Supplements
Quercetin: food flavonols, supplement forms, absorption, blood pressure and immune claims
On this page
Quercetin is a flavonol, a member of the plant flavonoid family, found in foods such as onions, apples, berries and tea. It is a chemical constituent of food, rather than a vitamin with an established daily requirement. Plants commonly attach sugars to quercetin; an isolated supplement can supply the unattached form. These distinctions matter before comparing a food study with a capsule study.[1]
Compare the actual quercetin preparation
| Preparation | What it means | Interpretation boundary |
|---|---|---|
| Food glycosides | Quercetin joined to sugars in a food matrix | Onion glucosides and rutin need not behave alike[2] |
| Aglycone | Quercetin without an attached sugar | Isolated powder differs from the entire food[1] |
| Rutin / isoquercetin | Different sugar-linked quercetin compounds | The sugar and attachment affect absorption[1] |
| Lecithin formulation / phytosome | A delivery system combining quercetin with phospholipid | Milligrams of formulation can differ from milligrams of quercetin[3] |
| Onion-skin extract | An extract supplying quercetin plus other constituents | A trial of the extract does not isolate every component[5] |
What an absorption experiment can establish
A 1995 crossover experiment studied nine adults with ileostomies, allowing researchers to compare intake with material leaving the small intestine. Estimated absorption was 52 ± 15% for onion glucosides, 17 ± 15% for rutin and 24 ± 9% for the aglycone. The estimates depended on correction for degradation in collection bags. They show that forms differ in this setting; they are not universal percentages for every food, digestive tract or commercial capsule. The endpoint was absorption, not disease prevention.[2]
The compound in a capsule is not necessarily what circulates
After absorption, quercetin is extensively metabolized, including into glucuronide, sulfate and methylated compounds. A laboratory dish exposing cells to free quercetin may therefore differ from concentrations and chemical forms reaching human tissues. The dietary review emphasizes that the activity of circulating metabolites is incompletely understood. A food containing more quercetin is not automatically the food delivering the largest biological effect.[1]
Evidence & uncertainty
Quercetin research measures different things: blood concentrations, blood pressure, respiratory symptoms and laboratory markers. An absorption result is not evidence that a product prevents illness. EFSA’s 2011 assessment did not establish a causal relationship for the submitted claim about protection of DNA, proteins or lipids from oxidative damage; that dated decision is narrower than a verdict on every later clinical question.[7]
Selected-source overview · Independent review pendingPublished questions
Is eating quercetin foods equivalent to taking a quercetin supplement?
Selected-source overview · Independent review pending. Read the evidence context.
No. Foods contain different glycosides and a food matrix; supplements can supply concentrated isolated aglycone. Equal labels do not establish equal absorption or clinical effects.[1]
Evidence behind this answer
- Dietary Quercetin and Kaempferol: Bioavailability and Potential Cardiovascular-Related Bioactivity in Humans
original narrative review · 2019
Selected original full-text definition, methods, results or discussion passages inspected on 2026-10-14 UTC; no independent systematic appraisal · 2019-09-25; sections 1–4: flavonols, dietary forms, glycosides and circulating metabolites
This answer summarizes selected sources. An independent appraisal of the complete evidence has not been completed.
Does every sugar-linked quercetin form absorb equally well?
Selected-source overview · Independent review pending. Read the evidence context.
No. In the small ileostomy experiment, onion glucosides, rutin and aglycone had different estimated absorption. That specific comparison should not become a universal percentage for all products.[2]
Evidence behind this answer
- Absorption of dietary quercetin glycosides and quercetin in healthy ileostomy volunteers
original human crossover absorption study · 1995
Original indexed abstract inspected on 2026-10-14 UTC; direct source opening was attempted but returned an access challenge or incomplete page; full text not inspected · 1995 Dec;62(6):1276–1282; nine ileostomy participants, collection-corrected absorption estimates
This answer summarizes selected sources. An independent appraisal of the complete evidence has not been completed.
Does a better-absorbed quercetin phytosome prove greater health benefits?
Selected-source overview · Independent review pending. Read the evidence context.
No. The 12-person crossover study measured single-dose blood exposure. Clinical outcomes require trials of the particular formulation and the particular health question.[3]
Evidence behind this answer
- Improved Oral Absorption of Quercetin from Quercetin Phytosome®, a New Delivery System Based on Food Grade Lecithin
original human pharmacokinetic trial · 2019
Selected original full-text definition, methods, results or discussion passages inspected on 2026-10-14 UTC; no independent systematic appraisal · First online 2018-10-16; volume 44 (2019), 169–177; methods 2.1–2.6 and results 3.2
This answer summarizes selected sources. An independent appraisal of the complete evidence has not been completed.
Did quercetin lower blood pressure in everyone in the 2007 trial?
Selected-source overview · Independent review pending. Read the evidence context.
No. The trial reported a response in participants with stage 1 hypertension but no change in its prehypertension group. It did not study prevention of cardiovascular events.[4]
Evidence behind this answer
- Quercetin reduces blood pressure in hypertensive subjects
original randomized crossover trial · 2007
Original PubMed abstract and bibliographic record inspected on 2026-10-14 UTC; full text not inspected · 2007 Nov;137(11):2405–2411; 730 mg/day, 28 days, subgroup-specific pressure and oxidation results
This answer summarizes selected sources. An independent appraisal of the complete evidence has not been completed.
Did the onion-skin extract trial show an overall blood pressure benefit?
Selected-source overview · Independent review pending. Read the evidence context.
No significant overall benefit was found. A hypertension subgroup had lower 24-hour systolic pressure; exploratory testing and the mixed extract limit generalization.[5]
Evidence behind this answer
- Effects of a quercetin-rich onion skin extract on 24 h ambulatory blood pressure and endothelial function in overweight-to-obese patients with (pre-)hypertension: a randomised double-blinded placebo-controlled cross-over trial
original randomized crossover trial · 2015
Selected original full-text definition, methods, results or discussion passages inspected on 2026-10-14 UTC; no independent systematic appraisal · 2015;114(8):1263–1277; methods, total-group results, hypertension subgroup and exploratory-analysis/extract limitations
This answer summarizes selected sources. An independent appraisal of the complete evidence has not been completed.
Has quercetin reliably prevented colds in a large community trial?
Selected-source overview · Independent review pending. Read the evidence context.
The 1,002-person trial found no significant overall respiratory outcome improvement. A fitter, older subgroup had favorable findings, which do not establish a population-wide effect.[6]
Evidence behind this answer
- Quercetin supplementation and upper respiratory tract infection: A randomized community clinical trial
original randomized controlled trial · 2010
Original PubMed abstract and bibliographic record inspected on 2026-10-14 UTC; full text not inspected · Online 2010-05-15; Sep 62(3):237–242; 1,002 participants, overall null and 325-person fitness/age subgroup
This answer summarizes selected sources. An independent appraisal of the complete evidence has not been completed.
Does calling quercetin an antioxidant establish protection against disease?
Selected-source overview · Independent review pending. Read the evidence context.
No. EFSA’s dated claim assessment did not establish the submitted oxidative-damage claim. Chemical activity and a demonstrated benefit in people are separate evidentiary questions.[7]
Evidence behind this answer
- Scientific Opinion on the substantiation of health claims related to quercetin and protection of DNA, proteins and lipids from oxidative damage, cardiovascular system, mental state and performance, and liver/kidneys
agency health claim assessment · 2011
Selected original full-text definition, methods, results or discussion passages inspected on 2026-10-14 UTC; no independent systematic appraisal · Adopted 2011-01-28; published 2011-04-08; pp. 2 and 7–8, evidence and conclusions for submitted claims
This answer summarizes selected sources. An independent appraisal of the complete evidence has not been completed.
Is high-dose quercetin proven safe long term or with medicines?
Selected-source overview · Independent review pending. Read the evidence context.
No. The safety review lacked adequate long-term high-dose data and described drug-bioavailability interactions. A particular product and medicine combination needs individual assessment.[8]
Evidence behind this answer
- Safety Aspects of the Use of Quercetin as a Dietary Supplement
original safety review · 2018
Original publisher abstract and reference list inspected on 2026-10-14 UTC; full-text request redirected to abstract; full text not inspected · First online 2017-11-11; 2018;62(1):1700447; original abstract on human adverse effects, duration limits, animal signals and drug bioavailability
This answer summarizes selected sources. An independent appraisal of the complete evidence has not been completed.
Does a one-week quercetin safety trial justify taking its highest dose indefinitely?
Selected-source overview · Independent review pending. Read the evidence context.
No. Short monitored exposure cannot resolve rare or delayed harms or show COPD treatment benefit. The amounts tested in research are not personal dosing instructions.[9]
Evidence behind this answer
- Randomised clinical trial to determine the safety of quercetin supplementation in patients with chronic obstructive pulmonary disease
original dose escalation safety trial · 2020
Original indexed abstract and bibliographic record inspected on 2026-10-14 UTC; an initial open exposed the record and later opens were incomplete; full text not inspected · 2020;7:e000392; one-week dose-escalation design and monitored adverse-event results
This answer summarizes selected sources. An independent appraisal of the complete evidence has not been completed.
Higher blood exposure is a separate product claim
An industry-affiliated randomized crossover study enrolled 12 healthy adults and compared single doses of unformulated quercetin with a lecithin formulation under fasting conditions. The formulated product produced substantially higher measured blood exposure over 24 hours. Its composition included about 40% quercetin: a 500 mg formulation was not simply 500 mg pure quercetin. This supports a pharmacokinetic difference for that preparation. It does not establish that a higher blood level gives better allergy control, longer life or fewer infections.[3]
Blood pressure: a response in one subgroup
In 2007, a placebo-controlled crossover trial tested 730 mg/day for 28 days in 19 adults with prehypertension and 22 with stage 1 hypertension under the study’s definitions. Blood pressure did not change in the prehypertension group. The hypertension group had reported systolic and diastolic reductions of 7 ± 2 and 5 ± 2 mmHg, respectively. Plasma and urine oxidant-stress markers did not improve. The finding concerns short-term pressure measurements; the trial did not test heart attacks, stroke or replacing prescribed treatment.[4]
A second trial preserves the overall null finding
A 2015 crossover study randomized 70 overweight-to-obese adults to onion-skin extract supplying 162 mg/day or placebo for six-week periods. Overall, neither 24-hour ambulatory nor office blood pressure changed significantly. In the hypertension subgroup, the mean treatment difference for 24-hour systolic pressure was −3.9 mmHg versus placebo. Lipids, glucose and several inflammation/oxidation measures did not show significant between-treatment changes. The authors described exploratory subgroup testing and could not exclude effects of other extract constituents. These limits prevent treating the subgroup result as a guarantee for every user.[5]
Immune claims: the whole trial and subgroup answer different questions
A randomized community study included 1,002 adults aged 18–85 taking 500 or 1,000 mg/day or placebo for 12 weeks. Daily symptom logs showed no significant overall difference in upper respiratory infection outcomes. An analysis of 325 people aged at least 40 who rated their fitness in the upper half found fewer sick days and lower symptom severity with the higher dose. That narrower finding does not erase the overall null result. Self-reported fitness, symptom reporting and subgroup selection limit a general claim that quercetin prevents colds.[6]
Short follow-up is a safety boundary
A 2018 safety review found that adverse effects reported in human intervention studies were usually mild, but adequate data were lacking for high-dose use of at least 1,000 mg/day beyond 12 weeks. It identified animal signals involving already damaged kidneys and estrogen-dependent tumors; these are reasons for investigation, not proof of the same harm in people. The review also described altered drug bioavailability in animal and short-term human studies. A pharmacist or clinician should assess a particular medicine combination rather than assuming that a food-derived compound cannot interact.[8]
A monitored COPD trial did not establish long-term treatment
A small dose-escalation trial in adults with COPD studied 500, 1,000 or 2,000 mg/day for one week. Investigators reported no study-drug-related severe adverse events in the monitored blood tests. A safety feasibility study over one week cannot establish effectiveness against COPD, detect uncommon delayed harms, or support indefinite use at its highest tested dose. The selected adult studies also do not establish supplement safety during pregnancy, breastfeeding or childhood. A tested amount is a description of research, not a personal recommendation.[9]
References
- Dietary Quercetin and Kaempferol: Bioavailability and Potential Cardiovascular-Related Bioactivity in Humans
Original narrative review
Dabeek and Ventura Marra · Content/publication date: 2019
Accessed: 2026-10-14 UTC · Selected original full-text definition, methods, results or discussion passages inspected on 2026-10-14 UTC; no independent systematic appraisal
Cited sections: 2019-09-25; sections 1–4: flavonols, dietary forms, glycosides and circulating metabolites - Absorption of dietary quercetin glycosides and quercetin in healthy ileostomy volunteers
Original human crossover absorption study
Hollman and colleagues · Content/publication date: 1995
Accessed: 2026-10-14 UTC · Original indexed abstract inspected on 2026-10-14 UTC; direct source opening was attempted but returned an access challenge or incomplete page; full text not inspected
Cited sections: 1995 Dec;62(6):1276–1282; nine ileostomy participants, collection-corrected absorption estimatesDo not substitute the 1997 summary’s differing printed uncertainty for the original 1995 indexed abstract.
- Improved Oral Absorption of Quercetin from Quercetin Phytosome®, a New Delivery System Based on Food Grade Lecithin
Original human pharmacokinetic trial
Riva and colleagues · Content/publication date: 2019
Accessed: 2026-10-14 UTC · Selected original full-text definition, methods, results or discussion passages inspected on 2026-10-14 UTC; no independent systematic appraisal
Cited sections: First online 2018-10-16; volume 44 (2019), 169–177; methods 2.1–2.6 and results 3.2Authors affiliated with formulation manufacturer; pharmacokinetic endpoints, not a disease-outcome trial.
- Quercetin reduces blood pressure in hypertensive subjects
Original randomized crossover trial
Edwards and colleagues · Content/publication date: 2007
Accessed: 2026-10-14 UTC · Original PubMed abstract and bibliographic record inspected on 2026-10-14 UTC; full text not inspected
Cited sections: 2007 Nov;137(11):2405–2411; 730 mg/day, 28 days, subgroup-specific pressure and oxidation results - Effects of a quercetin-rich onion skin extract on 24 h ambulatory blood pressure and endothelial function in overweight-to-obese patients with (pre-)hypertension: a randomised double-blinded placebo-controlled cross-over trial
Original randomized crossover trial
Brüll and colleagues · Content/publication date: 2015
Accessed: 2026-10-14 UTC · Selected original full-text definition, methods, results or discussion passages inspected on 2026-10-14 UTC; no independent systematic appraisal
Cited sections: 2015;114(8):1263–1277; methods, total-group results, hypertension subgroup and exploratory-analysis/extract limitations - Quercetin supplementation and upper respiratory tract infection: A randomized community clinical trial
Original randomized controlled trial
Heinz and colleagues · Content/publication date: 2010
Accessed: 2026-10-14 UTC · Original PubMed abstract and bibliographic record inspected on 2026-10-14 UTC; full text not inspected
Cited sections: Online 2010-05-15; Sep 62(3):237–242; 1,002 participants, overall null and 325-person fitness/age subgroup - Scientific Opinion on the substantiation of health claims related to quercetin and protection of DNA, proteins and lipids from oxidative damage, cardiovascular system, mental state and performance, and liver/kidneys
Agency health claim assessment
EFSA NDA Panel · Content/publication date: 2011
Accessed: 2026-10-14 UTC · Selected original full-text definition, methods, results or discussion passages inspected on 2026-10-14 UTC; no independent systematic appraisal
Cited sections: Adopted 2011-01-28; published 2011-04-08; pp. 2 and 7–8, evidence and conclusions for submitted claimsA dated regulatory claim assessment, not an assessment of every later trial.
- Safety Aspects of the Use of Quercetin as a Dietary Supplement
Original safety review
Andres and colleagues · Content/publication date: 2018
Accessed: 2026-10-14 UTC · Original publisher abstract and reference list inspected on 2026-10-14 UTC; full-text request redirected to abstract; full text not inspected
Cited sections: First online 2017-11-11; 2018;62(1):1700447; original abstract on human adverse effects, duration limits, animal signals and drug bioavailability - Randomised clinical trial to determine the safety of quercetin supplementation in patients with chronic obstructive pulmonary disease
Original dose escalation safety trial
Han and colleagues · Content/publication date: 2020
Accessed: 2026-10-14 UTC · Original indexed abstract and bibliographic record inspected on 2026-10-14 UTC; an initial open exposed the record and later opens were incomplete; full text not inspected
Cited sections: 2020;7:e000392; one-week dose-escalation design and monitored adverse-event results
Scope & review
- Population
- Source-specific adult research participants; evidence does not establish use in pregnancy, breastfeeding or children
- Exposure or intervention
- Food quercetin glycosides, isolated aglycone, lecithin formulation and onion-skin extract
- Comparison
- Each trial retains its own placebo or alternative formulation; foods, isolated supplements and proprietary extracts are not interchangeable
- Outcomes
- Form-dependent absorption, blood pressure, upper respiratory symptoms, safety and claim boundaries
- Timeframe
- Selected short-term trials and dated reviews; follow-up and product are retained for each study
- Jurisdiction
- International human research and specifically dated agency assessments; product authorization and labeling are jurisdiction-specific
Selected-source educational explanation. Official guidance, original study findings and editorial interpretation are distinguished. No exhaustive search, independent systematic synthesis or qualified medical review has been completed.
Sources checked on their individual access dates. Selected sources were checked; ongoing surveillance and qualified review have separate statuses on the coverage page.
Revision history
- 2026-10-14 UTC: Added a bounded supplement explanation; independent qualified review remains pending.
What this article still leaves open
- Do replicated formulation-specific trials demonstrate worthwhile clinical outcomes rather than blood exposure?
- What are the long-term safety and interaction profiles in pregnancy, childhood and kidney disease?
- Do exploratory subgroup findings reproduce in trials designed for those populations?
Suggest a correction
Submit a content correction to the editorial queue, or download a local note. Leave out personal medical details. Submitted text is stored privately with a browser receipt session and is visible to this browser and the editor. Clearing cookies removes access to your submission history. This is not an account or a medical service.
Your submitted corrections for this article
Explore the topic map
Browse the health domains connected to this article. The map also identifies topics whose articles remain unwritten.