent the main sources of polyphenols, but vegetables, leguminous plants, and cereals are also important sources. Polyphenols or polyphenol rich diets provide significant protection against the development and progression of many chronic pathological conditions including cancer, diabetes, cardio-vascular problems and aging. Although several biological effects based on epidemiological studies can be scientifically explained, the mechanism of action of some effects of polyphenols is not fully understood. However based on our current scientific understanding, polyphenols offer great hope for the prevention of chronic human diseases.
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