Quercetin 101: Getting to Know This Powerful Substance and How You Can Use it For Your Health

Quercetin, a potent flavonoid found in various plants, has gained prominence in nutritional and herbal medicine for its remarkable health benefits. What is quercetin used for? This versatile compound is renowned for its antioxidant and anti-inflammatory properties, making it a valuable asset in supporting overall well-being. Quercetin has been linked to a range of health advantages, including immune system support, heart health, and even allergy relief. Its ability to neutralize free radicals and modulate inflammatory responses has made it a popular choice in natural remedies and dietary supplements. As an essential component of plant-based nutrition and herbal medicine, quercetin continues to capture the attention of those seeking holistic approaches to health and wellness.

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Origin and Dietary Sources of Quercetin

Quercetin is a yellow flavonoid that can be found in many plant-based foods. Examples of natural dietary sources of quercetin include apples, onions, berries, citrus fruits, grapes, tomatoes, broccoli, and cabbage. Small amounts have also been isolated from botanical herbs, including Ginkgo biloba, St. John's wort, milk thistle, and elderberry. Quercetin is recognized for its potent antioxidant and anti-inflammatory properties. Consuming a diverse range of fruits and vegetables ensures a steady intake of quercetin, which plays a crucial role in supporting overall health and preventing chronic diseases. In standard Western diets, daily intakes of quercetin through diet have been estimated between 3-40 mg. In people with high fruit and vegetable consumption, daily quercetin intake can be as high as 250 mg. (24, 33) 

How Does Quercetin Work in the Body?

Quercetin is a naturally occurring flavonol, one of the six subsets of flavonoids. Flavonoids are phytochemical compounds found naturally in fruits, vegetables, herbs, nuts, and seeds with anticancer, antioxidant, anti-inflammatory, antimicrobial, neuroprotective, and cardioprotective properties. 

Quercetin's molecular structure (pictured below) consists of 15 carbon atoms arranged in a specific arrangement of rings. A phenolic hydroxyl group and double bonds in its chemical structure allow quercetin to have strong antioxidant activity. Quercetin is available in various forms, including quercetin aglycone (free quercetin), quercetin glycosides, and quercetin derivatives. These forms differ in their bioavailability and absorption rates due to their chemical structures. Quercetin aglycone is the pure, unbound form of quercetin. Quercetin glycosides, on the other hand, are quercetin molecules bound to sugar molecules. Overall, quercetin is considered to have low bioavailability. Supplemental quercetin is often modified into one of its derivatives or formulated with dietary fat to improve absorption. (16, 24, 33) 

The Top 5 Therapeutic Uses of Quercetin

Clinical studies have showcased the wide-ranging therapeutic benefits of quercetin. This article will discuss the benefits of using quercetin for treating inflammation, allergies, cardiovascular disease, infections, and oxidative stress.

1. Anti-Inflammatory Benefits of Quercetin

It is postulated that quercetin possesses the ability to inhibit the cyclooxygenase (COX) and lipoxygenase (LOX) enzymes, which limits the production and activity of inflammatory leukotrienes and prostaglandins, thereby exerting anti-inflammatory effects (16, 33). Animal studies also indicate that quercetin inhibits nuclear factor-kappa B, which controls gene expression of proinflammatory cytokines. 

Clinical studies show that quercetin helps to reduce inflammation. Subgroup analyses suggest that high-dose quercetin significantly reduces circulating C-reactive protein (an inflammatory marker) in patients diagnosed with inflammatory diseases, including rheumatoid arthritis, diabetes, and metabolic syndrome. 

An 8-week trial that included 50 women with rheumatoid arthritis observed that participants taking 500 mg of quercetin daily experienced significant reductions in morning joint stiffness, morning pain, and after-activity pain. Plasma hs-TNFα, an inflammatory marker, levels were also significantly reduced in the quercetin group compared to placebo.

The anti-inflammatory effects of quercetin are also likely responsible for the observed symptomatic improvement in men with chronic pelvic pain syndrome. 

2. Using Quercetin for Allergy Relief

Quercetin is used as the main ingredient of many potential anti-allergy drugs. Evidence suggests that quercetin works similarly to cromolyn, inhibiting the proliferation of mast cells and the mast cell-mediated release of histamine. Some studies show that quercetin can inhibit the proinflammatory cytokine IL-8 more effectively than cromolyn, in addition to suppressing the formation of IgE antibodies, which are immune proteins involved in allergic reactions. (19) 

The immune system operates through a delicate balance between two types of helper T cells, known as Th1 and Th2 cells. These cells play crucial roles in regulating the immune response. In allergic diseases, the immune system's balance between Th1 and Th2 cells is disrupted and shifts towards Th2 dominance. Quercetin may exert therapeutic benefit in treating allergies by stabilizing the Th1/Th2 balance. (13) 

Studies show that quercetin improves symptoms of allergic diseases, including allergic asthma, allergic rhinitis, and atopic dermatitis (13). For example, in a study of Japanese adults with allergic rhinitis, fatigue, sleep quality, sneezing, runny nose, and activity of daily living scores improved in patients taking 100 mg of quercetin twice daily for one month.

3. Using Quercetin for Cardiovascular Health

Research suggests quercetin may optimize heart health by mitigating endothelial dysfunction and reducing cardiovascular disease risk factors, including high blood pressure, high blood sugar, and atherosclerosis (29).  

Endothelial dysfunction describes a state of non-obstructive arterial disease that occurs when the cells that compose the inner lining of blood vessels don't function properly, leading to a narrowing of the vasculature. Endothelial dysfunction precedes the development of atherosclerosis and can lead to the loss of proper regulation of blood flow, blood pressure, and inflammation (11). Clinical trials studying the effects of quercetin on endothelial dysfunction have shown promising results. The positive benefits observed include enhanced endothelium-dependent vasodilation, reduced oxidative stress, and improved nitric oxide bioavailability (2, 7, 18). 

Quercetin supplementation also has modest blood pressure-lowering effects. A 2016 meta-analysis concluded that quercetin supplementation resulted in an average drop of 3/2 mmHg in blood pressure. The most significant blood pressure reductions were observed in patients taking at least 500 mg of quercetin daily. 

Studies have also demonstrated that quercetin effectively improves other parameters of cardiovascular health. Elevated levels of C-reactive protein (CRP) are associated with an increased risk of cardiovascular disease and adverse events; quercetin, especially when dosed above 500 mg daily, significantly reduces circulating CRP levels. Patients with polycystic ovary syndrome (PCOS) taking 1 gram of quercetin daily had significant reductions in fasting blood sugar, insulin, and HOMA-IR (a measure of insulin resistance) compared to those receiving placebo at the end of the 12-week trial. 

4. Using Quercetin for Immune Support

Studies have shown that quercetin has broad-spectrum antibacterial properties, expressing inhibitory effects on pathogenic bacteria, including Pseudomonas aeruginosa, Salmonella enteritidis, Staphylococcus aureus, Escherichia coli, Proteus, and Aspergillus flavus. Current research provides insights into quercetin's antibacterial abilities through mechanisms that involve destroying bacterial cell walls, inhibiting protein and DNA synthesis, and reducing enzymatic function. (33)

Additionally, studies have demonstrated its efficacy in treating viral infections by blocking viral replication. Quercetin has been shown to have antiviral activity against Herpes simplex virus (HSV), parainfluenza, respiratory syncytial viruses (RSV), and SARS-CoV2. (5) 

Studies have also shown quercetin to be a potent adjunctive therapy to the pharmacologic treatment of fungal infections (33).

5. Antioxidant Properties of Quercetin

Quercetin is popularly recognized as a potent antioxidant, crucial in combating oxidative stress. Oxidative stress occurs when there is an imbalance between free radicals, highly reactive molecules, and the body's ability to neutralize them. This imbalance can lead to cellular damage, inflammation, and chronic diseases. Quercetin acts as an antioxidant by scavenging these free radicals, preventing them from causing harm to cells and tissues. Quercetin contributes significantly to cellular health by mitigating oxidative stress, promoting overall well-being, and potentially reducing the risk of chronic diseases associated with oxidative damage. (29) 

Clinical trials have supported the therapeutic antioxidant benefits of quercetin. These studies have demonstrated its ability to reduce oxidative markers, enhance the activity of other antioxidants in the body, and protect cells from damage caused by free radicals. (8, 16, 21) 

Potential Side Effects of Quercetin and Considerations

While quercetin is considered safe when consumed in moderate amounts through dietary sources like fruits and vegetables, there are some precautions to consider when using it as a dietary supplement. Higher doses of quercetin supplements may cause headaches and upset stomach. Additionally, individuals with kidney disorders should exercise caution, as high doses have been reported to cause nephrotoxicity. There is limited data regarding the safety of quercetin administered in doses higher than 1,000 mg for longer than 12 weeks. (1, 23) 

Quercetin supplements may also interact with certain medications. Therefore, individuals should consult healthcare experts before starting quercetin supplementation, especially if they are taking any of the following medications (23) :

• Antibiotics
• Blood thinners
• Chemotherapy agents
• Corticosteroids
• Cyclosporine
• Digoxin

Healthcare professionals can provide personalized recommendations regarding the appropriate dosage and ensure the safe incorporation of quercetin supplements into one's routine. Adhering to recommended dosages and seeking professional guidance ensures the safe and effective utilization of quercetin for its potential health benefits.

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Summary

Embracing the health potentials of quercetin unveils many benefits for overall well-being. This natural flavonoid, abundant in fruits and vegetables, showcases its prowess as a powerful antioxidant, immune-booster, and anti-inflammatory agent. Its ability to combat oxidative stress, stabilize mast cells, and support endothelial function underscores its importance in maintaining cardiovascular health and managing allergies. Quercetin offers a natural approach to enhancing the immune system and promoting cellular health.