One of the most frequently studied beneficial effects of (poly)phenols is their ability to improve lipid profile. The enzymes and free radicals released by immune system cells, platelets, and endothelial cells modify native LDLs by oxidation. Oxidized LDLs (oxLDLs)
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are responsible for further development and destabilization of atherosclerotic plaque. Oxidative stress produces an increase in enzymes such as cyclooxygenase (COX) and lipooxygenase (LPO), which are implicated in the release of factors such as chemokines, pro- inflammatory substances, growth factors, free radicals, TF and proteolytic enzymes specialized in the digestion of connective tissue elements (MMPs) and other factors that have direct chemotactic properties for monocytes to adhere to the endothelium (Aviram et al. 1994). The accumulation of macrophage in this area eliminated the oxLDLs molecules but also provokes an inflammatory response, with requisite cell recruitment and proliferation accompanied by migration of smooth muscle cells. Oxidized LDL is preferentially taken up by macrophage cells via scavenger receptors, and they consequently become loaded with lipids and convert into “foam cell” (Aviram 1996). Extracellular matrix deposits increase around the inflamed area, and this permits the formation of so called atheroma plaque, which more or less blocks the vessel. Along with these processes, vasoconstriction episodes occur, caused by inhibition of NO formation and loss of arteries’ natural relaxation capacity (Ross 1999).
The beneficial effects of (poly)phenols on atherosclerosis have been widely studied and it was proposed that these compounds are able to attenuate the onset and development of the disease thanks to their ability to limit LDL oxidation. Numerous studies investigated the protective effect of flavan-3-ols, both monomeric and oligomeric, against LDL oxidation (Fuhrman et al. 2001; Auger et al. 2004). The development of foam cells in the aorta is a good model and indicator of atherosclerotic lesions. Using a hamster model of atherosclerosis, Vinson et al. (2002) found that grape seed proanthocyanidins induced a pronounced reduction in plasma cholesterol (25%) and triglyceride levels (up to 34%). Accompanying these changes was a reduction in the percentage of aorta covered in foam cells. The latter was reduced by 50% and 63% after supplementation of the animals with 50 and 100 mg/kg grape seed proanthocyanidins, respectively. Furthermore, these beneficial effects were associated with a significant decrease in plasma lipid peroxidation levels.
In a randomized, double-blind, placebo-controlled study, grape seed proanthocyanidin extracts was given to 40 hypercholesterolemic patients for 8 weeks (Bagchi et al. 2003). There was a significant reduction in LDL-c levels and total cholesterol levels for the grape seed proanthocyanidins group, but only when this group was additionally supplemented with niacin-bound chromium. This phenomenon suggested that the mechanisms of action are complex and may require other “factors” for nutritional benefit.
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Apolipoprotein E-/- (ApoE-/-)-deficient mice represent a good model for atherosclerosis. They are characterized by accelerated development of atherosclerosis and are more susceptible to oxidative stress. Grape extract effects were extensively considered. Fuhrman et al. (2005) investigated atherosclerotic lesions using ApoE-/-deficient mice following the dietary supplementation of freeze-dried extracts of fresh grapes. For ApoE-/-- deficient mice that consumed 150 μg total (poly)phenolics per day for 10 weeks, a 41% reduction in the atherosclerotic lesion area was observed compared with control (no supplements) or placebo (glucose and fructose supplementation) groups. This effect was associated with a significant reduction in serum oxidative stress as indicated by an 8% reduction in plasma lipid peroxide concentration and an increase in antioxidant capacity (16%-20%) as well as a reduction in macrophage uptake of oxidized LDL (33%). Another study by Frederiksen et al. (2007) using Watanabe heritable hyperlipidemic rabbits demonstrated that consumption of a red grape skin extract was associated with a delay of the development of aortic atherosclerosis in male rabbits but not females, as determined by cholesterol content within the abdominal aorta.
Anti-atherogenic effects were also observed with other type of fruits and beverages. Administration of pomegranate juice, rich in anthocyanins, and ellagitannins such as punicalagin, to apoE–/– mice resulted in dramatic reductions in lipid peroxides and macrophage accumulations, without significantly affecting plasma cholesterol. After three months of pomegranate juice supplementation, atherosclerosis was reduced by 44% (Aviram et al. 2008). In another study, administration of pomegranate by-product to apoE–/– mice attenuated atherosclerosis development as a result of decreased macrophage oxidative stress and cellular uptake of oxidized LDL (Rosenblat et al. 2006).
It has been demonstrated that resveratrol, impeded LDL oxidation and lowered cytotoxicity caused by oxidized LDL in endothelial cells (Delmas et al. 2005). Using the same apoE–/– mouse model, four-month supplementation of a regular chow diet with resveratrol led to a reduction in total plasma cholesterol and LDL-c, and an increase in HDL cholesterol. The mechanism for the reduction in plasma cholesterol was through a reduction in hepatic cholesterol synthesis, which may have stimulated LDL receptor-mediated uptake of LDL from plasma (Do et al. 2008). Berrougui et al. (2009) showed in an in vitro experiment that resveratrol could prevented lipid peroxidation and increased cholesterol efflux from macrophages. Through these mechanisms, resveratrol significantly reduced atherosclerotic plaque development in the aortic arch of apoE–/– mice.
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The reduction of vascular inflammation, the prevention of leukocyte adhesion, the inhibition of vascular smooth muscle proliferation and the stimulation of NO production may also contribute to the anti-atherosclerotic effects of (poly)phenols. Nie et al. (2006) showed that avenanthramides, polyphenols found in oats (Avena sativa L.), might contributed to the prevention of atherosclerosis through inhibition of smooth muscle proliferation and increasing NO production. A recent study by Choi et al. (2009), suggested that through its ability to inhibit type A scavenger receptors and CD36 (Cluster of Differentiation 36) on the macrophage surface, quercetin reduced oxLDLs uptake and absorption. Moreover, quercetin contributed to the production of pro-inflammatory and pro‑atherogenic vascular endothelial growth factor (VEGF) and inhibited the expression of MIP-2 (macrophage-inflammatory protein-2) and MCP-1. Quercetin has also been shown to reduce the activation of PPAR-γ (peroxisome proliferatoractivated receptor gamma) participating in the regulation of CD36 receptor expression on macrophages (Choi et al. 2009).
II.4.6. Conclusion on health effects
Although the beneficial effects, controversial results were found. Current limited evidence suggested that fruits containing relatively high concentrations of flavonols, anthocyanins and procyanidins, such as pomegranate, purple grapes and berries, were more effective at reducing CVD risk, particularly with respect to anti-hypertensive, inhibition of platelet aggregation and increasing endothelial-dependant vasodilatation than other fruits investigated. In fact, one of the reasons why it is difficult to draw a clear conclusion from the current evidence is the heterogeneity in study design. (Poly)phenols were mainly consumed in the form of fruit juices or fruits and a small number of studies provided (poly)phenols in the form of supplements (Conquer et al. 1998; Clifton 2004; Hubbard et al. 2004; Gorinstein et al. 2006). Between studies, duration of study period ranged from weeks to months and the dose of (poly)phenols investigated was not consistent between studies. Moreover, the dose administrated is often high and exceeds the dietary intake.
In addition, the types of subjects recruited differed, with some studies using healthy volunteers while others recruited subjects at risk of CVD or with CVD (Stein et al. 1999; Kurowska et al. 2000; Sumner et al. 2005; Gorinstein et al. 2006; Erlund et al. 2008; Wilson et al. 2008). It appears that observed effects were generally more marked in subjects with higher CVD risks. Moreover, subject compliance should also be considered because this could impact significantly the results. Difference in the degree of dietary compliance could
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account for inconsistencies in the results obtained from various studies. Most of the time, compliance including diet diaries, log books, frequent interviews by the researchers and biomarkers, was not assessed.
Besides, analytical methodologies should be standardized to allow valid comparison between studies. This is particularly important in the quantification of specific (poly)phenols in fruits. Differences in the concentration of the bioactive flavonoid components within the fruits ingested in the various studies have been consistently highlighted as a possible factor influencing the variable effects of similar types of fruits/fruit juices. In some investigations, details of the (poly)phenols content of the fruits investigated were omitted and thus, generating inconsistent results.
All these factors may be the reasons for heterogeneous results and have to be considered for an acute investigation of (poly)phenol health effects. Long intervention periods and subject’s compliance to treatments should be encouraged and evaluated in future studies. Improvements and standardization in the methodology for quantifying the (poly)phenolic content of fruits have to be taken into account.
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