One hundred grams of raw capers contain around 234mg of quercetin, compared with 4mg in the same amount of apple with its skin. Quercetin is the most abundant flavonol in the diet; supplements provide much more. Sources, absorption, trials, senescent cells and safety: here is what studies say.
Key takeaways
- Quercetin is a flavonol: 234mg per 100 g of raw capers, 39mg in red onion and 4mg in unpeeled apple.
- Its pure form is poorly absorbed; quercetin from onion is around three times more bioavailable than that from apple.
- In meta-analyses, systolic pressure decreases by an average of 2 to 4 mmHg, with no clear effect on cholesterol, glucose or weight.
- “Senolytic” trials mainly combine quercetin with dasatinib, a prescription cancer medicine; alone, it has not demonstrated this effect in humans.
What is quercetin?
Quercetin is a plant compound of the flavonoid family and the leading flavonol. In plants, it is mainly attached to sugars as glycosides: isoquercitrin carries glucose, rutin carries rutinose. Supplements often offer the free, sugar-free form, called the aglycone.
In the laboratory, it is a powerful antioxidant. But blood contains almost only its derivatives. Its antioxidant role in the body is therefore difficult to establish (see our articles on oxidative stress and antioxidants and glutathione, an antioxidant made by the body).
Which foods contain the most quercetin?
Capers lead the way, far ahead of shallots and red onion. Average contents come from the American USDA database, version 3.3 from 2018, except for shallots, which are absent from it and use Phenol-Explorer as the source.
| Food | Quercetin (mg/100 g) | Note |
|---|---|---|
| Raw capers | 234 | 173 in canned capers |
| Raw shallot | 61 | Only one sample analysed |
| Raw red onion | 39 | 6 in white onion |
| Raw kale | 23 | 0 to 56 depending on samples |
| Buckwheat grain | 15 | 3.5 in wholemeal flour |
| Raw apple with skin | 4 | 1 without skin |
| Brewed black tea | 2.2 | Mainly as rutin |
These contents vary substantially: 6 to 192mg/100 g depending on red onion samples. Apple skin is much richer than the flesh, with 19mg/100 g versus 1.
According to 1997 estimates cited again in 2024 by the Norwegian Scientific Committee for Food and Environment (VKM), food supplies an average of 2 to 53mg of quercetin per day.
Why is quercetin poorly absorbed?
Because its pure form, the aglycone, dissolves poorly in water, limiting absorption. Once absorbed, quercetin is modified by the intestine and liver through glucuronidation, sulfation and methylation. The unabsorbed portion is broken down into phenolic acids by the colonic microbiota.
The attached sugar also matters. In 1997, nine volunteers consumed onion, apple or pure rutin. Onion quercetin, attached to glucose, reached its blood peak in under 45 minutes. Apple quercetin and rutin were around three times less bioavailable, with peaks at 2.5 and 9 hours.
Which forms improve absorption?
A 2025 meta-analysis of 31 human studies compared forms. Oligoglucosides, isoquercitrin enzymatically enriched with glucose units, were absorbed around 20 times better than the aglycone and 10 times better than rutin. Compared with the aglycone, tested formulations multiplied absorption by:
- around 11 for isoquercitrin included in a cyclodextrin, a ring-shaped sugar;
- around 20 for a complex with lecithin;
- around 62 for an emulsion combining fenugreek fibre and lecithin.
These gains concern the quantity found in blood or urine, not a health effect.
Quercetin: what benefits do clinical trials show?
Measured human effects are modest and often come from small trials lasting a few weeks. No health claim is authorised for quercetin in the European Union: a supplement cannot claim any of the effects studied below.
| Area | Studied dose | Result | Evidence |
|---|---|---|---|
| Blood pressure | 100 to 1,000mg/day | −2 to −4 mmHg (systolic) | 2016 and 2023 meta-analyses |
| Cholesterol, blood glucose | Variable | No clear effect | 2023 review |
| Endurance | 1,000mg/day | Around +2% | 11 studies, 254 people |
| Respiratory infections | 1,000mg/day | Fewer cases in 40 cyclists; no overall effect in 1,002 adults | 2 trials |
| Allergies | Animal doses | Less IgE and histamine | 13 animal studies |
| Senescent cells | 1,000mg/day intermittently, with dasatinib | Mixed | Trials with 31 to 60 participants |
Blood pressure: a modest decrease
This is the best-documented effect. A 2016 meta-analysis of 7 trials with 587 participants over 4 to 10 weeks measured a reduction of 3 mmHg in systolic and 2.6 mmHg in diastolic pressure, significant only from 500mg per day (−4.5 mmHg systolic). Only one of these trials involved people with hypertension.
In 2023, a review of five meta-analyses concluded there was a smaller reduction, 1.9 mmHg systolic and none diastolic, with very low to moderate certainty depending on the outcomes. Medical follow-up remains essential for high blood pressure.
Cholesterol and glucose: no clear effect
The 2023 review finds no effect on cholesterol, triglycerides, fasting blood glucose, insulin resistance or inflammation. Only insulin levels fall slightly. Compare this with berberine, studied for blood glucose and cholesterol.
Endurance: a minimal gain
A meta-analysis of 11 studies with 254 participants, often using 1,000mg daily for around ten days, finds a significant effect when pooling VO2max, maximal oxygen consumption, and endurance. But its authors consider it “between negligible and small”: around 2% compared with placebo.
Respiratory infections in athletes
In 2007, 40 trained cyclists took 1,000mg of quercetin per day or placebo for three weeks, then through three days of intensive cycling and the following two weeks. Over those two weeks, 1 of 20 cyclists had an upper respiratory tract infection with quercetin, versus 9 of 20 with placebo. Immune markers did not differ.
In 2010, a trial in 1,002 adults taking 500 or 1,000mg per day for 12 weeks showed no overall effect on infections. Only a post hoc analysis restricted to people aged 40 or over who considered themselves fit found 31% fewer sick days with 1,000mg. The supplement also contained vitamin C and niacin, and the study was industry-funded.
Quercetin and senescent cells: where does research stand?
Senescent cells have stopped dividing but persist in tissues, where they accumulate with age and release inflammatory molecules. Senolytics aim to selectively eliminate them. In cells, quercetin is one such compound; in older mice, it has mainly been tested with dasatinib.
In humans, it has mainly been tested with dasatinib, a cancer medicine prescribed for certain blood diseases, using 1,000mg of quercetin per day for three consecutive days in spaced courses. Results are mixed:
- in 2024, among 60 postmenopausal women, 20 weeks of this regimen did not change the primary bone-resorption marker;
- in 2026, among 31 patients with steatohepatitis and fibrosis, biopsy-assessed fibrosis improved in 47% of treated patients versus 7% with placebo. Adverse effects were more frequent (82% versus 43%), all transient.
Alone, quercetin has not demonstrated a senolytic effect in humans. In 2025, among 97 coronary bypass patients taking 1 g per day for a few days, exploratory analyses showed reduced senescence signatures in men's arteries but not women's, in whom some inflammatory markers increased.
The 2024 trial authors note that people already self-medicate with it as a senolytic, without sufficient safety or efficacy data. Other ageing-related approaches are studied, such as NAD+'s decline with age and its precursors NMN and NR.
Quercetin: what side effects and risks apply?
At studied doses, quercetin is well tolerated. In 2024, the VKM analysed 23 randomised adult trials. Adverse effects were rare and mild, such as headache or transient tingling, over periods up to 10 months.
Interactions: in humans, 300 to 1,500mg per day increased blood levels of ciclosporin, pravastatin or fexofenadine, and tended to lower those of midazolam or talinolol. No notable effect was measured on warfarin, an anticoagulant, or digoxin. The increases might result from inhibition of CYP3A4 and P-glycoprotein (P-gp), a pump in the intestinal wall that rejects many molecules. If you take long-term medication, consult a doctor or pharmacist.
Caution also applies to oestrogen-sensitive cancers: according to the VKM, animal data suggest vulnerability.
Genotoxicity: what do official assessments say?
In the laboratory, quercetin causes mutations in certain tests, particularly in bacteria. In animals, it has not shown genotoxicity, and most long-term studies found no carcinogenic effect. In 1992, a two-year study by the American National Toxicology Program (NTP) observed mostly benign kidney tumours in male rats at very high doses of 400 and 1,900mg/kg per day. They were probably linked to a kidney disease specific to male rats and considered of little relevance to humans. The International Agency for Research on Cancer places quercetin in Group 3: “not classifiable as to its carcinogenicity to humans”.
Frequently asked questions
How much quercetin per day?
There is no recommended intake: quercetin is neither a vitamin nor a mineral. The VKM considers 500mg of quercetin dihydrate daily risk-free in adults for at least three months; beyond this, evidence is more uncertain.
Should you take quercetin with a meal?
Yes, if aiming for absorption: according to the 2025 meta-analysis, fats or fibre taken alongside it roughly doubled the absorbed quantity. This is an absorption effect, not a demonstrated health benefit.
Is quercetin a natural antihistamine?
This is not demonstrated in humans. A 2025 meta-analysis of 13 mouse and rat studies reports less IgE, histamine and inflammatory cells, with substantial differences between studies, and calls for clinical trials. Allergy or asthma diagnosis and management belong to a doctor.
Does quercetin help with weight loss?
No, according to available data: the 2023 review finds no effect on body-size measurements, or anthropometric indices. No such claim is authorised either.
Is quercetin harmful to the kidneys?
No human study has shown harmful kidney effects at the oral doses studied. In rats, however, very high doses worsened existing kidney disease. The VKM therefore considers people with chronic kidney disease potentially vulnerable. Medical advice is essential before any use.
Can you take quercetin during pregnancy?
As a precaution, it is best avoided because reassuring evidence is lacking. According to the VKM, quercetin probably crosses the placenta, and fused digits were observed in mouse pups exposed to high doses. Data are also lacking for breastfeeding and children.
Quercetin or resveratrol: what is the difference?
They are two polyphenols from different families: quercetin is a flavonol, resveratrol a stilbene. Very little of either circulates intact in blood, and human effects remain modest. See our article on resveratrol.
Conclusion
Quercetin is a common dietary flavonol, abundant in capers, shallots and red onion. In pure form it is poorly absorbed, and trials show only modest effects: a few mmHg less systolic pressure and a minimal endurance gain. Its senolytic reputation mainly rests on trials combining it with a cancer medicine. Well tolerated at studied doses, it can nevertheless interact with some treatments.
Sources
- Haytowitz DB, Wu X, Bhagwat S (2018). USDA Database for the Flavonoid Content of Selected Foods, Release 3.3. U.S. Department of Agriculture, Agricultural Research Service. ars.usda.gov
- Rothwell JA et al. (2013). Phenol-Explorer 3.0: a major update of the Phenol-Explorer database to incorporate data on the effects of food processing on polyphenol content. Database, 2013, bat070 (database accessed in version 3.6, October 2026). doi:10.1093/database/bat070
- Hollman PC et al. (1997). Relative bioavailability of the antioxidant flavonoid quercetin from various foods in man. FEBS Letters, 418(1-2), 152-156. doi:10.1016/s0014-5793(97)01367-7
- Liu L et al. (2025). Improving quercetin bioavailability: A systematic review and meta-analysis of human intervention studies. Food Chemistry, 477, 143630. doi:10.1016/j.foodchem.2025.143630
- Serban MC et al. (2016). Effects of Quercetin on Blood Pressure: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Journal of the American Heart Association, 5(7), e002713. doi:10.1161/JAHA.115.002713
- Arabi SM et al. (2023). The effects of Quercetin supplementation on cardiometabolic outcomes: An umbrella review of meta-analyses of randomized controlled trials. Phytotherapy Research, 37(11), 5080-5091. doi:10.1002/ptr.7971
- Kressler J et al. (2011). Quercetin and endurance exercise capacity: a systematic review and meta-analysis. Medicine & Science in Sports & Exercise, 43(12), 2396-2404. doi:10.1249/MSS.0b013e31822495a7
- Nieman DC et al. (2007). Quercetin reduces illness but not immune perturbations after intensive exercise. Medicine & Science in Sports & Exercise, 39(9), 1561-1569. doi:10.1249/mss.0b013e318076b566
- Heinz SA et al. (2010). Quercetin supplementation and upper respiratory tract infection: A randomized community clinical trial. Pharmacological Research, 62(3), 237-242. doi:10.1016/j.phrs.2010.05.001
- Farr JN et al. (2024). Effects of intermittent senolytic therapy on bone metabolism in postmenopausal women: a phase 2 randomized controlled trial. Nature Medicine, 30(9), 2605-2612. doi:10.1038/s41591-024-03096-2
- Koning M et al. (2026). Senolytics dasatinib and quercetin in metabolic dysfunction-associated steatohepatitis: a proof-of-principle randomized, controlled trial. Nature Metabolism, published online 1 October 2026. doi:10.1038/s42255-026-01643-4
- Mury P et al. (2025). Quercetin Reduces Vascular Senescence and Inflammation in Symptomatic Male but Not Female Coronary Artery Disease Patients. Aging Cell, 24(8), e70108. doi:10.1111/acel.70108
- VKM (2024). Risk assessment of quercetin dihydrate and rutin in food supplements – “Other substances”. VKM Bulletin 2024:09. Norwegian Scientific Committee for Food and Environment. vkm.no
- Lv Z et al. (2025). Quercetin exhibits multi-target anti-allergic effects in animal models: a systematic review and meta-analysis of preclinical studies. Frontiers in Pharmacology, 16, 1673712. doi:10.3389/fphar.2025.1673712
Food supplements do not replace a varied, balanced diet or a healthy lifestyle. Ask a healthcare professional for advice if you are taking medication, pregnant or breastfeeding.
