Quercetin: Broad-Spectrum ProtectionSeptember 2012
By Anne Buckley
Many of our most powerful medicines, including those for diabetes, chemotherapy, and cardiovascular health, are derived from plant extracts. One reason that plants hold so much potential for human health is that they are able to withstand a variety of destructive forces such as environmental radiation, oxidative damage, and chemical toxins due to their unique ability to manufacture complex molecules called flavonoids.1-4 For humans, these same conditions can be lethal. Yet when we consume plants as food and beverages, the protective benefits of these same flavonoids are readily transferred to our bodies!5
One flavonoid in particular, quercetin, is found in a broad range of foods, from grape skins and red onions to green tea and tomatoes. Quercetin is attracting intense scientific interest for its unique anti-aging and immune-boosting activities.6 Several recent studies show that organisms exposed to high levels of quercetin live longer, healthier lives.6--12 Laboratory models of aging, ranging from simple yeasts and primitive worms to cultured human cells, demonstrate that quercetin alone produces up to a 60% increase in life span!6,8-12 In addition, quercetin has been found to be cancer chemopreventive as well as reduce allergic reactions, boost immunity, and protect the cardiovascular system.
Due to its synergies with resveratrol, health-conscious people often obtain some quercetin in science-based resveratrol formulas they already use.
Quercetin Lowers Cholesterol, Fights Atherosclerosis
Quercetin is naturally abundant in plant foods and as a result is highly bioavailable.13 Nonetheless, most people do not obtain sufficient quercetin levels through their daily diets.14 This is unfortunate because large-scale epidemiological studies show that sustained high intake of quercetin and related flavonoids provides substantial protection against cardiovascular disease.15
In a study of 805 men aged 65-84 years, those with the highest quercetin and other flavonoid intake were 68% less likely to die from coronary heart disease than those with the lowest intake.14 A similar study found a reduction in death rates from all causes of 31% for women and 24% for men, with a 46% reduction in coronary death rates for women and a 22% reduction for men in those with the highest intakes.16
This dramatic cardiovascular protection is the result of a concerted synergy between several basic quercetin mechanisms, including its ability to lower cholesterol and reduce dangerous accumulations of abdominal and liver fat.
In one study on quercetin's impact on cholesterol levels, a group of otherwise healthy male smokers took 100 mg/day of quercetin or a placebo for 10 weeks, with blood tests done at baseline and at the end of the study.17 The supplemented group, but not the placebo group, had significant reductions in total and LDL cholesterol, and an increase in HDL cholesterol. They also had a significant reduction in blood sugar, another cardiovascular risk factor.
A larger study of non-smokers showed similar results, with an 18% reduction in total cholesterol, a 27% reduction in LDL, and an important 33% increase in beneficial HDL cholesterol.18
Animal and human studies also suggest that quercetin supplementation can reduce dangerous accumulations of abdominal and liver fat, the result of decreased oxidative stress and inflammation.19,20
Lowering cholesterol and reducing body fat are important first steps, but quercetin's cardiovascular benefits go farther, lowering blood pressure and increasing plasma levels of nitric oxide, thus providing more of the synergy mentioned earlier.
By blocking oxidation of LDL cholesterol, quercetin counteracts a major source of the blood vessel inflammation that precedes atherosclerosis.21,22 One human study found a 28% reduction in the rate of LDL oxidation following two weeks of supplementation with quercetin at just 30 mg/day.23
Higher doses of quercetin have shown the ability to modestly lower blood pressure.22,24 Quercetin supplements of 730 mg/day reduced hypertensive people's systolic (top number) blood pressure by 7 units (mmHg), and diastolic (bottom number) pressure by 5 units (mmHg).25 Those with normal blood pressure retained their healthy levels and did not experience a dangerous drop in blood pressure as is often the case with the use of anti-hypertensive medications. These results have been confirmed in other studies, with lower doses in the range of 100 - 200 mg/day.17,20,22,26
Quercetin also appears to exert its blood pressure-lowering effects by improving the health of your endothelium, the lining layer of the arteries that controls blood flow and pressure. Supplemental quercetin in hypertensive rats allowed "tight" blood vessels to relax, reducing their blood pressure.27
Quercetin supplements of 200 mg/day in healthy men increased blood levels of nitric oxide, a substance produced in endothelial cells that's required for vessels to dilate and lower blood pressure.28 Heart muscle that is overloaded by excess blood pressure loses its responsiveness to nitric oxide.29,30 It becomes overgrown, or hypertrophied and eventually loses its ability to pump blood effectively. This can lead to early death or disability.30 Quercetin completely blocks pressure overload-induced cardiac hypertrophy in animal models of hypertension which is the result of beneficial changes in gene expression in heart muscle tissue.30
Another of quercetin's cardiovascular benefits is its ability to inhibit platelet aggregation or "stickiness" in platelets. Oxidant stress and inflammation trigger this excessive "stickiness" in platelets and blood vessel walls. Untreated, the result can lead to a stroke or a heart attack following blood clot formation.
Human volunteers taking a highly bioavailable form of quercetin at both 150 and 300 mg/day rapidly (within 30 minutes) inhibited platelet aggregation.31 This effect proved to be the result of multiple changes in biochemical signaling pathways and gene expression, resulting in a healthier, "non-sticky" condition of the platelets.
Quercetin Shuts Down Cancer Cells Early
Cancers of all kinds are the second leading cause of death in the US, behind cardiovascular disease.32 Faced with the overwhelming challenge of achieving a cure, oncologists are increasingly turning to "chemoprevention" with nutrients as the most effective way to battle malignancies.33
Quercetin's multitargeted disease-fighting capabilities make it a natural choice as a chemopreventive compound. Strong evidence from epidemiological studies have shown that people with the highest quercetin intake enjoy substantially reduced risks for many of the leading causes of cancer death:
At the cellular level, quercetin interferes with the processes that turn healthy cells into malignant cancer cells. Quercetin's antioxidant and anti-inflammatory properties protect cellular DNA from dangerous cancer-inducing mutations.42 Quercetin "freezes" rapidly reproducing cancer cells into an early, non-productive phase of the cell replication cycle.43 This not only stops tumor growth but also triggers cell death by apoptosis—the innate programming healthy cells use to stop excessive reproduction.44 And quercetin favorably modulates chemical signaling pathways that are abnormal in cancer cells.45,46
All of these separate actions can dramatically reduce the chances that any given cancer cell will successfully form a real tumor. And that is precisely how chemoprevention is meant to work.
Here are details about quercetin's impact on some of the most common and dangerous human cancer types, from laboratory studies:
In one dramatic study, laboratory rats were treated with quercetin before exposure to the powerful environmental carcinogen, benzo(a)pyrene.33 This compound is found in cigarette smoke, charbroiled foods, and automobile (particularly diesel) exhaust, making it among the most common pollutants in our environment. Untreated rats developed lung cancers but rats supplemented first with quercetin showed no such findings.33
Many of the gravest cancer-related threats to liver tissue come from increased oxidant damage. Quercetin mitigates that damage by sharply increasing liver cells' production of natural protective antioxidant enzyme systems.51 The result, again, is a marked reduction in formation and replication of cancerous liver cells.