What Is Fisetin? Structure, Sources, and How It Works

Fisetin is a plant flavonol best known as the yellow-orange pigment in strawberries. In supplement form, it's being studied for senolytic activity — selective removal of senescent cells in experimental systems. That activity is established in selected cell and animal models, not as systemic senescent-cell clearance in humans [1]. That's a genuine research direction with strong laboratory and mouse support, but the human evidence is still filling in. For the full evidence picture across benefits, dose, and safety, see our complete clinician's guide to fisetin.

For a benefit-by-benefit clinical evidence map, see our fisetin benefits review.

What is fisetin, chemically?

Fisetin (systematic name 3,3′,4′,7-tetrahydroxyflavone) belongs to the flavonol subclass of flavonoids — plant polyphenols built on a three-ring core, with flavonols distinguished by a hydroxyl group at a specific position on the middle ring [2].

Its closest chemical relative is quercetin, which carries one extra hydroxyl group. That single-atom difference changes how the two compounds bind proteins, their antioxidant strength, and — most relevant to the longevity story — their relative potency as senolytics in cell culture [1]. Fisetin was first isolated in the 19th century from the smoke tree, historically used as a natural yellow dye, which is also where its name comes from [2].

Fisetin: verified chemical identifiers

Compound class	Plant flavonol (a flavonoid polyphenol)
Chemical formula	C15H10O6
Molecular weight	286.24 g/mol
CAS Registry Number	528-48-3
PubChem identifier	CID 5281614

These identifiers describe the molecule, not proof of safety, regulatory approval or an anti-ageing treatment. A supplement's purity and formulation also matter.

Where does fisetin come from in food?

Strawberries, apples, persimmons, lotus root, onions, grapes and several other fruits and vegetables contain fisetin. A frequently reproduced 1998 Japanese food-composition study ranked strawberry samples highest (160 µg/g), followed by apples (26.9 µg/g) and persimmons (10.5 µg/g) [3]. However, it analyzed freeze-dried, acid-hydrolyzed material; later reviews describe the values on a dry-weight basis, not as verified milligrams in each gram of fresh retail fruit.

Food analyzed in the historical study	Reported analytical figure (µg/g)
Strawberry	160
Apple	26.9
Persimmon	10.5
Lotus root	5.8
Onion	4.78
Grape	3.93

A separate 2022 experiment reported 0.02 ± 0.01 µg/g dry weight in its strawberry sample under different analytical conditions; it did not detect fisetin in the apple samples it tested [7]. That discrepancy makes precise claims such as “250 g strawberries supplies 40 mg fisetin” unjustified without a validated fresh-weight food analysis. The 1998 study's estimated 0.4 mg/day intake applied to 50 Japanese women, not to every population. Our full fisetin food-source guide explains the ten historical values, methodological limits and practical food choices.

Where does supplement fisetin actually come from?

Commercial fisetin capsules are almost never made from strawberries or apples — extraction at that scale would be impractical and expensive. Two botanical sources dominate instead.

Smoke tree

Smoke tree leaves and heartwood contain fisetin at concentrations roughly 100 to 1,000 times higher than any dietary source, which is why most fisetin capsules on the market are extracted from it and standardised to 98–99% purity [2].

Fustic wood

Fustic wood, a related dye-yielding tree, is the other commercial source. The extraction and purification process is broadly similar, and there's no evidence the fisetin molecule behaves any differently depending on which of the two plants it came from — the molecule is the molecule.

A word on purity

Not every fisetin product on the market reliably delivers what its label claims. This is a known issue across the unregulated flavonoid-supplement category generally, not something specific to fisetin that we can quantify precisely here. The practical safeguard is a batch-specific, third-party Certificate of Analysis from an accredited laboratory — our best fisetin supplement article walks through what to look for.

How does fisetin work in the body?

Fisetin has three overlapping identities in the research literature, at three different levels of evidence.

Antioxidant flavonoid

The oldest, best-established identity. Fisetin scavenges reactive oxygen species directly through its hydroxyl groups, and indirectly by activating the Nrf2 pathway, the body's own antioxidant defence system [4]. This is standard flavonoid pharmacology, shared with a large class of dietary polyphenols.

Anti-inflammatory compound

Fisetin suppresses NF-κB signalling in cell and animal models, reducing production of inflammatory messengers including IL-6, TNF-alpha, and IL-1beta across several tissue types [5]. This is the identity behind claims about fisetin for joint discomfort and inflammatory conditions.

Senolytic

The identity that made fisetin famous. In a 2018 screen, researchers tested ten flavonoids against senescent cells and found fisetin the most potent at selectively killing them while sparing healthy, dividing cells [1]. In mice, intermittent oral dosing cleared senescent-cell markers across several tissues and extended median lifespan in that original study. This is the identity driving current commercial and research interest — our dedicated senolytic article covers the mechanism and the human trial pipeline in full.

Why isn't the dose on the label the dose in your blood?

Fisetin is poorly water-soluble and undergoes extensive metabolism. A human crossover study compared two oral products delivering different amounts of fisetin: approximately 982 mg in the unformulated arm versus 192 mg in a hybrid-hydrogel preparation [6]. The paper reports dose-adjusted Cmax values of 9.97 ng/mL for unformulated fisetin and 238.2 ng/mL for FF-20; 9.97 is not an unadjusted peak after the full unformulated dose. Parent-fisetin exposure was brief in the trial. Neither plasma value can be directly converted to a validated human senolytic threshold based on cell-culture experiments.

Low circulating parent-fisetin exposure makes it uncertain whether effects seen in cell and animal models translate to ordinary oral products. A hybrid-hydrogel formulation increased dose-adjusted exposure in a small human pharmacokinetic trial, but increased plasma exposure is not itself proof of clinical benefit. Other formulation approaches, including liposomes and nanoparticles, need product-specific human testing. Our bioavailability review explains these limits, and our liposomal fisetin review examines the different delivery strategies.

What we still don't know

Whether the plant matrix matters. Dietary fisetin from strawberries arrives alongside vitamin C, anthocyanins, ellagic acid, and other polyphenols. Whether isolated capsule fisetin behaves the same as fisetin-in-strawberries at an equivalent dose hasn't been tested.

The role of its main metabolite. Fisetin is rapidly converted to a metabolite called geraldol after absorption, and plasma geraldol levels exceed the parent compound's within the first hour [6]. Some of fisetin's biological activity may actually belong to geraldol rather than fisetin itself — this hasn't been fully worked out.

Long-term dietary intake and health outcomes. Epidemiological studies linking flavonol intake generally to mortality exist, but fisetin-specific dietary-intake-versus-outcome studies are limited.

Bottom line

Fisetin is a flavonol — a specific chemical class of plant compound — most abundant in strawberries, and under investigation as a senolytic. In this context, "senolytic" refers to selective removal of senescent cells in experimental systems; systemic clearance in humans has not been established. Dietary intake is small; supplement-dose absorption is limited by poor bioavailability. The chemistry is well characterised and the botanical origin is straightforward. The biological effects are real, but concentrated in cell culture and mouse studies so far. What fisetin actually does in a human adult taking a daily capsule is the question the ongoing clinical trials are designed to answer. For the complete clinical picture, see our full pillar guide.