Isoquercitrin

Isoquercitrin is a quercetin-3-glucoside flavonol glycoside that demonstrates antiviral and vascular protective effects through inhibition of viral replication and stabilization of endothelial barrier function. Clinical trials show efficacy in reducing COVID-19 symptom duration and preventing dengue-related vascular leakage.

Category: Compound Evidence: 4/10 Tier: Moderate (some RCTs)

Origin & History

Isoquercitrin is a naturally occurring flavonoid glycoside consisting of quercetin linked to a glucose molecule, found in cotton, onions, apples, and mangosteen. It appears as a white to dark yellow powder with limited water solubility and can be extracted from natural sources or produced enzymatically from quercetin.

Historical & Cultural Context

While isoquercitrin itself lacks specific historical monographs, quercetin glycosides from plants like Sophora japonica have been documented in Traditional Chinese Medicine since ~200 AD for bleeding and inflammation. The compound occurs naturally in traditional remedies using onions for respiratory issues in Ayurveda, though modern interest stems from flavonoid bioactivity rather than traditional specific use.

Health Benefits

• Reduced COVID-19 symptom duration - Phase 2 RCT (n=140) showed faster symptom resolution (HR 1.36, p=0.018) with improved oxygenation
• Vascular protection in dengue fever - RCT (n=79) demonstrated reduced vascular leakage via plasma extravasation scores (p<0.05)
• Antioxidant activity - Potent radical scavenging with H-donor count 8, inhibiting nitric oxide synthase (IC₅₀ ~10 μM)
• Anti-inflammatory effects - Inhibits xanthine oxidase and modulates Nrf2 pathway for cytoprotection
• Endothelial stabilization - Reduces vascular permeability by stabilizing endothelial junctions including VE-cadherin

How It Works

Isoquercitrin inhibits viral replication by blocking 3CLpro and PLpro proteases essential for SARS-CoV-2 lifecycle. It stabilizes endothelial tight junctions through upregulation of VE-cadherin and claudin-5 proteins while reducing inflammatory cytokines IL-6 and TNF-α. The compound also scavenges reactive oxygen species via donation of electrons from hydroxyl groups on its flavonol backbone.

Scientific Research

Clinical evidence for isoquercitrin is limited but promising, with a phase 2 RCT (PMID: 34469660) showing reduced COVID-19 symptoms at 500 mg/day, and another RCT (PMID: 35622900) demonstrating vascular protection in dengue patients at 800 mg/day. A pharmacokinetic study (PMID: 32980913) confirmed safety at 50-200 mg single doses, though no large meta-analyses exist specifically for isoquercitrin.

Clinical Summary

A phase 2 randomized controlled trial (n=140) demonstrated that isoquercitrin reduced COVID-19 symptom duration with a hazard ratio of 1.36 (p=0.018) and improved oxygen saturation levels. In dengue fever patients, an RCT (n=79) showed significant reduction in vascular leakage measured by plasma extravasation scores (p<0.05). However, evidence remains limited to these two viral infection studies. Additional research is needed to establish broader therapeutic applications and optimal dosing protocols.

Nutritional Profile

Isoquercitrin (quercetin-3-O-β-D-glucopyranoside, C₂₁H₂₀O₁₂, MW 464.38) is a flavonol glycoside, not a macronutrient source. Key bioactive properties: 8 hydrogen-bond donors enabling potent radical scavenging (ORAC value ~5× higher than quercetin aglycone in aqueous systems). Water solubility is significantly improved over quercetin (~170× more soluble) due to the glucose moiety, yielding substantially higher oral bioavailability (Cmax approximately 3-5× that of quercetin in pharmacokinetic studies). Enzymatically modified isoquercitrin (EMIQ), produced via α-glucosylation, further enhances solubility and absorption (~17× greater bioavailability than quercetin). Typical supplemental doses range from 50-500 mg/day. No significant vitamin or mineral content; value is entirely as a polyphenolic bioactive. Undergoes deglycosylation by intestinal β-glucosidases (lactase-phlorizin hydrolase) in the small intestine, releasing quercetin aglycone for absorption, with additional colonic microbial metabolism producing ring-fission products (3,4-dihydroxyphenylacetic acid, etc.).

Preparation & Dosage

Clinically studied doses range from 50-800 mg/day orally, with COVID-19 trials using 500 mg/day for 14 days and dengue trials using 800 mg/day (divided doses) for 4 days. Pure powder or capsule forms are typically used, with enzymatically modified versions showing 5-17x better bioavailability than standard quercetin. Consult a healthcare provider before starting any new supplement.

Synergy & Pairings

Isoquercitrin pairs synergistically with Vitamin C (500-1000 mg), which regenerates oxidized quercetin back to its active antioxidant form while quercetin reciprocally spares vitamin C, amplifying total antioxidant capacity. Bromelain (500 GDU/150 mg) enhances intestinal absorption of flavonol glycosides and provides complementary anti-inflammatory activity via NF-κB modulation. Zinc (15-30 mg as zinc picolinate) works cooperatively, as isoquercitrin acts as a zinc ionophore facilitating intracellular zinc transport—a mechanism particularly relevant to its antiviral effects observed in the COVID-19 RCT. Rutin (500 mg) and hesperidin (250 mg) provide complementary vascular-protective synergy, collectively stabilizing endothelial tight junctions and reducing capillary permeability through overlapping but distinct mechanisms on VEGF signaling and glycocalyx integrity, which aligns with the dengue vascular leakage data.

Safety & Interactions

Isoquercitrin appears well-tolerated in clinical trials with no serious adverse events reported at doses up to 1000mg daily. Theoretical interactions may occur with anticoagulant medications due to potential effects on platelet aggregation. Safety during pregnancy and lactation has not been established in human studies. Individuals with known allergies to quercetin or citrus fruits should exercise caution as isoquercitrin is structurally related to quercetin.