Quercetin is a flavonoid found in capers, red onions, grapes, berries, citrus fruit, tomatoes, broccoli, kale, and many other plant foods. It is the most extensively studied flavonoid for its effects on allergy, inflammation, blood pressure, and endothelial function. The clinical evidence is robust

The Most Studied Flavonoid in the World

Quercetin is a flavonoid found in capers, red onions, grapes, berries, citrus fruit, tomatoes, broccoli, kale, and many other plant foods. It is the most extensively studied flavonoid for its effects on allergy, inflammation, blood pressure, and endothelial function. The clinical evidence is robust enough that quercetin is increasingly included in integrative cardiology and allergy protocols as a first-line botanical intervention alongside conventional treatments.

Blood Pressure Effects

Quercetin acts as a natural ACE inhibitor — it blocks the angiotensin-converting enzyme that raises blood pressure by promoting vasoconstriction and sodium retention. Human trials show that quercetin supplementation at 500-1000mg daily reduces both systolic and diastolic blood pressure by clinically meaningful amounts. A randomised controlled trial in hypertensive patients found that quercetin at 730mg daily for 28 days reduced systolic blood pressure by 7 points and diastolic by 3.5 points — effects comparable to starting a low-dose antihypertensive medication. The mechanism is not fully understood but appears to involve both ACE inhibition and improved endothelial nitric oxide production.

Allergy and Histamine Effects

Quercetin stabilises mast cells — the immune cells that release histamine and other inflammatory mediators during allergic reactions. This is the same mechanism as cromolyn sodium, the prescription mast cell stabiliser, but quercetin works through multiple pathways including inhibition of mast cell degranulation, reduction of IgE-mediated histamine release, and suppression of inflammatory cytokine production. For people with seasonal allergies, hay fever, or non-anaphylactic food allergies, quercetin at 500-1000mg daily is a evidence-based first-line intervention that works synergistically with antihistamines.

The NAD+ Connection

Quercetin is one of the most potent natural activators of the NAD+-boosting enzyme SIRT1 and AMP-activated protein kinase (AMPK). Through SIRT1 activation, quercetin increases cellular NAD+ levels and activates the mitochondrial biogenesis pathways that are central to metabolic health. Through AMPK activation, quercetin improves insulin sensitivity, promotes autophagy (cellular waste removal), and supports the metabolic flexibility that characterises metabolically healthy individuals. This places quercetin at the intersection of the flavonoid literature and the longevity literature, explaining its association with multiple health outcomes in population studies.

What You Can Do Today

Food-first quercetin is the best approach: capers are the richest dietary source (200mg per 100g), followed by red onions, apples with skin, berries, citrus fruit, and dark leafy greens. A diet rich in these foods provides the equivalent of several hundred milligrams of quercetin daily. Supplementation at 500-1000mg daily is appropriate for people with elevated blood pressure who want a non-pharmaceutical option, or for allergy sufferers who need additional support beyond antihistamines. Quercetin is fat-soluble — taking it with a meal containing fat improves absorption.

Quercetin and the Immune System

Beyond its effects on blood pressure and allergies, quercetin modulates immune function through effects on mast cells, basophils, and eosinophils — the effector cells of allergic and inflammatory responses. This broad immunomodulatory effect explains its traditional use for respiratory allergies, asthma, and inflammatory skin conditions. Studies in athletes show that quercetin reduces the incidence of upper respiratory tract infections following intensive exercise — a period when immune function is typically suppressed.

The antiviral properties of quercetin have gained attention following cell culture and animal studies showing activity against a broad range of RNA and DNA viruses. While human clinical data is limited, quercetin’s role as a zinc ionophore — facilitating zinc entry into cells — is relevant to its proposed antiviral mechanism, since intracellular zinc concentrations directly inhibit viral RNA polymerase activity.

Bioavailability and Absorption

Quercetin’s main limitation is its poor bioavailability — as a fat-soluble flavonoid, it requires dietary fat for optimal absorption and is subject to extensive first-pass metabolism in the liver. Quercetin glycosides from onions and apples are better absorbed than quercetin from most other food sources due to specific transport mechanisms in the gut. Supplements using quercetin dihydrate with bromelain (which improves absorption) or liposomal formulations address the bioavailability limitation for therapeutic applications.

What You Can Do Today

Food-first: one red onion and one apple daily provides 50-100mg of quercetin in highly bioavailable glycoside form. For therapeutic applications — hypertension, allergies, immune support — supplement at 500-1000mg daily with a fat-containing meal. Look for supplements that include bromelain or papain (proteolytic enzymes that improve quercetin absorption). People with allergies should begin supplementation 4-6 weeks before allergy season for maximum benefit.

Food vs Supplement: Getting Enough Quercetin

Dietary quercetin is found in a wide variety of plant foods, but certain foods are exceptionally rich sources. Capers contain approximately 180mg of quercetin per 100g — by far the richest food source known. Red onions contain approximately 40mg per 100g, with the highest concentration in the outer layers and the spring onion greens. Apples (particularly Red Delicious and Pink Lady varieties) provide 20-40mg per medium apple, with the quercetin concentrated in the skin. Blueberries, grapes, and citrus fruits provide 10-30mg per serving.

The absorption of quercetin from food is enhanced by the presence of fat and by the sugar:quercetin ratio in the food matrix. The quercetin glycosides in onions (quercetin-4′-O-glucoside) have higher bioavailability than the aglycone form found in supplements. This means that an onion sandwich with butter provides more usable quercetin than an equivalent dose of quercetin powder — the food matrix matters for absorption as well as for the other compounds that work synergistically with quercetin.

Quercetin and Athletic Performance

Quercetin’s effects on exercise performance have been studied with mixed results. Short-term studies (7-14 days of quercetin supplementation) show modest improvements in VO2 max and time-trial performance in non-athlete populations. The proposed mechanism involves improved mitochondrial biogenesis through activation of the PGC-1alpha pathway and improved calcium handling in skeletal muscle. However, these effects are relatively modest compared to the performance benefits of endurance training itself.