Quercetin

Anti-Inflammatory Mechanisms

Quercetin is a potent inhibitor of multiple inflammatory signaling pathways:

• NF-κB inhibition: Quercetin inhibits IKK (IκB kinase), blocking NF-κB translocation to the nucleus and suppressing transcription of TNF-α, IL-1β, IL-6, COX-2, and iNOS
• MAPK pathway inhibition: Suppresses JNK and p38 MAPK signaling, reducing inflammatory gene expression
• Inflammasome inhibition: Inhibits NLRP3 inflammasome assembly and activation, reducing IL-1β and IL-18 maturation
• Mast cell stabilization: Inhibits IgE-mediated mast cell degranulation, reducing histamine, prostaglandin D2, and leukotriene release — the mechanism for quercetin's antiallergic effects
• COX and LOX inhibition: Direct inhibition of cyclooxygenase and lipoxygenase enzymes, reducing prostaglandin and leukotriene synthesis

Senolytic Mechanism

Senescent cells resist apoptosis through upregulation of multiple pro-survival ("senescent cell anti-apoptotic pathways," SCAPs) including Bcl-2 family proteins, PI3K/AKT, and other pathways. Quercetin inhibits several of these survival pathways:

• Bcl-2 and Bcl-xL inhibition: Quercetin is a BH3 mimetic, binding to and inhibiting the anti-apoptotic Bcl-2 family proteins that senescent cells depend on for survival
• PI3K inhibition: Quercetin inhibits PI3K pathway signaling, reducing AKT-mediated pro-survival signaling
• Ephrin pathway inhibition: Blocks survival signals via ephrin receptors

In combination with dasatinib (which inhibits additional survival tyrosine kinases), quercetin produces synergistic senolytic activity that efficiently eliminates senescent cells while sparing healthy proliferating and quiescent cells. Importantly, senolytic dosing is typically intermittent (e.g., two consecutive days per month rather than daily), as a "hit-and-run" strategy — clearing accumulated senescent cells and then pausing to allow recovery.

CD38 Inhibition

Like apigenin, quercetin inhibits CD38 — the NAD+-consuming enzyme — and has been shown to increase intracellular NAD+ levels in preclinical models. This positions quercetin as a complementary compound to NAD+ precursor supplementation.

Antioxidant Mechanisms

• Direct free radical scavenging: Multiple phenolic OH groups donate hydrogen to neutralize ROS
• Metal chelation: Quercetin chelates iron and copper, reducing Fenton reaction-mediated hydroxyl radical generation
• Nrf2 activation: Indirect antioxidant effects via ARE gene induction (modest compared to sulforaphane)

Antiviral Properties

Quercetin inhibits viral entry and replication through multiple mechanisms, including inhibition of viral proteases, viral polymerases, and interference with viral receptor binding. It has demonstrated activity against influenza, SARS-CoV-2, Zika, and other viruses in cell culture and animal models, generating substantial interest during the COVID-19 pandemic.

Pharmacokinetics

Oral bioavailability of quercetin aglycone is approximately 1–3%. Quercetin glucosides (quercetin-3-glucoside from onions, rutin/quercetin-3-glucoside from buckwheat) are better absorbed via active sugar transporter mechanisms. Following absorption, quercetin is rapidly metabolized to phenolic acid metabolites and sulfate/glucuronide conjugates. The bioactive entity in plasma is primarily conjugated metabolites, not parent quercetin. Quercetin phytosome formulations achieve 20-fold improved bioavailability.

Early Flavonoid Research

Quercetin's scientific history begins with the broader story of flavonoid chemistry. In the 1930s, Hungarian-American biochemist Albert Szent-Györgyi (who had recently won the Nobel Prize for the discovery of vitamin C) proposed that a class of plant compounds he called "vitamin P" — which included flavonoids now identified as quercetin, rutin, and hesperidin — had vitamin-like activity in preventing capillary fragility. The "vitamin P" hypothesis was eventually discarded as flavonoids were not found to be essential nutrients in the classical sense, but the research generated early characterization of quercetin's chemical properties and biological effects.

Quercetin was first isolated in the mid-19th century from oak bark (Quercus species — the genus name is the etymological root of "quercetin"). The structure was characterized in the early 20th century, and the compound was recognized as one of the most widely distributed flavonoids in the plant kingdom.

Rise as a Research Subject

Quercetin attracted sustained research attention through the latter 20th century as the epidemiological association between dietary flavonoid intake and reduced cardiovascular disease and cancer risk drove interest in identifying the responsible phytochemicals. Quercetin, as the most abundant dietary flavonoid, became a primary research target.

Mechanistic research through the 1990s and 2000s characterized its NF-κB inhibitory, antioxidant, antihistaminic, and antiproliferative properties, generating thousands of published studies — though many relied on cell culture concentrations not achievable with oral supplementation.

Senolytic Discovery

The most significant recent development in quercetin research is its identification as a senolytic compound. This emerged from a systematic screen conducted at the Mayo Clinic by Kirkland, Tchkonia, and colleagues, published in Aging Cell in 2015, identifying quercetin (in combination with dasatinib) as one of the first compounds capable of selectively eliminating senescent cells in living organisms. The subsequent demonstration of improved physical function, reduced mortality, and extended healthspan in mice treated with senolytic quercetin + dasatinib, and the publication of the first human senolytic trial in 2019, have placed quercetin at the center of one of the most exciting areas of aging biology research.