What Is Quercetin, Anyway?

Quercetin is a potent antioxidant belonging to a group of plant nutrients called flavonoids, widely found in fruits and vegetables.¹ Studies show that the ingestion of flavonoids reduces the risk of cardiovascular diseases, diabetes, non-alcoholic fatty-liver disease, and certain types of cancer.² Food sources of quercetin include capers (the highest food source), apple skins, green vegetables, such as lettuce and kale, blueberries, onions, citrus fruits, red grapes, green tea, and red wine.^{3 4}

Our bodies cannot make quercetin, so we must obtain it from food or supplements. But food sources alone cannot supply optimal amounts of this powerful antioxidant—to achieve a dose of 500 mg, you’d have to eat 5 ½ cups of red onion or 28 cups of blueberries every day. With supplementation, you can obtain 500 mg with just one capsule.⁵ Typical doses in studies evaluating quercetin’s effectiveness range between 500 mg and 1,000 mg/day.⁶

A Respiratory Rockstar

Quercetin is a must-have this time of year, or anytime allergies and hay fever come a-knocking. It exerts an anti-allergy effect by inhibiting the release of pro-inflammatory compounds like histamine that cause allergy symptoms, including itching, sneezing, watery eyes, runny noses, and bronchoconstriction.^{7 8} But it does more than just quench a hay fever attack; it is also frequently used to treat a wide variety of allergic conditions, including asthma, eczema, and hives.⁹ Furthermore, quercetin is beneficial for the prevention and treatment of bronchitis. It relaxes airway smooth muscle, modulates inflammation, and supports healthy respiratory function in the respiratory tract, as well as the sinuses.

Quercetin not only supports healthy respiratory function during a change in seasons, but it can also offset the harmful effects of air pollution caused by carbon dioxide (CO2) and wildfire smoke, and even cigarette smoke.^{10 11} One study examining the effects of quercetin and vitamin C on bronchial cells when exposed to fine particulate matter (PM2.5), an especially dangerous type of air pollution, found that the combination of quercetin and vitamin C “strikingly” reduced PM2.5’s negative health effects to the cells, in part by reducing inflammation, oxidative damage, and mitochondrial damage.^{12 13} Similar results have also been found in studies investigating mice exposed to cigarette smoke, in which quercetin reduced airway inflammation, oxidative damage, and lung damage induced by second-hand cigarette smoke. The results lead the researchers of one study to write, “These findings suggest that quercetin has a potential for treating chronic airway diseases.”^{14 15}

An Anti-Aging Guru

Another reason to add quercetin to your supplement routine is that it is a potent antioxidant and anti-inflammatory. It not only protects the body from free-radical damage, it also quenches chronic, low-grade inflammation, which is becoming increasingly recognized as a significant contributor to accelerated aging. Low-grade inflammation contributes to the development of the common chronic diseases and conditions that account for more than 50 percent of all deaths in the world today.¹⁶ Ischemic heart disease, stroke, cancer, diabetes, chronic kidney disease, non-alcoholic fatty liver disease (NAFLD), and autoimmune and neurodegenerative conditions are all inflammation-related diseases.¹⁷

With quercetin’s natural anti-inflammatory effects, it may come as no surprise that it plays a direct role in reducing C-reactive protein (CRP) levels. CRP is a common way to measure both acute and chronic inflammation; when CRP levels remain high over an extended period, it can be an indicator of underlying disease.¹⁸ A 2017 meta-analysis of seven randomized controlled trials found that quercetin supplementation at doses greater than 500 mg/day, varying in length between 6 and 10 weeks, resulted in a “significant reduction in circulating CRP levels.”¹⁹

A Potent Antiviral

Most of us are familiar with the importance of vitamins C and D and zinc for immune health. In addition to these core supplements, quercetin is another valuable supplement to add to your immunity toolkit because it displays an impressive range of antiviral properties, including inhibiting virus entry, virus replication, and virus protein assembly.²⁰ This helps us fight upper respiratory infections like the common cold, as well as influenza. Quercetin also alleviates infection-related symptoms through its ability to reduce the release of inflammatory cytokines, proteins released by certain immune cells as part of the immune response.^{21 22 23}

In one study, participants 40 and older had a 36 percent reduction in upper respiratory infection severity and a 31 percent reduction in total sick days when taking 1,000 mg of quercetin, plus 1,000 mg of vitamin C and 40 mg of niacin, for 12 weeks, compared to the placebo group.²⁴

Quercetin also benefits the immune health of athletes. Competitive athletes can be more susceptible to respiratory infections as over-exercising can strain the immune system. In one study, participants who supplemented with 1,000 mg/day of quercetin for three weeks before, during, and two weeks after a three-day period of three hours of cycling, had a lower incidence of upper respiratory infections compared to the placebo group.²⁵

There are also preliminary studies in South Korea and China that have pointed to quercetin’s potential for inhibiting viral replication of SARS-CoV-2, the virus that causes COVID-19.²⁶ In a review entitled, “Traditional Chinese Medicine in the Treatment of Patients Infected with 2019-New Coronavirus (SARS-CoV-2),” researchers documented several key findings, noting quercetin’s potential effects as a treatment against coronaviruses.^{27 28} Specifically, quercetin had potent anti-SARS-CoV effects by “inhibition of viral cellular entry, adsorption, and penetration.”²⁹

A Cardiovascular Champion

Quercetin can also make your heart do a happy dance. It reduces the risk of cardiovascular disease (CVD) by lowering systolic and diastolic blood pressure and decreasing total and LDL cholesterol levels. It’s even been suggested to be a key component in the cardiovascular-protective element of eating the Mediterranean diet, which is associated with a decrease of cardiovascular diseases on the whole.³⁰ In a double-blind, placebo-controlled study, 93 overweight or obese subjects aged 25–65 with metabolic syndrome were randomly selected to receive 150 mg of quercetin or a placebo for six weeks. In contrast to the placebo group, quercetin decreased systolic blood pressure in all subgroups, including hypertensive subjects, and in the subgroup of younger adults aged 25–50 years. Quercetin also significantly decreased plasma concentrations of LDL cholesterol in the overweight subjects with high CVD risk factors.^{31 32}

Quercetin combats CVD in another key way. Atherosclerosis is the disease of the arteries characterized by the deposition of cholesterol plaques on their inner walls, thereby causing obstruction of blood flow.³³ In a study examining aortic samples with atherosclerotic disease, when quercetin was applied to the affected samples, the flavonoid specifically accumulated in the injured aorta. These observations suggest that quercetin exerts anti-atherosclerotic activities, including the inhibition of foam cell formation in the aorta, which is known to progress to atherosclerosis.^{34 35}

A No-Brainer for Healthy Brain Aging

Quercetin is one of a handful of nutrients that has the unique ability to cross the blood-brain barrier and support brain function; it scavenges free radicals, combats​ inflammation​, supports mitochondrial function and health,​ and reduces amyloid plaque formation​, a hallmark of Alzheimer’s disease.³⁶ Several in vitro human and animal studies show the neuroprotective effects of quercetin and its ability to delay brain aging. It protects the brain from the neurotoxic effect of chemicals, neuronal injury, and neurodegenerative diseases such as Alzheimer’s.³⁷ It also protects brain cells by stimulating cellular defenses against oxidative stress and reducing reactive oxygen species (ROS) in the hippocampus—a complex brain structure involved in learning and memory.³⁸

With its plethora of whole-body benefits, adding quercetin to your supplement routine might just be one of the best things you can do for your health span.

References

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