《食品化学》课程教学资源（文献资料）Polyphenol：food sources and bioavailability

Polyphenols:food sources and bioavailability12 Claudine Manach,Augustin Scalbert.Christine Morand.Christian Remesy,and Liliana Jimenez are abundant micronutrients in our diet and evidence es such as existing polyphenols are likely to prov ide the greatest protecti endon the amount and on their bioava ability.In.the nature and of the various poly tion of these compounds in our diet.Such knowledge will allow between the inta of these substances and the risk of develop rption and the influence of chemical structure (eg glycosyla- ively metabolized he d by h he inte in the catabolism of polyphe nols and the roduction of s ome active arget r the various polyphe s are co n the metabolism boeipopeiesorthe TYPES AND DISTRIBUTION OF POLYPHENOLS IN FOODS discus Final dan ups on arom atic rings) severa on Feb 粉 of the hundre generally involved in defense against ultraviolet radiationo that they fh structura that bind th 15.2012 ure 1 The ability flavonoids,which ng of 2 ar that form an o ated hete cle (r INTRODUCTION es as a function of the type of heteroc nce inour diet,an one another. tive stress s.such as cancer r and cular and neurodegen d C.Remes From the Inite des maladies metal ach rch.Pa elle.France found in many ddres ite d tivity of a reprin In th 6312c nra fr biological actions that are as yet poorly understood. :727-47.Printed in USA.004 American Society for Clinical Nutrition 727

Polyphenols: food sources and bioavailability1,2 Claudine Manach, Augustin Scalbert, Christine Morand, Christian Rémésy, and Liliana Jime´nez ABSTRACT Polyphenols are abundant micronutrients in our diet, and evidence for their role in the prevention of degenerative diseases such as cancer and cardiovascular diseases is emerging. The health effects of polyphenols depend on the amount consumed and on their bioavail￾ability. In this article, the nature and contents of the various poly￾phenols present in food sources and the influence of agricultural practices and industrial processes are reviewed. Estimates of dietary intakes are given for each class of polyphenols. The bioavailability of polyphenols is also reviewed, with particular focus on intestinal absorption and the influence of chemical structure (eg, glycosyla￾tion, esterification, and polymerization), food matrix, and excretion back into the intestinal lumen. Information on the role of microflora in the catabolism of polyphenols and the production of some active metabolites is presented. Mechanisms of intestinal and hepatic con￾jugation (methylation, glucuronidation, sulfation), plasma transport, and elimination in bile and urine are also described. Pharmacokinetic data for the various polyphenols are compared. Studies on the iden￾tification of circulating metabolites, cellular uptake, intracellular metabolism with possible deconjugation, biological properties of the conjugated metabolites, and specific accumulation in some target tissues are discussed. Finally, bioavailability appears to differ greatly between the various polyphenols, and the most abundant polyphenols in our diet are not necessarily those that have the best bioavailability profile. A thorough knowledge of the bioavailability of the hundreds of dietary polyphenols will help us to identify those that are most likely to exert protective health effects. Am J Clin Nutr 2004;79:727–47. KEY WORDS Polyphenols, flavonoids, phenolic acids, food sources, dietary intake, intestinal absorption, metabolism, bioavail￾ability INTRODUCTION Over the past 10 y, researchers and food manufacturers have become increasingly interested in polyphenols. The chief reason for this interest is the recognition of the antioxidant properties of polyphenols, their great abundance in our diet, and their probable role in the prevention of various diseases associated with oxida￾tive stress, such as cancer and cardiovascular and neurodegen￾erative diseases (Scalbert A, Manach C, Morand C, Rémésy C, Jime´nez L. Crit Rev Food Sci Nutr, in press). Furthermore, poly￾phenols, which constitute the active substances found in many medicinal plants, modulate the activity of a wide range of en￾zymes and cell receptors (1). In this way, in addition to having antioxidant properties, polyphenols have several other specific biological actions that are as yet poorly understood. Two aims of research are to establish evidence for the effects of polyphenol consumption on health and to identify which of the hundreds of existing polyphenols are likely to provide the greatest protection in the context of preventive nutrition. If these objectives are to be attained, it is first essential to determine the nature and distribu￾tion of these compounds in our diet. Such knowledge will allow evaluation of polyphenol intake and enable epidemiologic anal￾ysis that will in turn provide an understanding of the relation between the intake of these substances and the risk of develop￾ment of several diseases. Furthermore, not all polyphenols are absorbed with equal efficacy. They are extensively metabolized by intestinal and hepatic enzymes and by the intestinal micro￾flora. Knowledge of the bioavailability and metabolism of the various polyphenols is necessary to evaluate their biological activity within target tissues. The types and distribution of poly￾phenols in foods and the bioavailability of polyphenols are the topics of the present review. TYPES AND DISTRIBUTION OF POLYPHENOLS IN FOODS Several thousand molecules having a polyphenol structure (ie, several hydroxyl groups on aromatic rings) have been identified in higher plants, and several hundred are found in edible plants. These molecules are secondary metabolites of plants and are generally involved in defense against ultraviolet radiation or aggression by pathogens. These compounds may be classified into different groups as a function of the number of phenol rings that they contain and of the structural elements that bind these rings to one another. Distinctions are thus made between the phenolic acids, flavonoids, stilbenes, and lignans (Figure 1). The flavonoids, which share a common structure consisting of 2 ar￾omatic rings (A and B) that are bound together by 3 carbon atoms that form an oxygenated heterocycle (ring C), may themselves be divided into 6 subclasses as a function of the type of heterocycle involved: flavonols, flavones, isoflavones, flavanones, antho￾cyanidins, and flavanols (catechins and proanthocyanidins) (Figure 2). In addition to this diversity, polyphenols may be associated with various carbohydrates and organic acids and with one another. 1 From the Unité des Maladies Métaboliques et Micronutriments, INRA, Saint-Genès Champanelle, France (CM, AS, CM, and CR), and Danone Vitapole Research, Palaiseau cedex, France (LJ). 2 Address reprint requests to C Manach, Unité des Maladies Métaboliques et Micronutriments, INRA, 63122 Saint-Genès Champanelle, France. E￾mail: manach@clermont.inra.fr. Received June 3, 2003. Accepted for publication October 17, 2003. Am J Clin Nutr 2004;79:727–47. Printed in USA. © 2004 American Society for Clinical Nutrition 727 by guest on February 15, 2012 www.ajcn.org Downloaded from

728 MANACH ET AL Hydroxybenzoic acids Hydroxycinnamic acids "o R:及8eos 骨 及 尽：传oegm Flavonoids Isoflavones See Flpure 2 。 Stilbenes Lignans R:6:0aee Anthocyanidins R FIGURE 1.Chemical sture of polyphenos. Phenolic acids 及R:8t8 The hvdroxy zoic acid content of edible plants with milligrams Feb allic aci t such asst asp Y15.2012 FIGURE 2.Chemical structures of flavonoids. and are no rations ge rease during the course of ripening but crease as the ftutincrcascsintC ids are r rely found in the free undergone free acid.Catfeircandquimic cid combine to form chlorogenic aci a singl contain 70-350m nic acid and bran is the mn ofpolyphenols(3 loat f tely th the total hydroxycinnamic acid content of most fruit.Hydroxy- acid wheat rn() hough the hihest Cfceteaidclecndnereasandomtidencturs I pa

Phenolic acids Two classes of phenolic acids can be distinguished: deriva￾tives of benzoic acid and derivatives of cinnamic acid (Figure 1). The hydroxybenzoic acid content of edible plants is generally very 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 (2). Tea is an important source of gallic acid: tea leaves may contain up to 4.5 g/kg fresh wt (3). Furthermore, hydroxybenzoic acids are components of complex structures such as hydrolyzable tannins (gallotannins in mangoes and ellagitannins in red fruit such as strawberries, rasp￾berries, and blackberries) (4). 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 (5). The types of fruit having the highest content (blueberries, kiwis, plums, cherries, apples) contain 0.5–2 g hydroxycinnamic acids/kg fresh wt (Table 1) (6). 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. Hydroxy￾cinnamic acids are found in all parts of fruit, although the highest concentrations are seen in the outer parts of ripe fruit. Concen￾trations 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 rep￾resent up to 90% of total polyphenols (28, 29). 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 (30). Rice and oat flours containapproximatelythe same quantity of phenolic acids as wheat flour (63 mg/kg), although the content in maize flour is about 3 times as high (2). Ferulic acid is found chiefly in the transform, which is esterified to arabinoxylans andhemicellulosesinthealeuroneandpericarp.Only10%offerulic acid is found in soluble free form in wheat bran (29). Several dimers of ferulic acid are also found in cereals and form bridge structures between chains of hemicellulose. FIGURE 1. Chemical structures of polyphenols. FIGURE 2. Chemical structures of flavonoids. 728 MANACH ET AL by guest on February 15, 2012 www.ajcn.org Downloaded from

POLYPHENOLS:FOOD SOURCES AND BIOAVAILABILITY 729 TABLE I s in food Polyphenol conten So urce (rving size) By wt or vol By serving mg/g fresh w (or mg/四 Bla lic acid amic acids (2.5-7) acid le (200 0g1 .oat (75 g) nidin ols (1) 200g t(200g 老 200e20g on February 15.2012 es1-l2,14,18) 22-25) Tof (00 08 5.1.,.**1971.3 00E

TABLE 1 Polyphenols in foods Source (serving size) Polyphenol content By wt or vol By serving mg/kg fresh wt (or mg/L) mg/serving Hydroxybenzoic acids (2, 6) 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 (2, 5–7) 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 (8–10) 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 (11–18) 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 (11–12, 14, 18) 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 (19–21) 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 (22–25) 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 (6, 17, 26, 27) 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 POLYPHENOLS: FOOD SOURCES AND BIOAVAILABILITY 729 by guest on February 15, 2012 www.ajcn.org Downloaded from

代 MANACH ET AL flavonoids ids content of its man ively low concentrations gfresh wt.and contains between sugar moiety is very often seor rhamnose,but other sugars (catechins)and the may also s of fruit (a re th 6)Thes flay nols accumulate in the outer Table 1).The mg/L. s)becau se their biosynthesis is far the riches me ee and even between dif 381 ta gre zed during heating)of tea leaves to mor expo the cabbage,the glycoside concentrati nis≥l0 times as high n the green outer leaves as in the i ne olored lea ves(14).This atechin epiga cherry tomatoes than of use they have and more im ard ton ain see(7.39) st to other lasses of flav onoids,flavanol are n d to hea 15%ofh The only important edible ances are es of flavones ident vater at pH 5(40) The are dimer oligomers andp mers of catechins that are contain C 32-341 skin of citrus fruit conta arge qu tities of polvme d the mean degre of om dto 4 the for are the most hydrophobic natic plant an and uch as mint but ent in high tea The main agly cho centrations only in citrusfruit nes are naringe nin ingrap ando anpea hen th hes ripene this change has e ge impartsa bitter taste been whic s flavo unts for the apparent redu tion in tannin content that is com a single glass of ora dins are as rating th ery high flav tent, whol olar sap ofthe with struc b 9)7 7 and are not rms,both according to pH. cule. This c a ht H at a likely to d Degra and ester ids (citric products are the main lic acids In addition, citein. a concer are -0-glucosio OL d in red win malonylgluc side derivatives have unpleas and ishes).but they are most abundant in fruit Cvanidin is the most

Flavonoids Flavonols are the most ubiquitous flavonoids in foods, and the main representatives are quercetin and kaempferol. 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 1). 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, glucu￾ronic acid). Fruit often contains between 5 and 10 different fla￾vonol glycosides (6). 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 dif￾ferent sides of a single piece of fruit, depending on exposure to sunlight (31). 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 (14). 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 identi￾fied to date are parsley and celery (Table 1). Cereals such as millet and wheat contain C-glycosides of flavones (32–34). 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) (2). 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 con￾centrations only in citrus fruit. The main aglycones are naringe￾nin in grapefruit, hesperetin in oranges, and eriodictyol in lem￾ons. 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 flavor￾less. Orange juice contains between 200 and 600 mg hesperi￾din/L and 15–85 mg narirutin/L, and a single glass of orange juice may contain between 40 and 140 mg flavanone glycosides (20). 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 estro￾gens. Although they are not steroids, they have hydroxyl groups in positions 7 and 4' in a configuration analogous to that of the hy￾droxyls in the estradiol molecule. This confers pseudohormonal properties on them, including the ability to bind to estrogen recep￾tors, and they are consequently classified as phytoestrogens. Isofla￾vones are found almost exclusively in leguminous plants. Soya and its processed products are the main source of isoflavones in the humandiet.Theycontain3mainmolecules:genistein,daidzein,and glycitein, generally in a concentration ratio of 1:1:0.2. These isofla￾vones are found in 4 forms: aglycone, 7-O-glucoside, 6
-O-acetyl- 7-O-glucoside, and 6
-O-malonyl-7-O-glucoside (35). The 6
-O￾malonylglucoside derivatives have an unpleasant, bitter, and astringent taste. 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 man￾ufactured products varies greatly as a function of geographic zone, growing conditions, and processing. Soybeans contain between 580 and3800mgisoflavones/kgfreshwt,andsoymilkcontainsbetween 30 and 175 mg/L (25, 37). 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 1). 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 (38). 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 legu￾minous plants, in grapes, and more importantly in tea (27, 39). In contrast to other classes of flavonoids, flavanols are not glyco￾sylated 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 (40). Proanthocyanidins, which are also known as condensed tan￾nins, are dimers, oligomers, and polymers of catechins that are bound together by links between C4 and C8 (or C6). Their mean degree of polymerization in foods has rarely been determined. In cider apples, the mean degree of polymerization ranges from 4 to 11 (41). Through the formation of complexes with salivary pro￾teins, condensed tannins are responsible for the astringent char￾acter of fruit (grapes, peaches, kakis, apples, pears, berries, etc) and beverages (wine, cider, tea, beer, etc) and for the bitterness of choc￾olate (42). This astringency changes over the course of maturation andoftendisappearswhenthefruitreachesripeness;thischangehas been well explained in the kaki fruit by polymerization reactions with acetaldehyde (43). Such polymerization of tannins probably accounts for the apparent reduction in tannin content that is com￾monly seen during the ripening of many types of fruit. It is difficult to estimate the proanthocyanidin content of foods because proan￾thocyanidinshaveawiderangeofstructuresandmolecularweights. The only available data concern dimers and trimers, which are as abundant as the catechins themselves (26). 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 (9). They exist in different chem￾ical forms, both colored and uncolored, according to pH. Al￾though they are highly unstable in the aglycone form (anthocya￾nidins), while they are in plants, they are resistant to light, pH, and oxidation conditions that are likely to degrade them. Degra￾dation 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 fla￾vonoids (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, rad￾ishes), 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 730 MANACH ET AL by guest on February 15, 2012 www.ajcn.org Downloaded from

POLYPHENOLS:FOOD SOURCES AND BIOAVAILABILITY 731 fresh wt in blackberries (Table 1).These values wt and dge is often lim ted toone ora few varieties into various complex structures as the wine ages(10,44). particularly some exotic types of fruit and some Lignans include ripeness at the time of harvest.environmental factors. 2phenylpropane units(Figure 1).The e he hol(udry )f in lin s or f 109 1000 times as high as cctabolized to enteroiotand food tagcT degree s conside ncentra tions a our ing anin conc measured inplasmaandurine.Thus,thereare undou he though very few studies directly addressed this issue the poly without stress,such as those grown in conventional or hydro ables (garlic.asparagus.carrots).and fruit (ears.prunes) as minor sources. in theh tica nic effects hav own during oI m in s and which has were 03 ightorcli ate (54) 图 use resverat e Oxidation ctio sult in the tion of more o hanges in the qu with black tea)or harful 15200a VARIABILITY OF POLYPHENOL CONTENT OF FOODS nolic acids in qualitative terms,bu ons.)whe eas 58.A125 results ir a ep nidin B2 namy.chboogsaaxdandmlgnttesooherhndo vegetable cant portion of p ts tha in th may also h The polyp I pro frying(18).Steam cooking of vege les,wh havoids leac

fresh wt in blackcurrants or blackberries (Table 1). 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 (10, 44). Lignans Lignans are formed of 2 phenylpropane units (Figure 1). The richest dietary source is linseed, which contains secoisolaricir￾esinol (up to 3.7 g/kg dry wt) and low quantities of matairesinol. 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 (45). Lignans are metabolized to enterodiol and en￾terolactone 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 en￾terolactone and that have not yet been identified (46). Thompson et al (47) used an in vitro technique involving the fermentation of foods by human colonic microflora to quantitatively evaluate precursors of enterodiol and enterolactone. They confirmed that oleaginous seeds (linseed) 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. 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) (48–50). However, because resveratrol is found in such small quantities in the diet, any protective effect of this molecule is unlikely at normal nutritional intakes. VARIABILITY OF POLYPHENOL CONTENT OF FOODS Fruit and beverages such as tea and red wine constitute the main sources of polyphenols. Certain polyphenols such as quer￾cetin 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 hydroxy￾cinnamic acids, 2 glycosides of phloretin, several quercetin gly￾cosides, 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 vari￾eties of apples have notably been studied. The polyphenol pro￾files of all varieties of apples are practically identical, but con￾centrations 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 (41, 51). For many plant products, the polyphenol composition is much less known, knowledge is often limited to one or a few varieties, and data sometimes do not concern the edible parts. Some foods, particularly some exotic types of fruit and some cereals, have not been analyzed yet. Furthermore, numerous factors other than variety may affect the polyphenol content of plants; these factors include ripeness at the time of harvest, environmental factors, processing, and storage. Environmental factors have a major effect on polyphenol con￾tent. These factors may be pedoclimatic (soil type, sun exposure, rainfall) or agronomic (culture in greenhouses or fields, biolog￾ical culture, hydroponic culture, fruit yield per tree, etc). Expo￾sure to light has a considerable effect on most flavonoids. The degree of ripeness considerably affects the concentrations and proportions of the various polyphenols (6). In general, phenolic acid concentrations decrease during ripening, whereas anthocy￾anin concentrations increase. Many polyphenols, especially phe￾nolic acids, are directly involved in the response of plants to different types of stress: they contribute to healing by lignifica￾tion of damaged areas, they possess antimicrobial properties, and their concentrations may increase after infection (2, 6, 52). Al￾though very few studies directly addressed this issue, the poly￾phenol content of vegetables produced by organic or sustainable agriculture is certainly higher than that of vegetables grown without stress, such as those grown in conventional or hydro￾ponic conditions. This was shown recently in strawberries, blackberries, and corn (53). With the current state of knowledge, it is extremely difficult to determine for each family of plant products the key variables that are responsible for the variability in the content of each polyphenol and the relative weight of those variables. A huge amount of analysis would be required to obtain this information. For example, determination of the p-coumaric acid content of 500 red wines showed that genetic factors were more important than was exposure to light or climate (54). Storage may also affect the content of polyphenols that are easily oxidized. 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 (browningoffruit)toconsumeracceptability.Storageofwheatflour results in marked loss of phenolic acids (28). After 6 mo of storage, flours contained the same phenolic acids in qualitative terms, but their concentrations were 70% lower. Cold storage, in contrast, did not affect the content of polyphenols in apples (55, 56), pears (57), or onions (58). At 25 °C, storage of apple juice for 9 mo results in a 60% loss of quercetin and a total loss of procyanidins, despite the fact that polyphenols are more stable in fruit juices than is vita￾min C (59, 60). 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 poly￾phenols 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 (18). Steam cooking of vegetables, which avoids leaching, is preferable. Potatoes contain up to 190 mg chlorogenic acid/kg, mainly in the skin (61). Extensive loss occurs during cooking, POLYPHENOLS: FOOD SOURCES AND BIOAVAILABILITY 731 by guest on February 15, 2012 www.ajcn.org Downloaded from