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. According to an article in Current Research in Nutrition and Food Science, the bioavailability of quercetin absorbed by your body varies, depending on the source where you get it, as well as your individual body’s metabolism of it. Sugar and dietary fat and fiber also affect it.1
While the best way to get quercetin is through your diet, it also is available in supplement form, as pills or capsules. Sometimes, quercetin in supplement form is packaged with bromelain, an enzyme found in pineapples, because both have anti-inflammatory properties.2 Most dietary supplements contain a chemical compound, aglycone, which is the free form of quercetin.3
When looking for and using quercetin supplements, it’s important to maintain a nutritious, balanced diet with fiber and healthy fats in it, as research shows that accumulation of quercetin in the body is dependent on “factors such as glucose moieties, solubility, human factor, vitamin C status and food matrix.”4
Specifically, research also says that quercetin supplementation is best achieved when consumed with a fat-enriched diet.5 For these reason, I advise you to consider getting your quercetin from whole, fresh, real foods that contain quercetin, rather than resorting to supplements.
Foods Containing Quercetin You Should Try
There are foods high in quercetin that you can add to your diet. Arguably, red onions are some of the most potent sources of quercetin; one study found that absorption of onion-derived quercetin was 52% compared to only 24% in supplement form.6 In fact, onions can give you three times the amount of quercetin than you’d get from apples. Other food sources that are high in quercetin include:7,8,9
- Organically grown apples, grapes and tomatoes
- Dark cherries and dark berries like blueberries, blackberries and bilberries (eaten in moderation)
- Cocoa, unsweetened
- Hot green chili peppers
- Lingonberries
Herbs like tarragon, chives, coriander and dill weed, as well as fresh greens like watercress, spinach and kale also serve up healthy portions of quercetin. Green and black tea do, too, in lesser amounts, while moringa has much higher levels. And, if you’re a coffee lover, you’ll be pleased to know that researchers found that the quercetin in coffee has neuroprotective effects.10
Note: Many food sites claim that quercetin is also found in garlic, but researchers said in a study published in Food Chemistry that they found NO flavonols at all — kaempeferol, quercetin or myricetin — in garlic.11
Health Benefits of Quercetin
Quercetin has been linked to the following health benefits:
- Possessing anti-inflammatory, antioxidant properties that can help combat certain diseases.12,13 One study found that 500 mg of quercetin a day could improve symptoms of rheumatoid arthritis.14
- Helping prevent cancer cell growth from breast, colon, prostate, endometrial and lung.15,16,17,18
- Counteracting allergic reactions and helping with immune responses.19,20,21,22
- Helping to prevent osteoporosis23
- Addressing cognitive decline, including Alzheimer’s and dementia.24,25
- Assisting in control of blood pressure.26,27 A study in 2002 found that quercetin can have vasodilator effects.28
- Potentially helping to lower blood sugar levels. One study found that dosages of 500 mg per day “significantly reduced” fasting glucose levels.29
As you can see from the studies, 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. For that reason researchers are encouraging more human clinical studies with quercetin, while at the same time saying they believe:30
“Quercetin provides many health promoting benefits, including improvement of cardiovascular health, eye diseases, allergic disorders, arthritis, reducing risk for cancers and many more.”
What Are the Uses of Quercetin?
As mentioned, 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. Other research indicates it may also help address:
- Viral infections caused by influenza (A and B, and H1N1, H3N2 and H5N1)31,32 and dengue33
- Chronic prostatitis and pelvic pain34
- Hepatitis B35 and C36
- Mood disorders37,38
- Cataracts39
- Schizophrenia40
- Hay fever41
- Chronic fatigue syndrome (CFS)42
- Peptic ulcer43
- Gout44
Quercetin also may improve athletic performance, bone health and immune response, help prevent fatigue, boost energy levels and reduce recovery time.45 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.46
It has also been suggested that quercetin has potential effects for weight loss,47 particularly on exerting multiple effects directly to fat tissue, when taken alone or in combination with resveratrol and genistein.48
More Studies on Quercetin
Out of the different flavonoids, quercetin is the most frequently studied because of its potential health benefits.49 Some studies, such as the following, sought to discover if quercetin can address certain conditions:
• Treating liver damage in rats50 — 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 cold51 — 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.
• Lowering your risk for secondary infections and promoting increases in mitochondrial biogenesis in skeletal muscle.52
• Benefiting exercise performance, according to a 2012 study in Journal of Research in Medical Sciences.53
• Lowering oxidative stress — Nonprofessional 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.54
Ideal Dosage of Quercetin
For adults, a 500 milligram-dose (mg) twice a day can be considered, plus, many of the studies I’ve cited here use that dosage.55 However, remember that quercetin dosages can also vary, depending on your condition and your diet.
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.
Furthermore, research shows that supplementing with other bioflavonoids like resveratrol, genistein or green tea catechins alongside quercetin not only can increase the potency of quercetin synergistically, but also may allow you 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.
Quercetin Side Effects to Watch Out For
Headaches and an upset stomach are the common side effects of quercetin supplements, but other adverse side effects may also occur.56 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.
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.57
If you or someone you know is taking any of the following medications as treatment, avoid taking quercetin supplements without talking to your physician or doctor first.58
• 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.
Frequently Asked Questions About Quercetin
Q: What does quercetin do for the body?
A: Quercetin is an antioxidant with anti-inflammatory properties. It may be able to 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 prostatitis; preventing growth of cancer cells; and helping to combat chronic diseases. It's also has anti-inflammatory, anticarcinogenic 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 apples, moringa, leafy greens, dark cherries, dark berries and capers. This flavonoid antioxidant is also present in herbs like parsley and tarragon, 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 antihistamine? How much quercetin is effective for the allergies?
A: There is evidence suggesting that quercetin has antihistamine 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 better to get it from food or beverage sources.