Polyphenols are produced by plants to serve a multitude of roles including growth, reproduction, pigmentation and protection from pathogens. To date, over 8000 different polyphenolic structures have been elucidated (Bravo, 1998). It is found that they all share a common chemical structure and differ only in their additional linkages with other compounds, including sugars, amines, organic acids and lipids. As they are ubiquitous in the plant kingdom, these compounds are common constituents of the human diet (especially in fruits, vegetables, coffee, tea, wine, chocolate and soy). Studies indicate that polyphenols may be of great importance in disease treatment and prevention, mainly due to their antioxidant properties. Ingested polyphenols may be either absorbed in the small intestine or colon, and/or excreted in the faeces or urine. In the small intestine polyphenols can enter the mucosa directly while in the colon they first need to be digested into smaller structures (Bravo, 1998). Once absorbed, polyphenols and their metabolites are transported to the liver where further digestion may take place. Finally polyphenols or their metabolites may be transported to the tissues where they exert their effects or else be Figure 1.Chemical structures of polyphenols (Manach, Scalbert, Morand, Remesy, & Jimez, 2004) excreted by being shuttled to the kidneys or incorporated into bile. The beneficial effects of polyphenols were initially recognized from in vitro studies (which showed their antioxidant properties) and epidemiological studies (notably studies regarding the Mediterranean diet and the French paradox) which indicated that the consumption of polyphenol-rich foods correlated with a low incidence of cardiovascular disease. Clinical trials, animal experiments and mechanistic studies followed; the results from these are conflicting. In this essay the evidence (focusing on human studies) and possible mechanisms for the effects of polyphenols from five different foods/beverages (wine, tea, olive oil, coffee, and chocolate) on plasma lipoproteins will be reviewed in relation to cardiovascular health.
Polyphenols and LDL oxidation
As on can see from table 1, one of the most important outcomes found in most studies investigating the effects of consuming polyphenol-rich foods/beverages on plasma lipoproteins is a reduction in LDL oxidizability. It is believed that LDL oxidation plays a major role in early atherogenesis and in fact oxidized LDL has been detected in atherosclerotic lesions (Aviram, Maor, Keidar, & et al, 1995). LDL oxidation occurs within the arterial wall, where cells such as smooth muscles cells and macrophages can oxidize LDL. Moreover the interaction of LDL with macrophages under conditions of oxidative stress activates enzymes (cellular oxygenases) which in turn produce reactive oxygen/nitrogen species which are also capable of oxidizing LDL (Aviram, Rosenfeld, Etzioni, & Levy, 1996). Conversely, human serum paraoxonase protects LDL from oxidation. As polyphenols have been shown to be capable of protecting LDL from oxidation, they may have a very important preventive role in cardiovascular diseases. The protection against LDL oxidation is believed to be due to the increase in plasma phenolics following polyphenol-rich food consumption; these can bind to LDL and protect it from oxidation. Their ability to act as antioxidants in this scenario is related to their chemical structures as well to their localizaiton in the LDL particle. Several mechanisms may account for this inhibition of LDL oxidation: scavenging free radicals, chelating metal ions, sparing of other antioxidants and increasing/preserving paraoxonase activity (Cordova, Jackson, Berke-Schlessel, & Sumpio, 2005). Polyphenols may scavange free radicals by acting as reducing agents, as hydrogen atom donating molecules and as singlet oxygen quenchers. Polyphenols contain both lipophilic and hydrophilic moieties and thus are able to act against free radicals generated both in aqueous and lipid environments. In this respect, different polyphenols have different partition coefficients (aqueous/lipophylic compartments) in plasma and within the LDL particle and thus different polyphenols are expected to have different antioxidant capabilities. A study done with glabadrin (hydrophobic) and catechin (less lipophylic) found that after incubation of LDL with the two, only glabadrin was found to bind to LDL in significant amounts (Belinky, Aviram, Fuhram, & et al, 1998). The antioxidant effect of polyphenols might also be through the chelation of transition metal ions, thus reducing the ability of the metal to generate free radicals (Nardini, D'Aquino, Tamassi, & et al, 1995). Another mechanism is by sparing vitamin E and carotenoids in the LDL particle, thus enhancing the antioxidant effect. In vitro studies have demonstrated that incorporation of glabridin into LDL reduced the consumption of -carotene and lycopene by 41% and 50% respectively (Belinky, Aviram, Fuhram, & et al, 1998). Polyphenols might also inhibit LDL oxidation by protecting or elevating serum paraoxonase activity by decreasing the content of lipid peroxides, which can oxidise and thus inactivate paraoxonase (Fuhrman & Aviram, 2001). By protecting the levels of serum paraoxonase, the hydrolysis of arterial cells and LDL associated lipid peroxides is promoted. Polyphenol-induced attenuation of LDL oxidation could also result from inhibition of cellular oxygenases (such as NADPH oxidase) thereby reducing macrophage oxidative stress. This has been confirmed in vitro by incubating macrophages with glabridin, catechin and quercetin; all three lead to a reduction in the ability of the cell to oxidize LDL (Aviram & Fuhrman, LDL oxidation by arterial wall macrophages depends on the antioxidative status in the lipoprotein and in the cells: role of prooxidants vs. antioxidants., 1998) One must keep in mind that in foods, polyphenols exist in combination and thus they may act additively or synergistically, via several mechanisms to protect against LDL oxidation.
The French typically consume a diet which is high in saturated fats, yet they have a low mortality rate from ischemic heart disease (Renaud & de Lorgeril, 1992). This observation is often referred to as 'the French paradox' and is attributed to their high consumption of red wine. Though moderate [write amount and ref] consumption of alcohol in general has been found to help prevent arteriosclerosis [ref] red wine exerts a stronger effect than other alcoholic beverages[ref. Several components of red wine exert beneficiary effects on the cardiovascular system and the reason why red wine produces more pronounced effects compared to other alcoholic beverages is believed to be it s polyphenolic content. The polyphenols in red wine are mostly derived from grape skins and seeds and include flavanols (quercitin and myricetin), flavanols (catechin and epicatechin), anthocyannins, stilbenes (resveratrol), hydroxynnamates and the hydroxybenzoates (Cordova, Jackson, Berke-Schlessel, & Sumpio, 2005). Red wines generally contain between 1000 and 4000 mg phenolics/L (Burns, Gardner, O'Neil, & et al, 2000). It is believed that phenolic compounds in wine act as antioxidants protecting the body from damage caused by free radicals (Kanner, Frankel, Granit, German, & Kinsella, 1994). In vitro studies have also demonstrated that these compounds my prevent oxidative modifications of LDL (Frankel, Waterhouse, & Teissedre, 1995) On the other hand human intervention studies have yielded conflicting results, as can be seen in table 1. (Nigdikar, Williams, Griffin, & Howard, 1998) check all found that red wine consumption for two weeks led to a reduction in LDL oxidation (increase in lag time) while the white wine and vodka groups displayed no change. Total cholesterol, HDL, LDL and TAG levels were unaffected. Since it has been widely demonstrated that alcohol consumption increases HDL cholesterol (Hartung, Foreyt, & Reers, 1990) the authors of this study speculate that the lack of effect on this parameter was due to the duration of the study (2 weeks) being two short. (Avellone, et al., 2006) recorded an increase in HDL levels following 4 weeks of wine consumption, together with an increase in apoA1 and decreases in LDL/HDL ratio; TAG, total cholesterol and LDL levels were unaffected. This was a cross-over study and the altered parameters returned to baseline values after 4 weeks of cessation of wine intake. (Cordain, et al., 200) studied the potential positive effects of wine consumption in moderately obese (BMI) women with impaired insulin sensitivity, however no effects on plasma lipids were observed. This study however did not measure LDL oxidizability. The authors speculate that the lack of changes in plasma lipid parameters, especially HDL levels, might be due to the excess body fat negating the beneficial effects of alcohol consumption, as tends to happen for insulin sensitivity. This observation was corroborated by another study of over 600 women, where it was noted that alcohol consumption was correlated with an increase in HDL cholesterol in healthy weight women but not in obese women (Fricker, Fumeron, Chabchoub, & et al, 1990) (Cartron & et al, 2003) demonstrated that red wine lead to an increase in plasma antioxidant capacity (pAOC) after three to four hours following a single intake of red wine; however it was found that after 3 weeks of daily wine consumption this value actually decreased. The authors however commented that this apparent decrease might be due to the subject's high baseline values. apoA1 values increased which supports other studies reporting that HDL levels are increased following red wine consumption (Avellone, et al., 2006). This increase was not observed in subjects given white wine or champagne, thus indicating that polyphenols in red wine have a key role in this effect. Since previous studies have found that moderate alcohol intake in general has the ability to elevate apoA1 levels (Rimm, Williams, Fosher, Criqui, & Stampfer, 1999) then this study might suggest that alcohol and red wine polyphenols may act synergistically to increase apoA1plasma levels. After three weeks of red wine consumption, TAG concentration was found to be decreased, however LDL levels were unchanged. Unlike what has been reported by (Nigdikar, Williams, Griffin, & Howard, 1998), this study found no change in LDL oxidizability. There was also no detectable change in LDL vitamin E and ubiquinol whose levels usually reflect extent of oxidation (due to their redox properties). This study thus concludes that the beneficial effects of red wine with regards to CVD risk are due to the effects on lipid and lipoprotein constants and not by an antioxidant action.
Tea contains a number of different bioactive chemicals; however it particularly rich in polyphenols especially catechins and their derivatives. Fresh teal leaves are rich in epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG) and epigallocathechin gallate (EGCG). Tea leaves also contain plyphenol oxidase enzymes is separate layers of the leaves and when the leaves are broken during industrial tea production, the cathechins come in contact with this enzymnes are are subsequently oxidised, forming the flavanols theaflavins and thearubingins. Due to the different processes involved in the production of green tea and black tea, the former has a high concentration of catechins and a low concentration of theaflavins and theaarubingins while the latter is poor in catechins but rich in theaflavins and thearubingins. It has been demonstrated that catechins are absorbed intestinally (Okushio, Matxumuto, Kohri, Suzuki, Nanjo, & Hara, 1996) and that in humans, plasma levels peak two to four hours after ingestion (Yang, Chem, Lee, Balentine, Kuo, & Schantz, 1998). Tea polyphenols have been demonstrated to be powerful antioxidants in in vitro studies, and they may achieve this via five different mechanisms (Higdon & Frei, 2003). The first mechanism might be through free radical scavenging, especially considering the fact that EGCG and EGC have lower reduction potentials than vitamin E, thus indicating that they might be superior electron donors than vitamin E (Jovanavic, Steenken, & Simic, 1996). In vitro studies have demonstrated that green tea and black tea polyphenols are able to efficiently scavenge the nitric oxide radical, with green tea being five times more potent than black tea (Paquay, Haenen, Stender, Wiseman, Tijburg, & Bast, 2000). The antioxidant activity of catechins is believed to be due to 1) an ortho-dihydroxyl group in the B ring, which aids electron delocalization and stabilizes the radical form (Rice-Evans, Miller, & Paganga, 1996); 2) they are generally water soluble, however they are still able to effectively inhibit lipid peroxidation in liposomes and LDL (in in vitro studies) (Kondo, Kurihara, & Fukuhara, 2001). Tea polyphenols are able to chelate metal ions, and this may an important mechanism which explains their antioxidant effect since this implies that they are able to inhibit transition metal-catalyzed free radical formation (Rice-Evans & Miller, Antioxidant properties of phenolic compounds., 1997) Another possible route which has been demonstrated in vitro is through inhibition of transcription factor activation (Nuclear Factor-kB and Activator Protein-1); the former induces the transcription of inflammatory genes (Pan, Lin-Shiau, Ho, Lin, & Lin, 2000) while the latter's activity has been implicated in tumour promotion (Chung, Huang, Meng, Dong, & Yang, 1999). Tea polyphenols are also believed to be able to inhibit certain enzymes whose activity tends to increase oxidative stress. The three main enzyme of interest are inducible nitric oxide synthase (iNOS); lipoxygenases and cyclooxygenases; and xanthine oxidase [x1]. iNOS generates large amounts of nitric oxide which can react with oxygen in vivo to form harmful nitric oxide-derived oxidants. Lipoxygenases and cyclooxygenases may cooxidize other molecules, resulting in increased oxidative stress in some tissues. Xanthine oxidase catalyzes the oxidation of hypoxanthine and xantine to uric acid, producing reactive oxygen species as by products. Finally, in vitro studies have demonstrated that tea polyphenols may induce phase II enzymes and antioxidant enzymes (Higdon & Frei, 2003). Phase II enzymes generally have a detoxifying action while antioxidant enzymes such as catalase and superoxide dismutase carry out antioxidant roles in the body. Human studies regarding the effect of tea consumption on lipoproteins are scarce. (Ishikawa & et al, 1997) report a reduction in LDL oxidizability, which might be explained by the mechanisms explained above. The study found no change in HDL, TAG and total cholesterol levels. Similarly (Gomikawa & Ishikawa, 2000) found that consumption of green tea lead to a reduction in LDL oxidizability together with a decrease in LDL levels while HDL levels were unchanged. On the other hand (van het Hof, de Boer, Wiseman, Lien, Weststrate, & Tijburg, 1997) reported no change in LDL oxidizability or in LDL, HDL or TAG levels.
Olive oil contains a variety of bioactive molecules, the main beneficial ones believed to be oleic acid, phenolics and squalene (Owen, et al., 2000). These have all been found to posses antioxidant properties and the high levels of monounsaturated fatty acids found in olive oil have a cardiovascular benefit in their own right. The total phenolic content of olive has been measured to be in the range of 196-500 mg/kg, with extra virgin oil having higher phenolic content than refined virgin oil (Owen, et al., 2000). Major phenols in olive oil are hyroxytyrosol, tyrosol, oleuropein and ligstroside (Owen, et al., 2000). Hydroxytyrosol and oleuropein have been demonstrated to posses free radical scavenging properties and the ability to inhibit LDL oxidation in vitro (Owen, et al., 2000). Apart from their direct antioxidant activity, olive oil polyphenols have been shown to inhibit xanthine oxidase activity while seed oils had little effect (Owen, et al., 2000) It is believed that the reduction in LDL oxidizability brought about by olive oil phenolics is due to the binding of the polyphenols to the LDL particle and their subsequent antioxidant action. In fact it has been found that polyphenols bound to LDL increase in dose dependent manner with the amount of polyphenols in the olive oil consumed (Fuller & Jialal, 1994). Specifically it has been found that in humans, hyroxytyrosol and its metabolites are capable of binding to human LDL after olive oil consumption (De la Torre, Jauregui, Castellote, Lamuela-Raventos, Covas, & Casalas, 2006). Human intervention studies recording the positive effects of olive oil consumption on cardiovascular risk and lipoprotein parameters are many (Covas, 2007). In order for this effect to be ascribed to the polyphenol fraction, then one must look at studies comparing olive oils with similar MUFA levels but different polyphenols levels. (De la Torre R. , 2004) conducted a controlled, double blind, cross-over, randomized clinical trail using three similar olive oils with increasing phenolic concentration. It was found that when consuming olive oil without polyphenols, there was no change in LDL oxidizability; when consuming olive oil with 68mg/Kg polyphenols there was a reduction in LDL oxidizability; and when consuming an olive oil with 150mg/Kg polyphenols, the reduction in LDL oxidizability was even higher. Moreover only when consuming the olive oil containing the highest amounts of polyphenols was an increase in HDL levels recorded.
While there are over a thousand different compounds in coffee, the three main ingredients responsible for physiological effects are caffeine, the diterpene alcohols kahweol and cafestol and the polyphenols. The main polyphenol in coffee is chlorogenic acid, an ester of caffeic acid and quinic acid, and is an in vitro antioxidant (Natella, Nardini, Belelli, & Scaccini, 2007). It is estimated that a 180mL cup of brewed coffee contains 396mg of polyphenols while instant coffee around 316mg (Bonita, Mandarano, Shuta, & Vinson, 2007). In vitro studies indicate that coffee polyphenols can bind to LDL and thus protect them from oxidation (Bonita, Mandarano, Shuta, & Vinson, 2007) however human intervention studies are controversial. (Yukawa, et al., 2004) reported that after 1 week of coffee consumption, LDL oxidizability was reduced, together with a reduction in total and LDL cholesterol. Similarly (Natella, Nardini, Belelli, & Scaccini, 2007) demonstrated a reduction in LDL oxidizability after drinking a single cup of 200mL of filtered coffee. It seems that different types of coffees might have very different impacts on cardiovascular health. In fact while the studies mentioned above report a positive impact after the consumption of filtered coffee, a meta-analysis of randomized controlled clinical trials studying the effects of coffee consumption on serum cholesterol concluded that "consumption of unfiltered, but not filtered, coffee increases serum levels of total and LDL cholesterol" (Ha, He, Appel, Whelton, Suh, & Klag, 2001).
Cocoa and Chocolate
Cocoa is one of the richest flavinoid-conaining foods available; in fact over 10 percent of cocoa powder by weight is flavinoids (Keen, 2001). The flavinoids present in cocoa/chocolate are mainly catechin and epicatechin (in a ratio of about 6:1) which are also present in red wine and green tea. Studies in humans have shown that the concentration of these flavinoids in the plasma peaks after about one hour following ingestion and returns to baseline after about six hours (Keen, 2001). The observations from in vitro studies that cocoa/chocolate polypohenols act as antioxidants (Keen, 2001) have also been replicated in vivo: both (Mathur, Devaraj, Grundy, & Jialal, 2002) and (Osakabe, 2001) found that cocoa/chocolate consumption resulted in reduced LDL oxidizability. As for other polyphenols, it is believed that this effect is mainly brought about by 1) the direct antioxidant effect of the polyphenols and 2) the sparing effect on other antioxidants such as vitamin E and C. Other mechanisms described previously might also be in effect.
Various epidemiological studies have demonstrated an inverse association between the consumption of polyphenol-rich foods and the risk of cardiovascular diseases. With regards to plasma lipoproteins, the mechanism by which these compounds are thought to reduce the risk for cardiovascular disease is mainly by their ability to act as antioxidant, reducing the susceptibility of LDL to oxidation, which is a causative process in atherosclerosis. Polyphenols might also reduce total and LDL cholesterol and increase HDL in some cases. In vitro studies have confirmed that these compounds are powerful antioxidants and may indeed have the ability to reduce the susceptibility of LDL to oxidation. However, bioavailability studies have shown that ingested polyphenols are rapidly and extensively metabolized by the body, and this brings to question the usefulness of in vitro data. Human intervention studies focusing on the effects of polyphenols on lipoproteins have characteristically yielded conflicting results. Various factors might explain this. Firstly, bioavailability differs greatly among polyphenols and the same polyphenols are known to have different bioavailability according to the forms in which they are present in the food (Manach, Mazur, & Scalbert, 2005). Moreover high inter-individual variability in the efficacy of intestinal absorption and metabolism has been recorded (Meyer & et al, 2004). Most polyphenols have been shown to be rapidly eliminated once absorbed and thus whether the polyphenol-rich food/beverage is consumed as part of a meal or in fasting conditions may have a marked consequence on whether or not their effects are observed. Also, not all studies measure the concentration of the polyphenols in the food/beverage administered, and thus it is difficult to compare studies. It is known that for a given food item, the amounts and nature of polyphenols present may vary significantly according to seasonal variation, climate, ripeness, and food processing and preparation.