Quercetin is one of the predominant flavonoids in plants, and one of the most potent antioxidants of plant origin. Rich sources of dietary quercetin are onions, kale, unpeeled apples, berries, citrus fruits, and tea. Epidemiological studies report that quercetin is associated with a reduced risk of coronary heart disease and stroke.

 

    High blood pressure is one of the risk factors for cardiovascular disease and stroke. A few studies have shown the benefits of quercetin for lowering blood pressure.

 

    The effects of quercetin on blood pressure in overweight-to-obese patients with pre-hypertension (≥120–139 mmHg systolic blood pressure (SBP) and/or ≥80–89 mmHg diastolic blood pressure (DBP))  and stage I hypertension (≥140–159 mmHg SBP and/or ≥90–99 mmHg DBP) was investigated. Subjects (n= 70) were randomized to receive 162 mg/day quercetin from onion skin extract powder or placebo in a double-blinded, placebo-controlled cross-over trial with 6-week treatment periods separated by a 6-week washout period. Before and after the intervention, ambulatory blood pressure and office blood pressure were measured (1).

 

    Ambulatory blood pressure measurements throughout 24 hours could have better predictive power not only due to its higher number of readings, which increases the reliability of the measurement but also due to its ability to capture the impact of stressors and other environmental factors that occur in daily life and are likely to affect blood pressure.

 

    In the total group, quercetin did not significantly affect 24-hour ambulatory blood pressure parameters and office blood pressure. In the subgroup of hypertensives, quercetin decreased 24-hour systolic blood pressure by -3·6 mmHg (p=0·02) when compared with placebo (mean treatment difference, -3·9 mmHg; p<0.05). In addition, quercetin significantly decreased day-time and night-time systolic blood pressure in hypertensives. Quercetin significantly reduced the mean 24-hour systolic blood pressure by 3·6 mmHg in hypertensive participants, but not in pre-hypertensive participants, suggesting that a threshold of elevated blood pressure might be required to detect a blood pressure-lowering effect of quercetin. This study showed that supplementation with 162 mg/day of quercetin from onion skin extract lowers ambulatory blood pressure in patients with hypertension, confirming the cardio-protective effect of quercetin (1).

 

    In patients with high cardiovascular disease risk phenotype, chronic supplementation with a supra-nutritional dose of 150 mg/d quercetin also significantly reduced systolic blood pressure. The effects of quercetin supplementation on blood pressure, lipid metabolism, markers of oxidative stress, inflammation, and body composition in an at-risk population of ninety-three overweight or obese subjects aged 25-65 years with metabolic syndrome traits were studied. Subjects were randomized to receive 150 mg quercetin daily in a double-blinded, placebo-controlled cross-over trial with 6-week treatment periods separated by a 5-week washout period (2).

 

    Mean fasting plasma quercetin concentrations increased from 71 to 269 nmol/l (p < 0.001) during quercetin treatment. In contrast to placebo, quercetin decreased systolic blood pressure by 2.6 mmHg (p < 0.01) in the entire study group, by 2.9 mmHg (p < 0.01) in the subgroup of hypertensive subjects (systolic blood pressure ≥ 120 or diastolic blood pressure ≥ 80 mmHg) and by 3.7 mmHg (p < 0.001) in the subgroup of younger adults aged 25-50 years (2).

 

    Quercetin significantly decreased plasma concentrations of atherogenic oxidized LDL but did not affect TNF-alpha and C-reactive protein when compared with placebo. This study showed that quercetin reduced systolic blood pressure and plasma oxidized LDL concentrations in overweight subjects with a high-Cardiovascular risk phenotype. These findings provide further evidence that quercetin may provide protection against cardiovascular disease (2).

 

    Similar findings have been reported in hypertensive patients. Men and women with prehypertension (n = 19) and stage 1 hypertension (n = 22) were enrolled in a randomized, double-blind, placebo-controlled, crossover study to test the efficacy of 730 mg quercetin/d for 28 days vs. placebo. Blood pressure (mm Hg, systolic/diastolic) at enrollment was 137 ± 2/86 ± 1 in pre-hypertensives and 148 ± 2/96 ± 1 in stage 1 hypertensive subjects. Blood pressure was not altered in pre-hypertensive patients after quercetin supplementation. In contrast, reductions in systolic (-7 ± 2 mm Hg), diastolic (-5 ± 2 mm Hg), and mean arterial pressures (-5 ± 2 mm Hg) were observed in stage 1 hypertensive patients after quercetin treatment (p < 0.01). Indices of oxidant stress measured in the plasma and urine were not affected by quercetin. This study showed that quercetin supplementation could reduce blood pressure in hypertensive subjects. The quercetin-induced lowering of systolic blood pressure observed in stage 1 hypertensive subjects (-7 mm Hg) is clinically relevant because reductions of this magnitude are associated with a decrease in mortality of ~14% from stroke and ~9% from coronary heart disease (3).

 

    In another study in patients with stage 1 hypertension, one single high dose of quercetin aglycone (1095 mg) lowered systolic (142 ± 9 vs 135 ± 6), diastolic (91 ± 7 vs 88 ± 8 mm Hg), and mean (108 ± 7 vs 103 ± 7 mm Hg) blood pressure in hypertensive men (4). This ~7 mmHg reduction in systolic blood pressure is similar to what had been observed in the previous study (3).

 

    The vasodilating effect of quercetin was demonstrated in healthy humans. In a double-blind, randomized, placebo-controlled trial, fifteen healthy volunteers (26±5 years, 6 female) were given a capsule containing a placebo, 200 or 400mg of quercetin in random order in three consecutive weeks. At 2 hours after dosing, a dose-dependent increase in quercetin-3-O-glucuronide (a metabolite of quercetin) was observed in plasma (∼0.4 and 1μM for 200 and 400mg, respectively). No changes were observed in blood pressure. At 5 hours after dosing, quercetin induced an increase in brachial arterial diameter that correlated with the product of the levels of quercetin-3-O-glucuronide. Only quercetin ingestion of 400 mg significantly increased the plasma levels of glutathione (antioxidant marker) at 2 hours. The levels of urinary isoprostanes (oxidative stress marker) decreased in the three experimental groups after 5 hours. This decrease tended to be higher after 400 mg quercetin but the changes were not significantly different among groups. This study showed that quercetin had vasodilator effects in normotensive, normocholesterolemic human subjects (5).

 

    These studies showed that quercetin may have effects to prevent cardiovascular disease due to its antihypertensive and vasorelaxant properties. Quercetin is enriched in apples, berries, and onions. Eating more quercetin-rich foods can help to lower blood pressure and reduce the risk of coronary heart disease and stroke.

 

References:

1. Brüll V, Burak C, Stoffel-Wagner B, Wolffram S, Nickenig G, Müller C, Langguth P, Alteheld B, Fimmers R, Naaf S, Zimmermann BF, Stehle P, Egert S. Effects of a quercetin-rich onion skin extract on 24 h ambulatory blood pressure and endothelial function in overweight-to-obese patients with (pre-)hypertension: a randomised double-blinded placebo-controlled cross-over trial. Br J Nutr. 2015 Oct 28;114(8):1263-77. doi: 10.1017/S0007114515002950. Epub 2015 Sep 2. PMID: 26328470; PMCID: PMC4594049.

2. Egert S, Bosy-Westphal A, Seiberl J, et al. (2009) Quercetin reduces systolic blood pressure and plasma oxidised low-density lipoprotein concentrations in overweight subjects with a high-cardiovascular disease risk phenotype: a double-blinded, placebo-controlled cross-over study. Br J Nutr 102, 1065–1074. 

3. Edwards RL, Lyon T, Litwin SE, et al. (2007) Quercetin reduces blood pressure in hypertensive subjects. J Nutr 137, 2405–2411.

4. Larson A, Witman MA, Guo Y, Ives S, Richardson RS, Bruno RS, Jalili T, Symons JD. Acute, quercetin-induced reductions in blood pressure in hypertensive individuals are not secondary to lower plasma angiotensin-converting enzyme activity or endothelin-1: nitric oxide. Nutr Res. 2012 Aug;32(8):557-64. doi: 10.1016/j.nutres.2012.06.018. Epub 2012 Aug 16. PMID: 22935338.

5. Perez A, Gonzalez-Manzano S, Jimenez R, Perez-Abud R, Haro JM, Osuna A, Santos-Buelga C, Duarte J, Perez-Vizcaino F. The flavonoid quercetin induces acute vasodilator effects in healthy volunteers: correlation with beta-glucuronidase activity. Pharmacol Res. 2014 Nov;89:11-8. doi: 10.1016/j.phrs.2014.07.005. Epub 2014 Jul 27. PMID: 25076013.