Quercetin: A Reference Guide for Supplement Formulators

TL;DR

Quercetin is the most widely studied dietary flavonoid. It works on antioxidant, anti-inflammatory, antihistamine, and cardiovascular pathways. The single biggest formulation challenge is bioavailability — raw quercetin aglycone is poorly absorbed, so most modern supplements use phytosomal or EMIQ forms. Bulk material is extracted commercially from Sophora japonica flower buds (10–25% native content) or recovered as a by-product of onion processing.

See our Quercetin product page for COA, grades, and bulk pricing.

Chemistry

• Systematic name: 3,3',4',5,7-pentahydroxyflavone
• CAS: 117-39-5
• Molecular formula: C₁₅H₁₀O₇
• Molecular weight: 302.24 g/mol
• Appearance: Yellow crystalline powder

Quercetin is a flavonol (3-hydroxy-flavone backbone). The position-3 hydroxyl differentiates it from luteolin and gives it the characteristic flavonol antioxidant profile.

Natural sources

Commercial bulk quercetin almost always traces back to Sophora japonica flower bud, which is extracted as rutin and then hydrolyzed.

Mechanism summary

• Inhibits NF-κB and lipoxygenase pathways → anti-inflammatory
• Stabilizes mast cells → reduces histamine release (antihistamine effect)
• Inhibits xanthine oxidase → reduces uric acid synthesis (modest)
• Scavenges ROS via direct radical interaction and metal chelation

Dosage

Take with food to improve absorption. Combining with vitamin C or bromelain is common in commercial formulations.

Bioavailability strategies

Raw aglycone quercetin has bioavailability around 1%. Formulation options to improve this:

1. Phytosome (lecithin–quercetin complex) — 5–20× higher Cmax than aglycone
2. EMIQ (enzymatically modified isoquercitrin) — water-soluble glycoside, much higher absorption
3. Cyclodextrin inclusion — improves solubility
4. Co-administration with vitamin C — regenerates oxidized quercetin

Regulatory & safety

• US FDA: GRAS as a flavoring agent at low levels; for supplement use, falls under DSHEA. No drug claims allowed
• EFSA: not a Novel Food; ingredient considered to have a history of safe use
• LD₅₀ in mice >2 g/kg (oral); long-term human dosing at 1000 mg/day twice daily has been generally well tolerated for up to 12 weeks ¹

Bulk-buying qualification

1. Purity by HPLC — request COA with HPLC trace showing 95% or 98% quercetin (not just total flavonoid)
2. Source documentation — Sophora japonica vs onion-derived: affects allergen profile and labeling
3. Anhydrous vs dihydrate — anhydrous purity is calculated differently; clarify on COA
4. Heavy metals, microbial, solvent residues — same diligence as any flavonoid
5. For bioavailability-enhanced forms — separate qualification (phytosome ratio, EMIQ glycoside profile)

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References

These statements have not been evaluated by the FDA. This product is not intended to diagnose, treat, cure, or prevent any disease.