Antioxidants are of utmost importance in maintaining overall health because of their ability to fight free radicals in the body, which are precursors to devastating diseases. Antioxidants are also responsible for fixing damaged molecules, inhibiting metal radical production, delivering a “shield” effect and stimulating gene expression and endogenous antioxidant production.
Quercetin is one of the antioxidants that can deliver these health benefits, but not many are familiar with its effects on the body. Read on to learn what quercetin is, its uses and benefits, the various food sources of this antioxidant and the possible complications that can arise if you take quercetin supplements.
What Is Quercetin?
Quercetin is an antioxidant that belongs to a class of water-soluble plant substances called flavonoids, which are present in certain fruits and vegetables.
Quercetin is also available in supplement form, as pills or capsules. Sometimes, quercetin is packaged with bromelain, an enzyme found in pineapples, because both are said to have anti-inflammatory properties. Other forms of quercetin include hesperidin-methyl-chalcone (HMC) or quercetin chalcone.1
When looking for quercetin supplements, it’s said that dihydrate has the “apparent best bioavailability,” followed by glycosides, aglycone and rutinoside.2
However, research has actually a revealed major caveat linked to quercetin dihydrate — its poor oral bioavailability.3 If quercetin dihydrate is already considered to have the “best bioavailability,” then what does it say about the amount of quercetin present in other supplements? Hence, instead of relying on supplements to increase your body’s quercetin levels, I advise you to check out the following food sources containing substantial amounts of this antioxidant.
Foods Containing Quercetin You Should Try
There are foods high in quercetin that you can add to your diet. Arguably, red onions are one of the most potent sources of quercetin, and it’s said that onions can absorb quercetin twice more than tea, and thrice more than apples. Other food sources of quercetin include:4,5
• Organically grown apples, plums, red grapes and citrus fruits (eaten in moderation)
• Dark cherries and dark berries like blueberries, blackberries and bilberries (eaten in moderation)
Herbs like parsley, sage, elder flower, ginkgo biloba and St. John’s Wort, and green, black and buckwheat tea6 are also said to contain some amounts of quercetin. For a more thorough list of quercetin-rich foods, you can check out this Superfoodly article.7
Health Benefits of Quercetin
Quercetin has been linked to the following health benefits:8
Potentially lowering risk of atherosclerosis and lung cancer, especially among smokers
Assisting in preventing death from heart disease
Helping reduce blood pressure levels
Aiding with addressing symptoms of interstitial cystitis
Helping decrease prostatitis symptoms
Helping prevent cancer cell growth from breast, colon, prostate, endometrial and lung cancers
Possessing anti-inflammatory properties that can help combat chronic diseases
Aside from antioxidant properties, quercetin is known to have anti-carcinogenic and anti-artherogenic capabilities. It’s also said to be neuroactive, possessing the same abilities as caffeine, albeit less potent.9
What Are the Uses of Quercetin?
Quercetin is known for its potential to address conditions of the heart and blood vessels, such as atherosclerosis, high cholesterol levels, heart disease and circulation problems. It can also help address:10
Viral infections caused by influenza (A and B, and H1N1, H3N2 and H5N1),11,12 mouse hepatitis and dengue,13 and hepatitis B14 and C15,16
Chronic fatigue syndrome (CFS)
Chronic infections of the prostate
Quercetin may improve athletic performance, bone health and immune response, help prevent fatigue, boost energy levels and reduce recovery time.18 As mentioned earlier, quercetin has antioxidant properties that can scavenge free radicals in the body, which can damage cell membranes, tamper with DNA and trigger cell death.19
Researchers have also suggested that quercetin can be used against allergies by helping reduce symptoms like runny nose, watery eyes, hives and swelling of the face and lips. Test tube studies showed quercetin can work as an antihistamine by preventing immune cells from releasing histamines, or chemicals that cause allergic reactions. However, there is no evidence yet that shows this effect works on humans.
It has also been suggested that quercetin has potential effects for weight loss, particularly on exerting multiple effects directly to fat tissue, when taken alone or in combination with resveratrol and genistein.
According to Life Extension Magazine, quercetin can prevent fat accumulation in maturing human fat cells in culture, while inhibiting maturity of new fat cells and triggering apoptosis or programmed cell death in existing fat cells. Quercetin can also block the uptake of glucose from the blood, depriving fat cells of raw material that they need to produce.
Studies on Quercetin
Out of the different flavonoids, quercetin is the most frequently studied because of its potential health benefits.20 Some studies, such as the following, sought to discover if quercetin can address certain conditions:
• Reducing blood pressure levels:21 This 2012 Advances in Nutrition Study showed how quercetin can potentially deliver blood pressure-lowering effects in hypertensive humans. The researchers noted, however, that larger clinical trial studies may be needed to confirm this effect.
• Treating liver damage in rats:22 In this study published in Digestive Diseases and Sciences in 2003, researchers revealed that a quercetin treatment administered for three weeks improved liver histology of rats considered cirrhotic and had cell necrosis, fibrosis and inflammatory infiltration.
The quercetin-treated rats also had reduced collagen content, lipid peroxidation and inducible nitric oxide synthase (iNOS) expression.
• Alleviating the common cold:23 According to researchers of this 2014 Journal of Infectious Diseases & Preventive Medicine study, quercetin can be “a promising treatment for the common cold” because of its potential to prevent the virus from exerting effects on the body.
Quercetin was also shown to lower a person’s risk for secondary bacterial infections and promote increases in mitochondrial biogenesis in skeletal muscle.
A study published in 2007 conducted by researchers from the Appalachian State University and funded by the U.S. Department of Defense also discovered that quercetin helped reduce viral illness and promote better mental health performance in people who underwent periods of extreme physical stress.24 Meanwhile, other research focused on quercetin’s ability to trigger apoptosis or cell death towards cancer cells:
• Apoptosis of human gastric carcinoma cells:25 According to researchers from a 2012 Toxicology in Vitro study, quercetin was able to induce apoptosis on gastric carcinoma BGC-823 cells by changing the apoptopic protein expression.
• Apoptosis of human breast cancer cells:26 This study published in the International Journal of Oncology in 2001 revealed that quercetin can possibly inhibit growth of MCF-7 human breast cancer cells through at least two different mechanisms.
Lastly, quercetin was also examined for its possible benefits on exercise performance, as seen in a 2012 study in Journal of Research in Medical Sciences.
Non-professional athletes who exercised regularly and took quercetin-vitamin C supplements were observed for eight weeks to determine effects on oxidative stress and inflammatory biomarkers. The supplements were found effective in lowering oxidative stress and decreasing inflammatory biomarkers in healthy subjects.27
Ideal Dosage of Quercetin
For adults, a 500 milligram-dose (mg) twice a day can be considered.28 Quercetin dosages can also vary, depending on the patient’s condition. These are suggested values:29,30
• Hay fever: Between 200 and 400 mg per day
• Prostate pain and swelling (prostatitis): 500 mg twice a day
According to Examine, quercetin dosages are often in the range of 12.5 to 25 mg per kilogram body weight. This roughly translates to a range of 1,136 to 2,272 daily consumption of quercetin when used in isolation, which is quite a lot.
Since no optimal doses of quercetin have been established for any condition, I highly recommend that you consult a physician, doctor or other health professional first before taking quercetin supplements.31
Furthermore, it’s suggested that supplementing with other bioflavonoids like resveratrol, genistein or green tea catechins alongside quercetin can not only increase the potency of quercetin synergistically, but also allow the person to reap the benefits at a reduced level of intake.
Do take note that quercetin supplements may only be taken by adults, as there isn’t enough evidence supporting quercetin use in children. Likewise, pregnant and breastfeeding women and people with kidney disease must avoid taking quercetin supplements because they may experience negative side effects.32
Quercetin Side Effects to Watch Out For
Headaches and an upset stomach are the common side effects of quercetin supplements, but adverse side effects may also occur. Preliminary evidence suggested that a quercetin byproduct can result in a loss of protein function, and very high doses of quercetin may lead to kidney damage. It's advised that you take periodic breaks from taking quercetin.33
People with sensitive stomachs can also experience heartburn or acid reflux if they take larger doses on an empty stomach. This can be avoided by simply taking the quercetin in the middle of a meal. Furthermore, taking excessive amounts of supplements containing antioxidants such as quercetin and vitamin C at once can lead to a pro-oxidant effect. This means that there’s a tendency for the supplements to cause inflammation instead of preventing it.34
If you or someone you know is taking any of these medications as treatment, avoid taking quercetin supplements without talking to your physician or doctor first. Taking quercetin alongside these drugs can lead to contraindications:35
Antibiotics: There are concerns that quercetin can decrease the effectiveness of certain antibiotics.
Anticoagulants (blood thinners) such as Warfarin (Coumadin), Clopidogrel (Plavix) and Aspirin: Quercetin supplements can enhance the effects of these drugs and increase your risk for bleeding.
Chemotherapy: Test tube and animal studies have highlighted quercetin’s potential in enhancing the effects of two chemotherapy medications used to treat cancer, namely doxorubicin and cisplatin.
Ideally, talk to your oncologist first before taking any supplements during chemotherapy.
Corticosteroids: Quercetin supplements can cause these drugs to stay in the body for a longer period of time.
Cyclosporine: Quercetin can interfere with the body’s absorption of cyclosporine that’s often used to suppress the immune system.
Digoxin: Taking quercetin supplements simultaneously with digoxin can increase the risks of the latter.
Fluoroguinolones: Concomitant quercetin supplement use may reduce fluoroguinolones’ effectiveness.
Medications changed by the liver: Because quercetin can affect the liver, taking supplements with medications that are changed by the liver can change how the body metabolizes these medicines.
Quercetin is a potent antioxidant with many uses and benefits for the body. But even though studies on this particular antioxidant have yielded positive results, it's still better to acquire quercetin naturally from the food sources mentioned earlier.
Not only is the poor bioavailability of quercetin dihydrate supplements a major concern, but the side effects and interactions linked to quercetin supplements can result in health problems too.
Frequently Asked Questions About Quercetin
Q: What does quercetin do for the body?
A: Quercetin is an antioxidant with anti-inflammatory properties. It can help fight chronic diseases, assist in blood pressure level reduction and aid with addressing symptoms of prostatitis and interstitial cystitis. Quercetin's antioxidant properties also allow it to neutralize free radicals in the body, which are said to damage cell membranes, tamper with DNA and cause cell death.
Q: What is quercetin good for?
A: Quercetin can assist in reducing risk of diseases like atherosclerosis and lung cancer, lowering blood pressure levels and symptoms of interstitial cystitis and prostatitis, preventing growth of cancer cells and combatting chronic diseases. It’s also said to have anti-inflammatory, anti-carcinogenic and anti-artherogenic properties.
Q: What foods are high in quercetin?
A: Quercetin is found in high amounts in red onions, although it can also be found in fruits and vegetables such as garlic, organically grown apples and citrus fruits, dark cherries, dark berries and capers. This flavonoid antioxidant is also present in olive oil, herbs like parsley, sage, ginkgo biloba and St. John's wort, as well as in green, black and buckwheat tea.
Q: Is quercetin an anti-inflammatory?
A: Yes. Quercetin is a known anti-inflammatory, and can assist in fighting chronic diseases that may affect your body.
Q: Is quercetin an anti-histamine? How much quercetin is effective for the allergies?
A: There is evidence suggesting that quercetin has anti-histamine properties and can help with reducing common symptoms of an allergic reaction. Unfortunately, an ideal dose of quercetin supplements for allergy problems hasn't been established yet, so it's ideal that you talk to a physician, doctor or other health professional first before taking these.
Q: How do you take quercetin?
A: Quercetin is available in supplement form, although it’s more ideal to get it from food or beverage sources.