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What is Quercetin?
Quercetin is a plant pigment found in a variety of fruits, vegetables, nuts, and grains. Quercetin is a bioflavonoid known as a flavonol, important for its antioxidant properties.1 Its antioxidant properties help neutralize free radicals that may otherwise cause inflammation and contribute to a wide range of chronic diseases.
Quercetin is best known for its use in treating seasonal allergies, although research continues to demonstrate its versatility in other conditions. In fact, quercetin is known for having “anti-inflammatory, antihypertensive, antihistamine, vasodilator effects, antiobesity, antihypercholesterolemic and antiatherosclerotic activities.”1 Furthermore, it helps reduce blood clots and oxidative stress in the arteries and may protect against drug induced tissue damage.1
What is Quercetin used for?
Quercetin is most commonly used for preventing histamine-mediated allergic symptoms, i.e. seasonal allergies. Quercetin prevents the excessive release of histamine by inhibiting mast cells, the cells responsible for secreting histamine.2 Histamine is an inflammatory chemical that contributes to common allergy-like symptoms such as itching and sneezing.
Quercetin is also used to prevent and treat viral infections, protect cells from drug induced tissue damage,3 support cognition and brain health, inhibit cancer cell growth4, and combat chronic inflammation.2 In fact, quercetin is one of the most commonly used bioflavonoids for individuals with metabolic and/or inflammatory disorders.1 Documented benefits of quercetin include “cardiovascular protection, anticancer, antitumor, anti-ulcer, anti-allergy, anti-viral, anti-inflammatory, anti-diabetic, gastroprotective, antihypertensive, immunomodulatory, and anti-infective effects.”2 Just to name a few!
What are some food sources of Quercetin?
Quercetin can be found in a variety of fruits, vegetables, nuts and seeds. The most concentrated source of quercetin is caper berries. Other rich sources of quercetin include:
- Red grapes
- Green tea
- Brussels sprouts
- Citrus fruits
- Olive oil
It’s important to note that quercetin is primarily found in the peel or outer layer of these foods, so be sure to leave the peels on when possible! Quercetin is a lipophilic compound, meaning it’s best absorbed in the presence of dietary fat.5 Consider adding olive oil, avocado, or nuts and seeds to any of the foods listed above!
Quercetin supplements vs. food sources
The dose of quercetin typically used in clinical studies is 500-1000 mg per day.6 However, most people will only consume a small fraction of that amount from food sources. Although consuming more whole food sources of quercetin is wonderful for overall health and wellbeing, it may not be enough to generate significant improvements, especially in easing seasonal allergies. If the goal is to manage seasonal allergies or support the immune system in a meaningful way, a high quality supplement may be the best course of action.
The most commonly used quercetin dose is 500-1000 mg per day. However, clinical studies are conducted almost exclusively on adults. Generally children are prescribed 25-50% of the adult dose, depending on their age and size, but there is currently no consensus on a safe dose for children. It can take up to 6-8 weeks for effects to be seen, so initiating supplementation before allergy season begins is recommended.
Possible side effects of Quercetin
Documented side effects of quercetin include headache, numbness and tingling. In research studies where quercetin is given intravenously other side effects include shortness of breath, nausea, vomiting and kidney damage. Quercetin should not be administered intravenously unless under the supervision of a board-certified Naturopathic Oncology specialist (FABNO). For more information see fabno.org
In practice I find that quercetin supplementation is well tolerated. However, quercetin can cause agitation, anxiety or insomnia in some sensitive patients. I suspect these patients have COMT or MAOA polymorphisms, see below.
Quercetin medication interactions
Quercetin, like many bioflavonoids, may modulate the expression of many drug-metabolizing enzymes. This includes phase I and phase II enzymes like cytochrome P (CYP) 1A1, CYP1A2, CYP1B1, CYP2A6, CYP2C9, CYP2D6, CYP3A4, UDP glucuronosyltransferases (UGTs) and sulfotransferases (SULTs).12 The scope and impact of these phase I and phase II enzymes is well beyond the scope of this article. However, providers who prescribe quercetin to their patients should be aware that serum concentrations of medications may be significantly reduced when the patient is taking quercetin. Interactions with chemotherapeutics are particularly notable.
Quercetin is also suspected to have some MAOA and COMT inhibition. Though not proven, I strongly suggest avoiding quercetin administration in adults and children who are taking MAOI medications. Similarly, use caution or avoid quercetin in patients who have single-nucleotide polymorphisms that slow MAOA or COMT.
Is Quercetin safe for use in children?
Safety studies for quercetin in children are lacking. However, robust clinical trials for quercetin use in adults have been completed, and no significant side effects or toxicity have been observed, with the exception of intravenous quercetin use in cancer patients. Studies in adults show that quercetin is probably safe when used in doses up to 1,000 mg for 12 weeks. Because quercetin is a potent bioflavonoid, most research shows anti-cancer, neuroprotective, anti-inflammatory and other positive benefits. It is highly unlikely that effects in children would be dramatically different than those in adults.
Given the research in adults it is my medical opinion that quercetin is probably safe in children in reasonable doses. Quercetin use is possibly unsafe in patients taking medications that are affected by the above-listed medication interactions. Patients should be under the supervision of a knowledgeable physician if they are taking both medications and quercetin simultaneously. In some cases the benefits of quercetin administration may outweigh the risks of medication interactions, and/or the dosage of medication may be adjusted appropriately.
Quercetin is probably safe in children in weight-appropriate doses.
Quercetin is possibly unsafe in patients also taking medications that are metabolized by CYP1A2, CYP1B1, CYP2A6, CYP2C9, CYP2D6, CYP3A4, UDP glucuronosyltransferases (UGTs) and sulfotransferases (SULTs).
Quercetin should be used with caution in patients with MAOA SNP’s, or patients with severe insomnia or anxiety.
Quercetin uses in clinical practice
Quercetin is often used for allergic rhinitis due to its influence on numerous biological pathways and immune cells that contribute to the allergic immune response. Its antioxidant activity helps scavenge free radicals, stimulate the immune system, inhibit histamine release, decrease pro-inflammatory cytokines and leukotrienes, and suppress interleukin IL-4 production. Quercetin may help improve a skewed Th1/Th2 balance and suppress antigen-specific IgE antibody formation. Additionally, quercetin inhibits enzymes such as lipoxygenase and peroxidase, as well as other inflammatory mediators. All of these actions contribute to an overall anti-inflammatory and immunomodulating effect that reduces the symptoms of seasonal allergies.6
Viral Respiratory Infections
Quercetin may be used to prevent and treat viral infections like the common cold and flu. Rhinovirus is responsible for the majority of common colds and quercetin has been found to block viral replication and inhibit virally stimulated cytokine expression in vitro. In human trials, quercetin has been shown to inhibit viral replication and decrease lung inflammation. Quercetin supports the immune system, prevents viral entry into healthy cells, and inhibits viral replication. Quercetin works synergistically with other immune supporting nutrients like zinc and vitamin C to further boost the immune system and reduce the severity and duration of the common cold and flu.7
The antioxidant effect of quercetin makes it a promising tool for combating the chronic inflammation that contributes to so many diseases. Studies have found that 500 mg of quercetin a day lowers levels of c-reactive protein (CRP), a biomarker used to measure inflammation. CRP is often elevated in diseases such as heart disease, rheumatoid arthritis, metabolic syndrome, and diabetes.8
Cognitive Function, Autism Spectrum Disorder, ADHD and Depression.
Animal studies have found that quercetin supplementation reduces several markers associated with Alzheimer’s disease and improves performance on various memory tasks.10 Although studies on quercetin and cognitive function are limited and inconclusive in humans, we do know that it supports the production of new mitochondria in the brain which may enhance cognition and protect against neurodegenerative diseases.1
Where neurological conditions are caused or worsened by oxidative stress, quercetin can be helpful to prevent progression or even reduce symptoms. One review discusses a potential role of quercetin in the treatment of autism spectrum disorder and ADHD. Further studies need to be completed to determine the proper dosage and treatment effect, if any.13
Clinically I find quercetin to be helpful in some late-childhood to adolescent-aged children with depression. Specifically quercetin seems to help children or teens who have low dopamine levels. As previously mentioned, the positive effect on mood is probably due to some mild MAOA or COMT inhibition. Again, use caution in patients with anxiety or insomnia.
Other promising uses
Quercetin has been shown to have a positive effect on systolic and diastolic blood pressure in adults with diabetes, preventing LDL cholesterol oxidation, and reducing overall cholesterol levels.11 Research on quercetin is ever evolving and there are promising results for its use in everything from interstitial cystitis, to athletic performance, to longevity in general!
Quercetin is a plant compound known as a flavonol that possesses important antioxidant properties. It can be found in a variety of fruits, vegetables, nuts, seeds, and whole grains, but a supplement may be necessary for therapeutic purposes. Quercetin is best known for its use in treating and preventing seasonal allergies. It may also be used to prevent and treat viral respiratory infections, protect against drug induced tissue damage, and manage chronic inflammation. Quercetin continues to show promising effects in a wide variety of conditions and emerging research will guide future recommendations. Food and supplemental sources of quercetin pose little risk, although mild headaches, GI discomfort, and agitation may occur. If symptoms persist, discontinue supplementation and speak with your physician.
- Anand David AV, Arulmoli R, Parasuraman S. Overviews of Biological Importance of Quercetin: A Bioactive Flavonoid. Pharmacogn Rev. 2016;10(20):84-89. doi:10.4103/0973-7847.194044
- Lakhanpal P, Rai D. Quercetin: A Versatile Flavonoid. Internet Journal of Medical Update. 2007;2(2).
- Begum AN, Terao J. Protective effect of quercetin against cigarette tar extract-induced impairment of erythrocyte deformability. J Nutr Biochem. 2002;13(5):265-272. doi:10.1016/s0955-2863(01)00219-4
- Wang R, Yang L, Li S, et al. Quercetin Inhibits Breast Cancer Stem Cells via Downregulation of Aldehyde Dehydrogenase 1A1 (ALDH1A1), Chemokine Receptor Type 4 (CXCR4), Mucin 1 (MUC1), and Epithelial Cell Adhesion Molecule (EpCAM). Med Sci Monit. 2018;24:412-420. Published 2018 Jan 21. doi:10.12659/msm.908022
- Kaşıkcı M. B, Bağdatlıoğlu N. Bioavailability of Quercetin. Curr Res Nutr Food Sci 2016;4(Special Issue Confernce October 2016).
- Mlcek J, Jurikova T, Skrovankova S, Sochor J. Quercetin and Its Anti-Allergic Immune Response. Molecules. 2016; 21(5):623. https://doi.org/10.3390/molecules21050623G
- Shyamala Ganesan S, Faris AN, Comstock AT, et al. Quercetin inhibits rhinovirus replication in vitro and in vivo. Antiviral Res. 2012;94(3):258-271. doi:10.1016/j.antiviral.2012.03.005
- Mohammadi-Sartang M, Mazloom Z, Sherafatmanesh S, Ghorbani M, Firoozi D. Effects of supplementation with quercetin on plasma C-reactive protein concentrations: a systematic review and meta-analysis of randomized controlled trials. Eur J Clin Nutr. 2017;71(9):1033-1039. doi:10.1038/ejcn.2017.55
- Javadi F, Ahmadzadeh A, Eghtesadi S, et al. The Effect of Quercetin on Inflammatory Factors and Clinical Symptoms in Women with Rheumatoid Arthritis: A Double-Blind, Randomized Controlled Trial. J Am Coll Nutr. 2017;36(1):9-15. doi:10.1080/07315724.2016.1140093
- Sabogal-Guáqueta AM, Muñoz-Manco JI, Ramírez-Pineda JR, Lamprea-Rodriguez M, Osorio E, Cardona-Gómez GP. The flavonoid quercetin ameliorates Alzheimer’s disease pathology and protects cognitive and emotional function in aged triple transgenic Alzheimer’s disease model mice. Neuropharmacology. 2015;93:134-145. doi:10.1016/j.neuropharm.2015.01.027
- Serban M, et al. Effects of Quercetin on Blood Pressure: A Systematic Review and Meta‐Analysis of Randomized Controlled Trials. Journal of the American Heart Association. 2016;5:e002713.
- Liu, et al. Impact of quercetin-induced changes in drug-metabolizing enzyme and transporter expression on the pharmacokinetics of cyclosporine in rats. Molecular Medicine Reports. 2016;14.4: 3073-3085.
- Alvarez-Arellano L, Salazar-García M, Corona JC. Neuroprotective Effects of Quercetin in Pediatric Neurological Diseases. Molecules. 2020 Nov 28;25(23):5597. doi: 10.3390/molecules25235597