Quercetin

Table of Contents

What is Quercetin?

Quercetin is a flavonoid, a type of plant pigment found in many fruits, vegetables, and grains. It is known for its antioxidant properties and potential health benefits. Quercetin is commonly found in foods such as apples, onions, berries, citrus fruits, red wine, and tea.

Quercetin is available in several forms, including:

Quercetin supplements: These are available in pill or capsule form and are often derived from sources like onions, apples, or citrus fruits. They are commonly taken for their antioxidant and anti-inflammatory properties.
Quercetin dihydrate: This form of quercetin is a crystalline powder that contains two molecules of water for every molecule of quercetin. It’s often used in research studies and as an ingredient in dietary supplements.
Quercetin aglycone: This is the pure form of quercetin, without any attached sugars. It is often used in research and is believed to have greater bioavailability compared to other forms of quercetin.
Quercetin glycosides: These are quercetin molecules that are attached to sugar molecules. They are naturally occurring in many fruits and vegetables and are a common form of quercetin found in foods.
Quercetin complex: Some supplements contain a combination of quercetin with other flavonoids or antioxidants for enhanced benefits.

What are the Benefits of Quercetin?

Quercetin offers several potential health benefits, although more research is needed to fully understand its effects. Some of the reported benefits of quercetin include:

Antioxidant properties: Quercetin is a potent antioxidant, meaning it helps neutralize harmful free radicals in the body. Free radicals can cause oxidative damage to cells, leading to inflammation, aging, and various diseases. By scavenging free radicals, quercetin may help protect against oxidative stress and reduce the risk of chronic diseases such as heart disease, cancer, and neurodegenerative disorders.
Anti-inflammatory effects: Quercetin has been shown to possess anti-inflammatory properties, which can help reduce inflammation in the body. Chronic inflammation is linked to the development of many diseases, including heart disease, diabetes, and arthritis. By inhibiting inflammatory pathways, quercetin may help alleviate symptoms of inflammation and potentially lower the risk of related diseases.
Immune system support: Some research suggests that quercetin may help support the immune system by modulating immune cell activity and enhancing the body’s response to infections. It may also help reduce the severity and duration of colds and respiratory infections by exerting antiviral effects.
Heart health: Quercetin may have beneficial effects on heart health by improving cardiovascular function and reducing the risk of heart disease. It has been shown to lower blood pressure, improve blood vessel function, reduce inflammation in the arteries, and inhibit the oxidation of LDL cholesterol—all of which contribute to a healthier heart.
Allergy relief: Quercetin is believed to have antihistamine properties, which may help alleviate symptoms of allergies such as hay fever, asthma, and allergic rhinitis. It may help stabilize mast cells and inhibit the release of histamine, a compound involved in allergic reactions.
Brain health: Some studies suggest that quercetin may have neuroprotective effects and could potentially help prevent or slow the progression of neurodegenerative diseases like Alzheimer’s and Parkinson’s disease. Its antioxidant and anti-inflammatory properties may help protect brain cells from damage and reduce the risk of cognitive decline.
Exercise performance: Quercetin supplementation has been investigated for its potential to enhance exercise performance and recovery. Some studies suggest that it may improve endurance, reduce fatigue, and enhance mitochondrial function, although results have been mixed.

What happens if I don’t get enough Quercetin (Deficiency)

Quercetin is not considered an essential nutrient, meaning the body does not require it for basic functioning, and there is no established dietary reference intake (DRI) for quercetin. However, quercetin-rich foods are often part of a healthy diet, and obtaining sufficient amounts of quercetin through dietary sources or supplementation may offer various health benefits, as discussed earlier.

While there isn’t a specific set of diseases directly caused by a deficiency of quercetin, inadequate intake of quercetin-rich foods may contribute to an increased risk of certain health problems associated with oxidative stress, inflammation, and immune dysfunction. These conditions may include:

Chronic diseases: Quercetin’s antioxidant and anti-inflammatory properties help protect against oxidative damage and inflammation, which are underlying factors in many chronic diseases such as heart disease, cancer, diabetes, and neurodegenerative disorders. Inadequate intake of quercetin-rich foods may potentially increase the risk of developing these conditions.
Allergies and respiratory issues: Quercetin has been studied for its potential to alleviate allergy symptoms and reduce inflammation in the respiratory tract. A deficiency in quercetin may contribute to increased susceptibility to allergic reactions, asthma, and other respiratory issues.
Immune system dysfunction: Quercetin has been shown to modulate immune cell activity and enhance the body’s response to infections. Inadequate intake of quercetin-rich foods may weaken the immune system, potentially increasing the risk of infections and impairing immune function.
Cognitive decline: Some research suggests that quercetin may have neuroprotective effects and could help prevent or slow the progression of neurodegenerative diseases like Alzheimer’s and Parkinson’s disease. A deficiency in quercetin may contribute to increased oxidative stress and inflammation in the brain, potentially worsening cognitive decline over time.

Recommended Dietary Allowances (RDAs) for Quercetin

There is no established recommended daily intake (RDI) or dietary reference intake (DRI) for quercetin by health authorities such as the Institute of Medicine (IOM) or the National Institutes of Health (NIH). Quercetin is considered a dietary flavonoid rather than an essential nutrient, so there are no official guidelines for its daily intake.

Best Food Sources for Quercetin

Quercetin is found in a variety of plant-based foods. Some of the best food sources of quercetin include:

Onions: Red onions tend to have higher levels of quercetin compared to other onion varieties. Raw onions contain more quercetin than cooked onions.
Apples: Quercetin is found primarily in the skin of apples, so consuming apples with the skin on provides the most quercetin.
Berries: Many berries, including cranberries, blueberries, strawberries, and raspberries, are good sources of quercetin.
Citrus fruits: Citrus fruits like oranges, grapefruits, lemons, and limes contain quercetin, particularly in the peel and white pith.
Leafy greens: Dark, leafy greens such as kale, spinach, and Swiss chard contain quercetin, although in lower amounts compared to some other foods.
Tomatoes: Quercetin is present in tomatoes, especially in the skin and seeds.
Red grapes: Red grapes, as well as red wine made from them, contain quercetin, primarily in the skin and seeds.
Broccoli: Broccoli and other cruciferous vegetables like kale and Brussels sprouts contain quercetin, along with other beneficial compounds.
Cherries: Cherries, particularly tart cherries, are a good source of quercetin.
Tea: Both green tea and black tea contain quercetin, with higher levels found in unfermented green tea.

Quercetin Interaction with Medication/Overconsumption

While quercetin is generally considered safe when consumed in typical dietary amounts found in foods, taking excessive amounts of quercetin supplements may lead to potential adverse effects. Some possible effects of excessive quercetin intake include:

Digestive issues: High doses of quercetin supplements may cause gastrointestinal discomfort, including stomach pain, diarrhea, or nausea, in some individuals.
Kidney damage: There is some evidence to suggest that extremely high doses of quercetin may have nephrotoxic effects and could potentially cause kidney damage.
Interactions with medications: Quercetin may interact with certain medications, particularly drugs that are metabolized by the liver or that affect blood clotting. Some potential interactions include:
- Blood-thinning medications: Quercetin may enhance the effects of anticoagulant or antiplatelet medications, such as warfarin (Coumadin), aspirin, or clopidogrel (Plavix), increasing the risk of bleeding.
- Cytochrome P450 substrates: Quercetin may affect the activity of cytochrome P450 enzymes in the liver, which are involved in the metabolism of many drugs. It may interact with medications metabolized by these enzymes, such as certain statins, antiretrovirals, and antidepressants, potentially altering their efficacy or side effects.
- Chemotherapy drugs: Quercetin may interfere with the efficacy of certain chemotherapy drugs, so individuals undergoing cancer treatment should consult with their healthcare provider before taking quercetin supplements.
- Cyclosporine: Quercetin may reduce the absorption of cyclosporine, an immunosuppressive medication used to prevent organ rejection in transplant recipients.
Allergic reactions: In rare cases, quercetin supplements may cause allergic reactions, particularly in individuals who are sensitive to flavonoids or have pollen allergies.