Search for fisetin and gout and you will find the claim that fisetin lowers uric acid by inhibiting xanthine oxidase, the enzyme allopurinol targets. That claim is mostly wrong, and tracing why is worth doing, because the real finding underneath it is more specific, more interesting, and points at a different safety consideration than the one people assume.
Key Takeaways
- In a screen of 23 dietary flavonoids against URAT1, the transporter that reabsorbs urate from urine back into the blood, fisetin was the most potent inhibitor tested, with an IC50 of 7.5 µM. Quercetin was second at 12.6 µM.[1]
- In hyperuricemic mice, oral fisetin at 50 and 100 mg/kg lowered blood uric acid and shifted the expression of the kidney urate transporters URAT1, OAT1, OAT3 and ABCG2, with allopurinol as the positive control.[2]
- That is a uricosuric mechanism, meaning more urate excreted, not a production-blocking one like allopurinol or febuxostat. The distinction matters for kidney stone risk.
- The widely repeated xanthine oxidase claim traces back to papers that tested pedalitin and quercetin, not fisetin.[3]
- The URAT1 result is a cell-based assay. 7.5 µM is well above the plasma concentration a fisetin capsule is thought to produce.
- No human study has measured what fisetin does to serum uric acid. None.
Where the xanthine oxidase claim comes from
There is a genuine and long-standing literature on flavonoids inhibiting xanthine oxidase. It is frequently cited in fisetin articles. Read the actual papers, though, and fisetin is usually not the compound being tested.
One commonly cited 2006 study isolated antioxidants from Rabdosia japonica and found pedalitin and quercetin were competitive xanthine oxidase inhibitors that reduced uric acid formation. Fisetin does not appear in that result.[3] A 2004 structure-activity study included fisetin, but its finding was that a synthetic flavonol, 3′,4′-dihydroxyflavonol, was significantly more potent than fisetin and the other natural compounds on the vascular and antioxidant endpoints measured.[4] Fisetin was the comparison that lost.
This is a routine failure mode in supplement writing: a class-level finding about flavonoids gets attributed to whichever flavonoid the page is selling. Restrict a PubMed search to fisetin in the title or abstract alongside xanthine oxidase and you get six results, most of which are reviews or studies where fisetin is one item in a list.
The finding that is actually about fisetin
Toyoda and colleagues took a different approach entirely. Rather than looking at urate production, they looked at urate reabsorption. Most of the urate filtered by your kidneys is pulled back into the blood by a transporter called URAT1, and blocking it increases net urate excretion. This is how uricosuric drugs such as probenecid and lesinurad work.
They screened 162 plant extracts and then tested 23 individual dietary compounds in a cell-based urate transport assay. Fisetin came out strongest, with an IC50 of 7.5 µM, ahead of quercetin at 12.6 µM.[1] For a compound not being studied for this purpose, topping a screen of the flavonoids people actually eat is a genuinely notable result.
An in vivo study supports the same direction. Ren and colleagues induced hyperuricemic nephropathy in mice with potassium oxonate and adenine, then gave oral fisetin at 50 or 100 mg/kg, either alongside the induction or after the condition was established. Both regimens lowered uricemia and protected kidney function and structure, and the authors attributed the urate lowering to changed expression of URAT1, OAT1, OAT3 and ABCG2.[2] Allopurinol at 10 mg/kg was the positive control, so the comparison is against a real drug rather than against nothing.
The same group later reported that fisetin protects fibrotic kidney tissue through a separate route, blocking ACSL4-mediated ferroptosis in kidney tubule cells.[5] That is a kidney-protection finding rather than a urate finding, but it is worth separating the two, because articles citing fisetin for gout often blur kidney protection into urate lowering as though they were the same result.
Why the mechanism distinction is not pedantry
Allopurinol and febuxostat reduce how much urate your body makes. Uricosurics increase how much you excrete. Both lower serum uric acid, and they are not interchangeable in terms of risk.
Pushing more urate through the kidneys raises urinary urate concentration, and urate stones form in urine, not in blood. This is why uricosuric drugs are generally avoided in people with a history of kidney stones or reduced kidney function, and why they come with advice about fluid intake. If fisetin’s urate effect in humans works the way the animal and cell data suggest, it belongs in the uricosuric category, and the relevant caution is a stone caution, not a liver caution.
To be clear about the strength of that inference: it is an inference. Nobody has measured serum or urinary uric acid in a human taking fisetin.
The concentration problem
An IC50 of 7.5 µM is the concentration needed in the assay dish to halve transporter activity. Whether an oral fisetin dose puts anything close to that concentration in front of your kidney transporters is a separate question, and fisetin’s poor oral bioavailability makes the answer likely to be no. Plasma levels after oral fisetin are low, and it is cleared quickly.
The in vivo mouse doses of 50 and 100 mg/kg are also high. Scaled by body surface area, they are far above a typical human fisetin dose, and they were given daily rather than in the pulsed schedule most senolytic protocols use.
What to take from this
If you have gout or hyperuricemia, fisetin is not a treatment and there is no human evidence that it lowers your uric acid. Urate-lowering therapy has established options with decades of outcome data behind them.
Two things are still worth knowing. First, if you take a uricosuric drug or have a history of urate stones, the mechanism here is at least theoretically additive, and it is reasonable to mention fisetin to your doctor. Second, if you read an article claiming fisetin inhibits xanthine oxidase, you now know to check which compound the cited study actually tested. It is a useful habit well beyond this one topic.
Frequently Asked Questions
Does fisetin lower uric acid?
No human study has measured this. In a cell-based assay, fisetin was the most potent inhibitor of the urate reabsorption transporter URAT1 among 23 dietary flavonoids tested, with an IC50 of 7.5 micromolar. In hyperuricemic mice, oral fisetin at 50 to 100 mg/kg lowered blood uric acid and altered kidney urate transporter expression.
Does fisetin inhibit xanthine oxidase like allopurinol?
This claim is common and largely unsupported for fisetin specifically. The frequently cited xanthine oxidase studies tested other flavonoids such as pedalitin and quercetin. Fisetin’s documented urate mechanism is inhibition of the URAT1 reabsorption transporter, which increases excretion rather than reducing production.
Is fisetin safe if I have gout or kidney stones?
Discuss it with your doctor. Fisetin’s urate mechanism in animal and cell studies resembles a uricosuric drug, which increases urate excretion through the kidneys. Uricosurics are generally used with caution in people with a history of urate stones or reduced kidney function. No human data on fisetin exists here.
References
- Identification of Inhibitory Activities of Dietary Flavonoids against URAT1, a Renal Urate Re-Absorber: In Vitro Screening and Fractional Approach Focused on Rooibos Leaves. Nutrients (2022). PMID 35276934
- Natural flavonol fisetin attenuated hyperuricemic nephropathy via inhibiting IL-6/JAK2/STAT3 and TGF-β/SMAD3 signaling. Phytomedicine (2021). PMID 33994251
- Antioxidants from Rabdosia japonica. Phytother Res (2006). PMID 16521110
- Vascular and anti-oxidant actions of flavonols and flavones. Clin Exp Pharmacol Physiol (2004). PMID 15566394
- Fisetin ameliorates fibrotic kidney disease in mice via inhibiting ACSL4-mediated tubular ferroptosis. Acta Pharmacol Sin (2024). PMID 37696989
These statements have not been evaluated by the Food and Drug Administration. This information is not intended to diagnose, treat, cure, or prevent any disease. Content is for informational purposes only and is not medical advice; consult a qualified healthcare provider before starting any supplement. As an Amazon Associate we earn from qualifying purchases.