What Is Quercetin & How Can It Help Support The Immune System? 

So many aspects of modern life can impair immune system functioning (poor sleep, excessive sugar intake, chronically elevated stress levels—you get the picture), making it harder to stay healthy. Identifying your immune-sapping triggers and adjusting your lifestyle accordingly is a great start, but no one's perfect—so, we often reach for supplements like vitamin C and vitamin D for extra immune strength. But there's another less well-known nutrient gaining attention for its immune-supporting potential: quercetin. 

You've most likely ingested this bioactive plant compound before in the form of apples, onions, or tea, but increasing evidence suggests it might be worth supplementing. Here, we break down the benefits of quercetin, how to get more in your diet, and whether supplementing is right for you. 

Quercetin is part of a group of plant pigments called flavonoids, and it's found in a range of foods such as fruits, vegetables, nuts, and grains. In supplement form, quercetin is often derived from the dried flowers and buds of the Sophora japonica (or Japanese pagoda) tree. 

Like all flavonoids, quercetin is an antioxidant, meaning it helps scavenge and neutralize free radicals that might otherwise accelerate aging, contribute to inflammation, and fuel a range of chronic diseases from cancer to heart disease. 

Quercetin may be most well known for its ability to ease seasonal allergy symptoms, but it's proving to be quite the versatile plant compound. "Quercetin is a powerful antioxidant providing anti-inflammatory, antihistamine, and antiviral properties, as well as serving the ability to reduce blood clots and oxidative stress in arteries," says Ali Miller, R.D., L.D., CDE, registered dietitian and integrative functional medicine practitioner. "The current pandemic makes this nutrient even more of an important player as we see oxidative stress, inflammation, and clotting factors as concerns."

Like any nutrient, quercetin isn't a magic bullet for health or immunity in isolation. But the body of existing research on quercetin is promising. 

"The main reason I've prescribed quercetin for over 30 years is because it works, it is safe, and its use is based on solid published evidence," says Robert Rountree, M.D., renowned integrative physician. "I also prescribe it a lot because it is so versatile as a preventive or treatment for so many kinds of health conditions." 

Here are some specific ways quercetin may enhance health:

Quercetin may be useful in the prevention and treatment of a wide range of viral infections, including upper respiratory tract infections (URTIs) such as the common cold and the flu. In one study published in the Journal of Clinical & Experimental Cardiology, supplementation with 1,000 mg quercetin per day reduced the incidence of URTIs1 after a period of intense exercise—three hours of cycling, three days in a row, during winter. 

This flavonoid has even helped reduce the frequency and severity of herpes outbreaks among Rountree's patients. According to Rountree, quercetin's mechanism of combating viruses isn't totally understood, but it appears to be a combination of immune modulation, prevention of viral entry into cells, and inhibition of viral replication.  

Quercetin also acts synergistically with other immune-boosting nutrients. For example, "Quercetin acts as a zinc shuttle, getting natural antiviral zinc into cells where it can help mitigate viral replication," Vincent Pedre, M.D., integrative physician, recently told mbg. 

It's too early to say whether or not quercetin would help against COVID-19—but research is beginning to be published2 on quercetin's potential value in prevention and treatment, and it's been one of Rountree's top recommendations to support the immune system during the pandemic. Several studies are underway to explore this, so stay tuned.  

"Quercetin's most obvious and immediately noticeable benefit is prevention of histamine-mediated allergic symptoms," says Rountree. "Over the years I have treated numerous individuals with chronic allergies who were either able to stop or greatly reduce their use of pharmaceutical agents."

More research is needed in humans, but lab studies—including one published in the journal Molecules—show that quercetin can help combat conditional allergic rhinitis3. How exactly? It seems to help prevent the overactivation of mast cells, which secrete histamine—an inflammatory chemical that's involved in allergy symptoms like itching and sneezing. Vitamin C may enhance quercetin's allergy-fighting potential. 

Free radicals in the body promote inflammation, but antioxidants help scavenge and neutralize free radicals. So it makes sense that quercetin (a flavonoid antioxidant) would have anti-inflammatory properties. 

In fact, several studies like this one published in the European Journal of Clinical Nutrition have found that supplementing with about 500 mg of quercetin4 per day helps lower levels of the inflammatory biomarker c-reactive protein (CRP), which is elevated in conditions like heart disease, rheumatoid arthritis, metabolic syndrome, and diabetes. Another study5 published in the Journal of the American College of Nutrition, which looked at 50 women with rheumatoid arthritis, found that 500 mg of quercetin per day for eight weeks helped reduce morning stiffness and pain.

Dementia, Alzheimer's, and other neurodegenerative diseases can be exacerbated by oxidative stress, caused by an overabundance of free radicals in the body. Not only does quercetin have antioxidant properties that counter these effects, but it supports mitochondrial biogenesis (the production of new mitochondria, the energy-revving "powerhouse" of cells) in the brain for enhanced cognition, says Roundtree. 

More studies are needed on humans, but research on mice6 has found that quercetin injections reduced several markers of Alzheimer's disease and improved performance on memory tasks. 

Numerous flavonoid antioxidants, not just quercetin, have demonstrated anti-cancer properties, which is one reason eating loads of colorful fruits and veggies is so dang good for you. But quercetin, specifically, has been shown to inhibit cancer cell growth and proliferation, or multiplication. 

Abundant lab and animal studies demonstrate quercetin's ability to curb cancer cell growth and even prompt cancer cell death in multiple types of cancer (breast7, colon8, ovarian9, lung10, and prostate to name a few), but more research is needed on humans. 

Quercetin's cardiovascular benefits go above and beyond its ability to fight inflammation. Here are a few more reasons it may support your heart:

In addition to the health benefits above, there's evidence to suggest quercetin may help with the following:

Before we dive into info on supplementing, you should know that quercetin is present in loads of healthy, delicious plant-based foods12 (often in the peel or outer layer). While upping your intake of quercetin-rich foods may not treat the ailments listed above, it will support overall health.

Quercetin is present in many healthy foods, and eating a variety of the quercetin-rich foods listed above poses no risk (unless you're allergic or have a sensitivity, of course!). Supplements are quite safe too. "I have found that quercetin is very well tolerated with minimal side effects, the most common of which is mild upset stomach," says Rountree.  

Some research suggests15 that quercetin may cause headaches or tingling nerve sensations, but Rountree says he has not seen this among his patients and that quercetin is on his "A-list of supplements" that most people could benefit from taking every day.

Who should avoid quercetin? If you're currently taking another medication, it's always a good idea to talk to your health care provider about potential interactions. For example, because quercetin can thin the blood and help reduce blood clots, it may interfere or amplify the effects of blood thinners; there's also some evidence it could interfere with medications that have a narrow therapeutic window such as transplant anti-rejection drugs.  

Pregnant women should also steer clear since no good studies exist on the safety of quercetin during pregnancy (food sources are totally fine). 

Most people will realistically only hit up to 80 mg of quercetin per day from dietary sources, according to Rountree. So supplements are necessary if you want to fight allergies, support immunity, or experience any of the potential quercetin health benefits listed above. In fact, "the dose for prevention of allergic symptoms is from 500 to 3,000 mg daily," he says.  

A typical dose of quercetin is 500 to 1,000 mg per day (this is the dosage often used in clinical studies16). However, quercetin is somewhat poorly absorbed through the intestinal wall, so Rountree recommends quercetin phytosome—a form where quercetin is attached to a phospholipid complex17, which has been shown to dramatically enhance absorption18. 

Another way to get more nutritional bang for your quercetin buck: Combine it with vitamin C. "I recommend a form of quercetin with a food-derived vitamin C source like acerola cherry and bioflavonoids from citrus," says Miller. "This will provide some synergistic effects, which will get outcomes at a lower dosage."

Quercetin is one of those nutrients that's pretty great for overall wellness—as it supports immunity and has anti-inflammatory perks—and it may be particularly beneficial for people struggling with seasonal allergies. Food sources are abundant (think fruits, veggies, nuts, teas) and contain a variety of other nutrients, but to help manage or prevent a specific ailment, supplementation at doses of 500 to 1,000 mg is typically necessary. Just remember, as with all supplements, it's important to check in with your doctor first if you have questions or concerns. 

Keep in mind: When it comes to supporting overall immunity, quercetin should be just one component of your routine (nothing works in isolation, after all). To give your immune system an extra leg up, get seven to nine hours of sleep a night, eat immune-boosting foods, consider additional expert-backed supplements for immune health, stay active, and manage stress or any existing health conditions you may have.