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Does quercetin reduce cholesterol?

Quercetin-rich supplementation significantly reduced serum concentrations of total cholesterol (P < 0.05) and LDL-cholesterol (P < 0.01), whereas these effects were not shown in the placebo group.

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A prospective study reported that the consumption of dietary quercetin is inversely associated with total mortality and cardiovascular mortality [ 22 ]. Among the mechanisms explaining these associations include the effects of quercetin on lipid metabolism [ 10 12 ], oxidative stress [ 6 ], and inflammation [ 14 15 ]. Several plausible pathways as to how quercetin affects lipid metabolism have been suggested by animal studies. Quercetin increases fecal excretion of cholesterol and bile acids, thereby, decreasing serum cholesterol levels and total liver cholesterol content in rats [ 12 ]. Kamada et al. [ 11 ] reported that quercetin glucoside significantly reduces the amounts of total cholesterol, triacylglycerol, and total fatty acids in both the plasma and aortas of rats fed a high cholesterol diet. Additionally, recent experimental data further supports the hypolipidemic effect of quercetin. Gnoni et al. [ 23 ] demonstrated that quercetin induces reductions in both de novo fatty acid and TG synthesis and the resulting decreases in VLDL-TG formation in hepatocytes of normal rats suggests a cardioprotective role for dietary quercetin. These results indicate that quercetin may be involved in lipid metabolism in rats by reducing hepatic fatty acid synthesis and by inhibiting cholesterol biosynthesis [ 24 ]. However, the hypolipidemic effects of quercetin in human clinical studies have been inconclusive. Supplements of quercetin-rich onion powder [ 25 ], grape juice [ 26 ], and grape powder [ 27 ] favorably influence blood lipid profiles in humans. In addition to the results of controlled clinical studies, epidemiological evidence indicates that dietary quercetin intake is negatively associated with plasma levels of LDL-cholesterol after adjusting for total energy intake, dietary cholesterol intake, age, and BMI [ 28 ]. In contrast, as shown by Egert [ 29 ], a 2-week supplementation with quercetin did not change blood lipid concentrations in healthy humans. It was also reported that quercetin supplementation increases plasma quercetin concentrations without any effect on blood lipids [ 30 ], which is consistent with very recently published data [ 31 ]. In the present study, supplementation with quercetin from onion peel extract for 10 weeks significantly reduced serum concentrations of total cholesterol and LDL-cholesterol and significantly increased serum concentrations of HDL-cholesterol in male smokers. These differences can be attributed to the duration of administration, the characteristics of the study subjects, and the amount and/or food sources of ingested quercetin. It is likely that the 10-week study period was long enough to demonstrate such effects of quercetin on the studied parameters. Furthermore, our subjects were all males and current smokers who were at increased cardiovascular risk. Smokers are under a high and sustained free radical load, which may overwhelm the antioxidant defense system, resulting in oxidative damage [ 32 ]. Additionally, the associations between chronic cigarette smoking and hypertension and low grade chronic inflammation and endothelial dysfunction are well recognized [ 33 ]. While the exact mechanism by which flavonoids, particularly quercetin, influences the cardiometabolic risk components has not been elucidated, a biological smoking interaction may be working to increase quercetin bioavailability in smokers who confer relatively higher risks for cardiovascular disease. Therefore, it would appear that smokers in particular, may benefit from quercetin supplementation. Currently, the prevalence of smoking is estimated at 47.7% in men. Although the prevalence of smoking among Korean men was 64.9% in 1998 and has since declined, it still remains very high [ 34 ]. Considering that smoking is one of the major independent risk factors for cardiovascular disease [ 35 ], these results indicate that daily quercetin supplementation provides beneficial effects on blood lipid profile measurements, especially in smokers who are exposed to a high degree of oxidative stress and are particularly susceptible to the development of coronary heart disease.

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