Quercetin Supplement – Effects, Dosage and Benefits

What Is a Quercetin Supplement?

Quercetin is a naturally occurring flavonoid – a class of plant-derived polyphenols – found in a wide variety of fruits, vegetables, and herbs. As a dietary supplement, it is available in concentrated form as capsules, tablets, or powder, and has attracted significant scientific attention due to its broad range of biological activities.

Dietary Sources of Quercetin

Quercetin is found in high concentrations in the following foods:

• Onions (especially red onions)
• Apples (particularly in the skin)
• Capers
• Broccoli and other cruciferous vegetables
• Berries (e.g., blueberries, cranberries)
• Green and black tea
• Red wine

Through a normal diet, adults typically consume around 10–100 mg of quercetin per day. Dietary supplements often provide significantly higher amounts, typically between 250 mg and 1,000 mg per daily dose.

Mechanism of Action

Quercetin exerts its biological effects through several mechanisms:

• Antioxidant activity: Quercetin neutralises free radicals and reduces oxidative stress in cells, helping to protect against cellular damage.
• Anti-inflammatory activity: It inhibits various inflammatory mediators, including cytokines such as TNF-alpha and interleukin-6, as well as the enzymes COX-1 and COX-2.
• Immunomodulatory activity: Quercetin influences the activity of immune cells, particularly mast cells, potentially reducing allergic responses.
• Antiviral properties: Laboratory studies suggest that quercetin may inhibit the entry of certain viruses into cells.
• Cardiovascular-protective effects: Quercetin may reduce arterial stiffness and positively influence blood pressure.

Applications and Health Benefits

Immune Support and Infection Prevention

Quercetin supplements are commonly taken to support immune function. Some clinical studies suggest they may reduce the frequency and duration of respiratory tract infections, particularly in physically active individuals.

Allergies and Histamine Sensitivity

By inhibiting mast cell degranulation, quercetin can reduce the release of histamine, thereby alleviating allergy symptoms such as runny nose, itching, and skin redness. It is therefore often discussed as a natural alternative to antihistamines.

Cardiovascular Health

Studies indicate that quercetin may modestly lower both systolic and diastolic blood pressure. It also has antithrombotic effects and may improve vascular function.

Athletic Performance and Recovery

Some research suggests that quercetin may enhance aerobic endurance performance and support recovery after intense exercise by reducing exercise-induced muscle inflammation.

Metabolic Syndrome and Blood Sugar Regulation

Quercetin may improve insulin sensitivity and help stabilise blood glucose levels, making it potentially beneficial for individuals with metabolic syndrome or type 2 diabetes.

Bioavailability and Combinations

A well-known challenge with quercetin is its relatively low bioavailability – the proportion that is actually absorbed and utilised by the body. To improve absorption, several strategies are commonly used:

• Quercetin phytosomes: Quercetin bound to phospholipids for enhanced absorption.
• Combination with bromelain: The enzyme from pineapple may improve quercetin absorption.
• Combination with vitamin C: Supports antioxidant activity and may enhance uptake.
• Combination with zinc: Quercetin acts as a so-called zinc ionophore, potentially transporting zinc into cells and amplifying the antiviral effects of both substances.

Recommended Dosage

There is no officially established recommended daily dose for quercetin supplements. Clinical studies have commonly used doses of 500 mg to 1,000 mg per day, divided into two servings. The optimal individual dose may vary depending on health status and intended use. Consulting a doctor or pharmacist is advisable.

Side Effects and Safety

Quercetin is generally considered well tolerated and safe at moderate doses. At very high doses (above 1,000 mg daily), the following side effects may occur:

• Headaches
• Tingling in the extremities (paraesthesia)
• Gastrointestinal issues such as nausea or diarrhoea

At very high intravenous doses – which are not reached through oral supplementation – nephrotoxic effects have been observed in animal studies. At typical oral supplement doses, quercetin is considered safe.

Interactions

Quercetin may interact with various medications:

• Antibiotics (fluoroquinolones): Quercetin may reduce their effectiveness.
• Anticoagulants (e.g., warfarin): Possible potentiation of blood-thinning effects.
• Chemotherapy agents: Possible interactions when taken concurrently.
• Immunosuppressants: Quercetin may alter their effects.

Individuals taking prescription medications should seek medical advice before starting a quercetin supplement.

Contraindications

Quercetin supplements should be avoided or only taken under medical supervision in the following situations:

• Pregnancy and breastfeeding (insufficient safety data)
• Kidney disease
• Use of immunosuppressants or anticoagulants

References

1. Formica JV, Regelson W. Review of the biology of Quercetin and related bioflavonoids. Food and Chemical Toxicology, 1995; 33(12):1061–1080.
2. Boots AW, Haenen GR, Bast A. Health effects of quercetin: from antioxidant to nutraceutical. European Journal of Pharmacology, 2008; 585(2–3):325–337.
3. Mlcek J, Jurikova T, Skrovankova S, Sochor J. Quercetin and Its Anti-Allergic Immune Response. Molecules, 2016; 21(5):623. DOI: 10.3390/molecules21050623.