Flavonols

PhytochemicalFlavonoidBest with a meal

What is it

Flavonols are a subclass of flavonoids with a 3-hydroxyflavone backbone. Major members include quercetin, kaempferol, myricetin, and isorhamnetin. They are abundant in onions, kale, broccoli, berries, and tea.

Evidence for 2 uses

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Cardiovascular markers

Good Evidence

Cohort studies and small RCTs show flavonol-rich diets reduce blood pressure and cardiovascular risk.

Seasonal allergies (quercetin)

Limited Evidence

Small RCTs suggest modest histamine-stabilizing effects; evidence not robust.

How it works

Flavonols are absorbed mainly as their aglycones after gut microbial deglycosylation. They are potent antioxidants in vitro, modulate cell signaling (NF-kB, AMPK), inhibit mast-cell histamine release (quercetin), and support endothelial function. Habitual dietary flavonol intake correlates with lower cardiovascular mortality in cohort studies. Quercetin is the most-studied isolated flavonol supplement.

Dosage

Quercetin: 500-1000 mg/day. Mixed flavonol supplements: 200-500 mg/day. Whole-food intake estimated at 20-50 mg/day in typical Western diets.

When and how to take it

Take with meals; some evidence for enhanced absorption with bromelain or fat.

2 commercial forms

Compare the main delivery options and what they’re best suited for.

Quercetin dihydrate

Standard supplement form.

Low absorption (<5%) without enhancement.

Quercetin phytosome (Quercefit)

Preferred for clinical doses.

10-20x higher absorption than standard.

Safety

Whole-food flavonols are very safe. High-dose quercetin supplements (>1 g) may cause headache, GI upset, and at very high doses transient kidney effects.

Who should be cautious

Patients on CYP3A4 substrates or P-gp substrates should consult a pharmacist before high-dose flavonol supplements.

Interactions

Quercetin can inhibit CYP3A4 and P-glycoprotein at high doses, affecting medications like cyclosporine, digoxin, and certain statins.

Food sources

Red onion (1 medium)

Amount
~50-100 mg quercetin
%DV
—

Kale (1 cup)

Amount
~20-40 mg flavonols
%DV
—

Berries (1 cup)

Amount
~10-30 mg flavonols
%DV
—

Frequently asked questions

Is quercetin worth taking?⌄

Whole-food sources have stronger epidemiology. High-dose supplements have specific niche uses (allergies, exercise inflammation).

Do flavonol supplements interact with drugs?⌄

At high doses, yes, especially with CYP3A4 substrates. Consult a pharmacist.

References

Flavonols on Wikidata — Wikidata link

Flavonols (ChEBI:28802) — ChEBI link

Flavonols on NIH DSLD (US supplement label database) — NIH Dietary Supplement Label Database link

Research on Flavonols (PubMed search) — PubMed link

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Evidence-based·How we grade evidence

Disclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you’re pregnant, breastfeeding, on medications, or managing a chronic condition.