Quercetin is a flavonoid, a type of plant polyphenol with antioxidant activity, found above all in onion, apple and capers. It is studied for its role in oxidative balance and, in human studies, for its relationship with blood pressure and some markers of inflammation. Here you will find what it is, where it is abundant in the diet and what the evidence shows, with its nuances.
What is quercetin and what is it used for?
Quercetin is a plant flavonoid from the flavonol subgroup, the same pigments that give colour to many fruits and vegetables. It is part of the polyphenols (plant compounds with chemical groups able to neutralise free radicals under laboratory conditions). In the body it does not behave as an essential nutrient: there is no recommended daily amount nor an officially recognised physiological function for it.
Its interest lies elsewhere. It is one of the most studied flavonoids, and research has explored its relationship with blood pressure, the lipid profile and oxidative balance. It is worth reading those results sensibly, because promising findings coexist with limitations of quality and dose, as we will see later.
Which foods contain quercetin?
Quercetin reaches our diet through everyday vegetables. Onion is one of the richest and most common sources, where it mainly appears in the form of glycosides (the molecule bound to a sugar, a bond that conditions how it is absorbed). These are some of the usual dietary sources:
- Onion, especially the red variety and the outer layers of the bulb: the most concentrated source in the common diet.
- Apple, above all in the skin.
- Capers, one of the densest sources per gram.
- Green tea and black tea.
- Grapes, broccoli and leafy greens.
- Red berries such as blueberries and blackberries.
Culinary processing also counts: the way onion is cooked and chopped modifies how quercetin is distributed within the bulb, so two dishes made from the same raw ingredient do not necessarily provide the same amount.
Why is quercetin considered an antioxidant?
An antioxidant is a molecule able to donate electrons to neutralise free radicals, unstable particles that can damage the structures of cells. Quercetin, as a polyphenol, has that capacity in laboratory assays (in vitro, that is, outside the organism): its chemical structure traps those radicals efficiently.
That said, what happens in a test tube does not translate directly to the body. After being ingested, quercetin is transformed intensively, so its antioxidant role within the organism is more subtle than the laboratory experiments suggest. That is why it is worth separating two things that are often mixed up: its chemical potency measured in vitro and what has actually been observed in people, which is where we turn next.
What has research observed about quercetin?
Here we enter the realm of nuance. Quercetin has no authorised health claims, so everything that follows comes from the scientific literature and is presented as exactly what it is: study findings, with their figures and their limitations.
Blood pressure and cardiometabolic markers
A meta-analysis of seven clinical trials (nine treatment arms and 587 participants in total) found that supplementation with quercetin was associated with a mean reduction in systolic pressure of 3.04 mmHg (95% confidence interval: −5.75 to −0.33) and in diastolic pressure of 2.63 mmHg (−3.26 to −2.01). The relevant detail is the dose: the effect was only significant from 500 mg per day; below that amount no clear change was observed.
A later umbrella review, which gathers and compares several meta-analyses, tempered the enthusiasm. It confirmed a reduction in systolic pressure of around 1.9 mmHg and a decrease in insulin levels, but found no clear effect on the rest of the cardiometabolic parameters. Its certainty of the evidence ranged from very low to moderate, which calls for caution.
If we widen the focus to flavonols, the chemical family to which quercetin belongs, the consumption of this group of compounds has been associated with modest changes in several biomarkers: total cholesterol −0.10 mmol/L, HDL +0.05 mmol/L, fasting glucose −0.18 mmol/L and systolic pressure −4.84 mmHg. It is worth remembering that these data correspond to the flavonol class as a whole, not to quercetin in isolation.
Why form matters: the journey of quercetin
The amount in a food does not equal the amount that reaches the organism in active form.
In the food
Quercetin is usually bound to sugars (glycosides), above all in onion.
Release
The body separates the sugar to obtain the free form (aglycone), the one that can be absorbed.
Metabolism
In the intestine and the liver it is transformed intensively before passing into the blood.
Active fraction
Only a part reaches the tissues in active form: real bioavailability is modest.
Inflammation and oxidative balance
On the inflammation front, sixteen randomised trials documented a decrease in C-reactive protein (a marker of inflammation measured in blood) associated with quercetin, with a standardised mean difference of −0.64 (95% confidence interval: −1.03 to −0.25). That same analysis found no significant changes in other inflammatory molecules, such as interleukin-6 (IL-6) or tumour necrosis factor alpha (TNF-alpha). In other words: a signal in one marker, not in all of them.
There is another interesting pathway. After being ingested, quercetin undergoes extensive metabolism, but it can still influence the redox balance (the balance between the production of free radicals and the body's capacity to neutralise them) by inducing the antioxidant and detoxifying enzymes that the organism itself produces. Seen this way, part of its interest would lie not in acting as a direct antioxidant, but in stimulating the internal defences.
Is quercetin well absorbed? The question of bioavailability
Bioavailability is the proportion of a substance that reaches the blood and the tissues in active form after consuming it. And with quercetin this point is decisive. In foods it is usually in the form of glycosides (bound to sugars); the body first needs to release the free, sugar-free form (the aglycone), and from there it transforms it intensively in the intestine and the liver. A good part of what we ingest is metabolised before exerting any effect.
That is why form matters so much. The amount of quercetin contained in an onion does not translate directly into quercetin available to the organism, and different presentations (food, extract or combined form) are absorbed differently. It is one of the reasons why results vary between studies and why high doses, from those 500 mg per day, appear more frequently in the trials that do show an effect.
Quercetin, polyphenols and cellular vitality
Quercetin rarely acts alone. In the diet it coexists with other polyphenols and, in the field of supplements, it is usually combined with compounds that share that interest in cellular vitality. It is the approach of formulas such as PLENIAGE PRO NAD+, which combines NADH with a triad of polyphenols —resveratrol, berberine and quercetin— together with hyaluronic acid and vitamin C.
The presence of vitamin C is no coincidence. Unlike quercetin, vitamin C contributes to the protection of cells from oxidative damage, an officially recognised function. You can see the detail in its ingredient profile and, if you are interested in how the organism sustains its own antioxidant defence, also in that of glutathione.
This piece is part of our content on vitality. If you want an overview of the topic, we have brought it all together in the cellular vitality guide.
Frequently asked questions
What is quercetin used for?
Quercetin is a flavonoid studied for its role in the body's oxidative balance. It has no officially recognised health functions, but human research has explored its relationship with blood pressure and with markers of inflammation, always with results that are worth reading with nuance.
Which foods have the most quercetin?
Onion is one of the richest and most common sources, where it concentrates above all in the outer layers of the bulb. Capers, apple (in the skin), tea, grapes, broccoli and red berries such as blueberries and blackberries also provide notable amounts.
Is quercetin a good antioxidant?
In the laboratory, quercetin neutralises free radicals efficiently. In the body the picture is more nuanced: it is heavily metabolised after being ingested, so part of its effect could be due to it stimulating the antioxidant enzymes that the organism itself produces, rather than to a direct action.
How much quercetin is used in studies?
In the meta-analyses on blood pressure, significant effects appeared with doses from 500 mg per day; below that amount no clear change was observed. These are not consumption recommendations, but the doses used in research, which should be assessed with a healthcare professional.
- Serban MC, et al. Effects of Quercetin on Blood Pressure: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Journal of the American Heart Association. 2016. PMID: 27405810.
- Arabi SM, et al. The effects of Quercetin supplementation on cardiometabolic outcomes: An umbrella review of meta-analyses of randomized controlled trials. Phytotherapy Research. 2023. PMID: 37654199.
- Menezes R, et al. Impact of Flavonols on Cardiometabolic Biomarkers: A Meta-Analysis of Randomized Controlled Human Trials to Explore the Role of Inter-Individual Variability. Nutrients. 2017. PMID: 28208791.
- Tabrizi R, et al. The effects of quercetin supplementation on lipid profiles and inflammatory markers among patients with metabolic syndrome and related disorders: A systematic review and meta-analysis of randomized controlled trials. Critical Reviews in Food Science and Nutrition. 2020. PMID: 31017459.
- Nemeth K, et al. Food content, processing, absorption and metabolism of onion flavonoids. Critical Reviews in Food Science and Nutrition. 2007. PMID: 17457724.