Dietary Polyphenols and Their Biological Significance

Introduction

Oxidative stress results in oxidative alteration of biological macromolecules such as lipids, proteins and nucleic acids. It is considered to play a pivotal role in the pathogenesis of aging and degenerative diseases [ 1 – 3 ]. In order to cope with an excess of free radicals produced upon oxidative stress, human bodies have developed sophisticated mechanisms for maintaining redox homeostasis. These protective mechanisms include scavenging or detoxification of reactive oxygen species (ROS), blocking ROS production, sequestration of transition metals, as well as enzymatic and nonenzymatic antioxidant defenses produced in the body, that is, endogenous [ 4 , 5 ], and others supplied with the diet, namely, exogenous ones. Among them, dietary polyphenols have been widely studied for their strong antioxidant capacities and other properties by which cell functions are regulated [ 6 , 7 ].

Dietary polyphenols represent a group of secondary metabolites which widely occur in fruits, vegetables, wine, tea, extra virgin olive oil, chocolate and other cocoa products. They are mostly derivatives, and/or isomers of flavones, isoflavones, flavonols, catechins, and phenolic acids. Dietary polyphenols exhibit many biologically significant functions, such as protection against oxidative stress, and degenerative diseases. Experimental data indicate that most of these biological actions can be attributed to their intrinsic antioxidant capabilities. Dietary polyphenols may offer an indirect protection by activating endogenous defense systems and by modulating cellular signaling processes such as NF-κB activation, AP-1 DNA binding, glutathione biosynthesis, PI3-kinase/Akt pathway, MAPK proteins (ERK, JNK and P38) activation, and the translocation into the nucleus of Nrf2 [ 8 – 10 ].

Classification and occurrence of dietary polyphenols

Dietary polyphenols are the most abundant antioxidants in human diets. With over 8,000 structural variants, they are secondary metabolites of plants and denote many substances with aromatic ring(s) bearing one or more hydroxyl moieties. They are subdivided into groups ( Figure 1 ) by the number of phenolic rings and of the structural elements that link these rings [ 11 ]: (1) The phenolic acids with the subclasses derived from hydroxybenzoic acids such as gallic acid and from hydroxycinnamic acid, containing caffeic, ferulic, and coumaric acid; (2) the large flavonoid subclass, which includes the flavonols, flavones, isoflavones, flavanones, anthocyanidins, and flavanols; (3) the stilbenes; and (4) the lignans and the polymeric lignins.

The main dietary sources of polyphenols include some common fruits, vegetables and beverages. Phenolic acids account for about one third of the total intake and flavonoids account for the remaining two thirds. The most abundant flavonoids in the diet are flavanols (catechins plus proanthocyanidins), anthocyanins and their oxidation products. The main polyphenol dietary sources are fruit and beverages (fruit juice, wine, tea, coffee, chocolate and beer) and, to a lesser extent vegetables, dry legumes and cereals. Most of dietary polyphenols and their sources in our diets were shown in Table 1 .

Phenolic acids

A major class within the phenolic compounds is the hydroxycinnamic acids, which are widely distributed in plant kingdom. The major hydroxycinnamic acid is caffeic acid, which occurs in foods mainly as an ester with quinic acid called chlorogenic acid (5-caffeoylquinic acid). Chlorogenic acid and caffeic acid are antioxidants in vitro and they might inhibit the formation of mutagenic and carcinogenic N -nitroso compounds for the inhibitory effect on the N -nitrosation reaction in vitro .

Flavonoids

Flavonoids are the most abundant polyphenols in human diets, and are mainly divided into: (a) anthocyanins, glycosylated derivative of anthocyanidin, present in colorful flowers and fruits; (b) anthoxanthins, a group of colorless compounds further divided in several categories, including flavones, flavans, flavonols, flavanols, isoflavones, and their glycosides. Flavonols are mainly represented by myricetin, fisetin, quercetin and kaempferol.

Stilbenes

Stibenes are structurally characterized by the presence of a 1,2-diphenylethylene nucleus with hydroxyls substitued on the aromatic rings, and exist in the form of monomers or oligomers. The best known compound is trans-resveratrol, possessing a trihydroxystilbene skelelton.

Tannins

Tannins are a group of water-soluble polyphenols having molecular weights from 500 to 3,000 which are subdivided into condensed and hydrolisable tannins, and commonly found complexed with alkaloids, polysaccharides and proteins, particularly the latter. On the basis of structural characteristics there are two groups, gallotannins and ellagitannins of hydrolysable tannins.

Diferuloylmethanes

Diferuloylmethanes are a small group of phenolic compounds with two aromatic rings substitued with hydroxyls, and linked by aliphatic chain containing carbonyl groups. There are also some other polyphenols such as hydroxytyrosol, a simple polyphenol presenting in olive fruits and olive oil [ 12 , 13 ].

Bioactivities of dietary polyphenols

Oxidative stress is considered to play a pivotal role in the pathogenesis of aging and several degenerative diseases, such as atherosclerosis, cardiovascular disease, type II diabetes and cancer [ 1 – 3 ]. In order to cope with an excess of free radicals produced upon oxidative stress, humans have developed endogenous and exogenous mechanisms in order to maintain redox homeostasis. Among these, dietary polyphenols have been largely studied for their strong antioxidant capacities and other properties by which cell activities are regulated ( Figures 2 and 3 ).

Antioxidant and free radical scavenging properties

In order to combat and neutralize the deleterious effects of ROS, various antioxidant strategies have evolved either by increasing the endogenous antioxidant enzyme defenses or by enhancing the non-enzymatic defenses through dietary or pharmacological means ( Table 2 ). Dietary polyphenols have been reported to possess potent antioxidant activity by endogenous and exogenous mechanisms.

Dihydrocaffeic acid was able to scavenge free radicals (superoxide anion, hydroxyl and peroxyl radicals) in human EA.hy926 endothelial cells [ 42 ]. Curcumin and quercetin increased several antioxidant enzyme activities such as glutathione peroxidase (GPx), superoxide dismutase (SOD), catalase (CAT) or glutathione reductase (GR) in vivo and in vitro [ 8 , 9 , 44 ], and activated endogenous defense systems in vitro [ 40 , 45 ]. Hydroxytyrosol could increase CAT and SOD activities in rats fed a cholesterol-rich diet [ 35 ].

The transcription factor Nrf2 regulates the basal and inducible expression of numerous detoxifying and antioxidant genes. The Nrf2–Kelch-like ECH-associated protein 1 (Keap1)-ARE system is now recognized as one of the major cellular defence mechanisms against oxidative and xenobiotic stresses [ 46 ]. (−)-Epigallochatechin gallate (EGCG) and (−)-epichatechin gallate (ECG) induced ARE-mediated gene expression through the activation of MAPK proteins (ERK, JNK and p38) in HepG2-ARE-C8 cell [ 10 ]. Tanigawa et al. reported that quercetin-induced ARE activity involves upregulation of Nrf2 through the regulation of both transcription and posttranscription sites and repression of Keap1 by affecting the posttranscription site in HepG2 cells [ 48 ]. Curcumin could increase the expression of glutathione S-transferase P1 (GSTP1) by activing ARE and Nrf2 in HepG2 cells [ 40 ].

Anti-atherosclerosis and cardioprotection

Studies have shown that some of dietary polyphenols exerted anti-atherosclerosis and cardioprotection ( Table 3 ). Oleuropein inhibited the oxidation of low density lipoprotein (LDL) in vitro [ 61 ]. Quercetin decreased lipid peroxidation, upregulated the expression of serum high density lipoprotein (HDL)-associated paraoxonase 1(PON-1) in the HuH7 human hepatoma cell line [ 66 ], inhibited oxidized LDL (oxLDL)-triggered apoptosis, and increased intracellular glutathione (GSH) downregulation in COS-1 cells [ 68 ].

Proanthocyanidin could significantly reduce cardiomyocyte apoptosis by inhibiting ischemia/reperfusion-induced activation of JNK-1 and c-Jun in Male Sprague Dawley rats [ 74 ]. Furthermore, proanthocyanidin could regulate the levels of CD36 mRNA and protein in oxLDL treated peripheral blood mononuclear cells [ 73 ]. Resveratrol showed that in vitro it could decrease the expression of vascular cell adhesion molecule-1 (VCAM-1) [ 64 ], cyclooxygenase-2 (COX-2) [ 55 ], and matrix metalloproteinase-9 (MMP-9) mRNA [ 56 ] through suppression of activation of nuclear factor AP-1 [ 55 ]. Hydroxytyrosol could not only lower serum total cholesterol (TC) and low density lipoprotein cholesterol (LDL-C), but also slow the lipid peroxidation process in rats fed a cholesterol-rich diet [ 35 ].

Neuroprotective effects on anti-aging and neurodegenerative diseases

Recently, there has been considerable interest in the neuroprotective effects of dietary polyphenols ( Table 4 ), especially in the context of their modes of action as antioxidants [ 6 ]. Resveratrol had an impact on cognitive deficits by activating the phosphorylation of protein kinase C (PKC), secreting transthyretin to prevent Aβ aggregation in cultured rat hippocampal cells [ 77 ], and stimulating AMP kinase activity in Neuro2a cells and primary neurons [ 75 ]. EGCG stimulated the deacetylase activity of recombinant silent information regulator two ortholog 1 (SIRT1) protein in human HT29 cells [ 80 ]. Curcumin could disrupt existing plaques and restore distorted neurites in an Alzheimer mouse model [ 84 ]. They had been considered as therapeutic agents for altering brain aging processes, and as possible neuroprotective agents in progressive neurodegenerative disorders such as Parkinson’s and Alzheimer’s diseases.

Anti-inflammatory properties

Oxidative stress induced inflammation is mediated by the activation of NF-kB and AP-1. It affects a wide variety of cellular signaling processes leading to generation of inflammatory mediators and chromatin remodeling [ 95 , 96 ]. The latter allows expression of pro-inflammatory genes such as interleukin-1beta (IL-1β), IL-8, tumor necrotic factor alpha (TNF-a), and inducible nitric oxide synthase (iNOS). The undesired effects of oxidative stress have been found to be controlled by the antioxidant and/or anti-inflammatory effects of dietary polyphenols such as curcumin and resveratrol in vivo and in vitro [ 88 – 90 , 95 , 97 ] ( Table 5 ). Resveratrol inhibited pro-inflammatory gene expression via inhibition of inhibitory κB (IκB), thus inhibiting NF-κB transactivation, as well as restoring transrepressive pathways through the activation of histone deacetylases in RAW 264.7 cells [ 89 ].

On the other hand, to counter the effects of oxidative stress, the cells also concomitantly express protective antioxidants such as glutamate cysteine ligase (GCL), manganese superoxide dismutase (MnSOD), and heme oxygenase-1(HO-1). In addition, expression of these antioxidant genes via modulation of MAPK-ARE-Nrf2 pathway is upregulated by EGCG and ECG in HepG2-ARE-C8 cell [ 10 ]. Apigenin, luteolin and quercetin had also been reported to inhibit inflammatory responses by downregulating the expression of iNOS and adhesion molecules in NR8383 macrophages and human endothelial cells [ 91 – 93 ].

Antimutagenic/anticarcinogenic properties

Dietary polyphenols could modulate diverse biochemical processes involved in carcinogenesis ( Table 6 ). Curcumin exerted antitumor activities by inhibition of cellular proliferation and angiogenesis, blockade of tumor cell cycle progression, and induction of programmed cell death in vivo and in vitro [ 109 , 110 ]. Cellular signaling cascades mediated by NF-κB or AP-1 acted as a centerplay in regulating many of aforementioned biochemical processes [ 102 , 110 ].

Resveratrol could block the activation of MAPKs and AP-1 in the skin of mice [ 102 ]. Consumption of berries and red fruits rich in polyphenols contributed to the reduction of cancer through many mechanisms such as in vitro inhibiting human cytochrome P450-dependent monooxygenases 1A1 (CYP1A1) activities [ 26 ], blocking the epidermal growth factor receptor (EGFR) tyrosine kinase activity [ 107 ], and decreasing protein kinase CKII activity [ 103 ].

Maintenance of gastrointestinal health and effects on digestive enzymes

It had been reported that digestive enzymes such as lipase, α-amylase, and α-glucosidase, were inhibited by proanthocyanidins and tannins in young chicks, which decreased the digestibility of protein, starch and lipid [ 119 , 120 ]. Resveratrol could inhibit pancreatic bile salt-dependent lipase (BSDL) activity, expression and secretion in the rat pancreatic AR4-2J cells [ 121 ]. Cyanidin-3α-O-rhamnoside and quercetin-3α-O-rhamnoside could inhibit α-glucosidase and advanced glycation end product (AGE) formation in vitro [ 123 ]. The inhibition of digestive enzymes by dietary polyphenols may represent an under-reported mechanism for delivering some of the health benefits attributed to a diet rich in fruit and vegetables.

Modulation of signal transduction pathways

Dietary polyphenols may not merely exert their diverse biological effects as free radical scavengers, but may also modulate cellular signaling processes by affecting signal transduction pathways [ 122 ] ( Table 7 ). Studies have been reported that curcumin could in vitro modulate NF-κB activation [ 124 ], AP-1 DNA binding [ 126 ], signal transducer and activator of transcription-3 (STAT3) phosphorylation [ 118 ]. Resveratrol exerted protection in vitro through PI3-kinase/Akt pathway, MAPK proteins (ERK, JNK and P38) activation [ 58 ], and the translocation into the nucleus of Nrf2 [ 132 ]. Resveratrol could also upregulate the expressions of GCL, MnSOD, and HO-1 against oxidative stress via MAPK-ARE-Nrf2 pathway in PC12 cells [ 132 ].

Improvement of endothelium functions

Several studies have indicated that red wine polyphenolic compounds (RWPCs) were able to inhibit proliferation and migration of vascular cells ( Table 8 ). RWPCs induced nitric oxide (NO)-mediated endothelium-dependent relaxations in isolated arteries. The activation of endothelial NO synthase (eNOS) was due to two distinct mechanisms: (a) an increase in [Ca2+] i and (b) a phosphorylation of eNOS by the PI3-kinase/Akt pathway [ 137 ]. In addition, RWPCs caused endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxations of isolated arteries consecutively to a localized and controlled formation of superoxide anions leading to the activation of the PI3-kinase/Akt pathway [ 136 ]. RWPCs also increased endothelial prostacyclin release and inhibited the synthesis and the effects of endothelin-1 in endothelial cell [ 139 , 141 ].

RWPCs could prevent matrix metalloproteinases-2 (MMP-2) activation and vascular endothelial growth factor (VEGF) expression in vascular smooth muscle cells (VSMCs) [ 133 , 134 ]. All these mechanisms might contribute to explain the vasodilatory, vasoprotective and anti-hypertensive effects of polyphenols in vivo.

Cyanidin-3-glucoside (Cy3G) and EGCG could enhance vascular eNOS activity and improve vascular endothelial function in bovine vascular endothelial cells [ 142 ]. Catechins had anti-angiogenic effects by reducing the vascularization on the chicken chorioallantoic membrane (CAM) [ 145 ].

Protective effect on immune cell functions

Dietary polyphenols appear to have a protective effect on immune cell functions. Alvarez et al . showed that leukocyte functions were improved in prematurely aging mice after five weeks of diet supplementation with polyphenol-rich cereals [ 149 ]. They could increase macrophage chemotaxis, phagocytosis, microbicidal activity, and natural killer function, and increase lymphoproliferation and IL-2 release in response to concanavalin A and lipopolysaccharide.

Curcumin could prevent tumor-induced T cell apoptosis by downregulating Bax level and augmenting Bcl-2 expression and restore cytokine-dependent Jak-3/Stat-5a signaling pathway in T cells of tumor bearer [ 150 ]. Caffeic acid, ellagic acid, and ferulic acid could inhibit apoptosis through the Bcl-2 independent mechanism in normal human peripheral blood mononuclear cells [ 116 ]. Thus, regular intake of these compounds will protect and improve quality of life.

Antiallergic activity

The incidence of type I allergic disorders have been increasing worldwide, particularly, the hypersensitivity to food. Akiyama and his coworkers reported that the apple condensed tannins intake would inhibit the development of the oral sensitization, and the inhibition could correlate with the rise in the population of TCRγδ-T cells in the intestinal intraepithelial lymphocytes [ 151 ]. Moreover, the apple condensed tannins could inhibit the release of histamine from rat basophilic leukemia (RBL-2H3) cells stimulated by the antigen-stimulation and from rat peritoneal mast cells stimulated by compound 48/80. They also inhibited hyaluronidase activity and increase in intracellular free calcium concentration in RBL-2H3 cells stimulated with the antigen [ 152 ].

Antidiabetic effects

Johnston and coworkers demonstrated that glucose uptake into cells under sodium-dependent conditions was inhibited by flavonoid glycosides and non-glycosylated polyphenols in polarised Caco-2 intestinal cells [ 154 ]. Under sodium-free conditions, aglycones and non-glycosylated polyphenols inhibited glucose uptake whereas glycosides and phenolic acids were ineffective. These data suggest that aglycones inhibit facilitated glucose uptake whereas glycosides inhibit the active transport of glucose. The non-glycosylated dietary polyphenols appeared to exert their effects via steric hindrance, while EGCG, ECG and (−)-epigallochatechin were effective against both transporters.

More recently, Koboyashi et al. have shown that the green tea polyphenols EGCG and ECG also inhibited glucose transport, possibly by sodium-dependent glucose transporter 1 (SGLT1) inhibition in the rabbit small intestine [ 155 ]. Song et al have presented evidence for quercetin-mediated inhibition of the facilitated diffusion glucose transporter 2 (GLUT2) in Chinese hamster ovary cells [ 156 ].

Anthocyanins inhibited α-glucosidase activity and reduced blood glucose levels after starch-rich meals. This is a proven clinical therapy for controlling type II diabetes [ 158 ] ( Table 9 ).

Regulation of cell cycle progression

It was demonstrated that resveratrol and proanthocyanidins could regulate cell cycle progression by upregulating p21 expression, G1 phase arrest and downregulating cyclin D1/D2–Cdk6 in vitro [ 163 – 165 , 170 ] ( Table 10 ).

Modulation of hormonal effects and contraceptive activity

Some studies showed that dietary polyphenols could modulate the level of hormone. Resveratrol could exert mixed estrogen agonist/antagonist activities in mammary tumor models. It could affect the expression of 17β-estradiol-responsive progesterone receptor (PR) and presnelin 2 proteins in vitro and in vivo [ 159 ]. Bhat et al. showed that resveratrol exhibited antiestrogenic properties and inhibited the levels and activity of PR by downregulating α (1)-integrin expression in human endometrial adenocarcinoma cells [ 160 ].

Otake and his coworkers demonstrated that quercetin and resveratrol potently reduced estrogen sulfotransferase (EST) activity and inhibited sulfation of 17β-estradiol in normal human mammary epithelial cells [ 161 ]. Both of the compounds potently inhibited recombinant human EST. In fact, they could serve as substrates for EST. Gossypol, a polyphenolic compound from cotton seed, had contraceptive activity and could inhibit 11β-hydroxysteroid dehydrogenase and cause hypokalemia in some men [ 162 ].

Effect in the treatment of chronic obstructive pulmonary disease (COPD)

Since a variety of oxidants and free radicals are implicated in the pathogenesis of COPD, it is possible that therapeutic administration of multiple antioxidants will be effective in the treatment of COPD. Various approaches to enhance lung antioxidant capacity and clinical trials of dietary polyphenols in COPD are discussed. Resveratrol, EGCG, and quercetin could inhibit inflammatory gene expression by controling NF-κB activation and regulate GSH biosynthesis and chromatin remodel in human airway epithelial A549 cells [ 171 , 172 ]. Curcumin could decrease protein/mRNA expressions of pulmonary type I collagen (Col-I) and TGF-β1 in rats [ 173 ].

Other bioactive effects

It has been demonstrated that dietary polyphenols have other bioactive effects ( Table 11 ), such as antibacterial activity of Gnemonol B and gnetin E [ 174 ], anti-HIV effect of proanthocyanidins [ 176 ], hepatoprotective ability of a novel proanthocyanidins IH636 [ 178 ], and angiogenesis effect of proanthocyanidins [ 177 ].

Prooxidant activity and cellular effects of the phenoxyl radicals of dietary polyphenols

Dietary polyphenols have beneficial antioxidant, anti-inflammatory and anticancer effects. However, at higher doses or under certain conditions these compounds may exert toxic prooxidant activities [ 181 ]. Galati et al. [ 182 ] have observed that dietary polyphenols with phenol rings were metabolized by peroxidase to form prooxidant phenoxyl radicals which, in some cases were sufficiently reactive to cooxidize GSH or NADH accompanied by extensive oxygen uptake and reactive oxygen species formation. Polyphenols with catechol rings also cooxidized ascorbate, likely mediated by semiquinone radicals. Incubation of hepatocytes with dietary polyphenols containing phenol rings was found to partially oxidize hepatocyte GSH to GSSG while polyphenols with a catechol ring were found to deplete GSH through formation of GSH conjugates.

Dietary polyphenols with phenol rings also oxidized human erythrocyte oxyhemoglobin and caused erythrocyte hemolysis more readily than polyphenols with catechol rings. It is concluded that polyphenols containing a phenol ring are generally more prooxidant than polyphenols containing a catechol ring. Subsequent studies revealed that [ 183 ] B-ring catechol-type flavonoids showed swift formation of their two electron oxidized quinone type metabolites, even upon their one electron oxidation by peroxidases. Enzymatic and/or chemical (auto) oxidation of the flavonoid generates the flavonoid semiquinone radical, which may be scavenged by GSH, thereby regenerating the flavonoid and generating the thiyl radical of glutathione. This thiyl radical may react with GSH to generate a disulfide radical anion which rapidly reduces molecular oxygen to superoxide anion radicals.

Huisman et al . [ 184 ] found that wine polyphenols and ethanol do not significantly scavenge superoxide nor affect endothelial nitric oxide production. Studies showed that flavonoids can induce oxidative damage and nick DNA via the production of radicals in the presence of Cu and O (2). Al, Zn, Ca, Mg and Cd have been found to stimulate phenoxyl radical-induced lipid peroxidation [ 185 ]. As a result of such enzymatic as well as non-enzymatic antioxidant reactions, phenoxyl radicals are formed as the primary oxidized products. Phenoxyl radicals can initiate lipid peroxidation. It is concluded that the prooxidant cytotoxicity of diet polyphenols is due to formation of ROS [ 186 ], role of phenoxyl radical/phenol redox couple [ 187 ], and stimulation by metals [ 185 ].

Bioavailability of dietary polyphenols

Polyphenols are the most abundant antioxidants in the human diet. They show a considerable structural diversity, which largely influences their bioavailability [ 188 ]. The biological properties of polyphenols depend on the amount consumed and on their bioavailability. Bioavailability appears to differ greatly between the various polyphenols, and the most abundant polyphenols in our diet are not necessarily those leading to the highest concentrations of active metabolites in target tissues [ 189 ]. Both isoflavones and phenolic acids like caffeic acid and gallic acid are the most well absorbed polyphenols, followed by catechins, flavanones, and quercetin glucosides, but with different kinetics. The least well-absorbed polyphenols are large molecular weight polyphenols such as the proanthocyanidins, the galloylated tea catechins, and the anthocyanins [ 190 ].

Ellagic acid was detected in human plasma at a maximum concentration (31.9 ng/mL) after 1 h postingestion [ 191 ]. Absorption of flavanols such as catechins was enhanced when tea polyphenols were administered as a green tea supplement in capsule form when consumed in the absence of food and led to a small but significant increase in plasma antioxidant activity compared with when tea polyphenols were consumed as black tea or green tea [ 192 , 193 ]. No differences were found in plasma EGCG concentrations and trolox equivalents determined by the trolox equivalent antioxidant capacity assay after administration as a single large dose in the form of either purified EGCG or as green tea extract (Polyphenon E) [ 194 ]. Hydroxytyrosol, the major olive oil phenolic compound, is dosedependently absorbed from olive oil [ 195 ]. Tuck et al. showed that hydroxytyrosol intravenously and orally administered oil-based dosings resulted in significantly greater elimination of the phenolics in urine within 24 h than the oral, aqueous dosing method. Oral bioavailability estimates of hydroxyl Int. tyrosol when administered in an olive oil solution and when dosed as an aqueous solution was 99% and 75%, respectively [ 13 ].

Once absorbed, polyphenols are conjugated to glucuronide, sulphate and methyl groups in the gut mucosa and inner tissues. Non-conjugated polyphenols are virtually absent in plasma. Such reactions facilitate their excretion and limit their potential toxicity. EGCG and ECG were present in plasma mostly as the free form, whereas epicatechin and epigallocatechin were mostly present as the glucuronide and sulfate conjugates [ 192 ]. Recent data suggest that beta-glucosidases and maybe also lactase phlorizin hydrolase (LPH) in the small intestine are capable of hydrolysing flavonoid glucosides and these compounds are thus taken up as the free aglycon and not as the intact glycosides [ 196 ]. It has been reported that around 98% of hydroxytyrosol is present in plasma and urine in conjugated forms, mainly glucuronoconjugates, suggesting an extensive first pass intestinal/hepatic metabolism of the ingested primary forms [ 197 – 199 ] and the 3- O -glucuronide of hydroxytyrosol shows stronger activity as a radical scavenger than hydroxytyrosol itself [ 200 ]. The major metabolites identified in in vitro and in vivo studies were an Omethylated derivative of hydroxytyrosol, glucuronides of hydroxytyrosol and a novel glutathionyl conjugate of hydroxytyrosol [ 200 , 201 ]. It has been recently reported that hydroxytyrosol and its metabolites are capable of binding human LDL after olive oil ingestion [ 202 ].

The polyphenols reaching the colon are extensively metabolised by the microflora into a wide array of low molecular weight phenolic acids. It has been shown that the plasma concentrations of total metabolites ranged from 0 to 4 μmol/L with an intake of 50 mg aglycone equivalents, and the relative urinary excretion ranged from 0.3% to 43% of the ingested dose, depending on the polyphenol [ 189 ]. The biological properties of both conjugated derivatives and microbial metabolites will be essential to better assess the health effects of dietary polyphenols. Alternatively, some health effects of polyphenols may not require their absorption through the gut barrier. Their role as iron chelators in the gut lumen is briefly discussed. Tannic acid and catechin both interact with the gut but only catechin appears able to traverse the gut. In addition, they provide evidence for binding of tannic acid and catechin by endogenous proteins in the intestinal lumen. This may limit their absorption from the small intestine [ 203 ].

Conclusions

Consumption of polyphenol-rich fruits, vegetables, and beverages derived from plants, such as cocoa, red wine and tea, represents a diet beneficial to human health. Some dietary polyphenols possess antioxidative and anti-inflammatory properties, to some extent, contributing to their cancer chemopreventive potential. These phenolic substances have the ability to abrogate various biochemical processes induced or mediated by the tumor promoters. Some dietary polyphenols also induce apoptosis in premalignant or cancerous cells, and suppress growth and proliferation of various types of tumor cells via induction of apoptosis or arrest of a specific phase of the cell cycle.

However, the specific mechanism(s) by which these compounds affect human health remains unclear, despite extensive research conducted in this area in recent years. Most of that research has focused on the antioxidant properties of dietary polyphenols, which are well characterized and well established in vitro . The in vitro data often conflict with results obtained from in vivo studies on the antioxidant capacity of plasma or the resistance of plasma and lipoproteins to oxidation ex vivo after the consumption of polyphenols-rich foods by human subjects. These inconsistencies between the in vitro and the in vivo data are likely explained by the limited bioavailability of dietary polyphenols and their extensive metabolism in humans. Most of them exert multifacet action, and any clinical applications using these substances should be based on the precise understanding of the physiologically relevant action mechanisms.

Acknowledgements

This project was supported by a grant from the National Natural Science Foundation of P.R.China (No. 30472072)

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