Flavonols: Food Sources, Quercetin & What Research Shows

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What Are Flavonols?

Flavonols are one subclass within the broader family of plant compounds called flavonoids.

Other flavonoid subclasses include:

• anthocyanins;
• flavan-3-ols;
• flavones;
• flavanones;
• isoflavones; and
• related flavonoid families.

Flavonols share a common chemical backbone but differ in the number and position of hydroxyl groups and in the sugars or other chemical groups attached to them.

Those structural differences can influence:

• color and behavior in plants;
• solubility;
• chemical stability;
• absorption;
• metabolism; and
• activity in laboratory systems.

Flavonols are produced by plants as part of normal plant metabolism and can participate in processes involving pigmentation, ultraviolet protection, signaling, and responses to environmental stress.

Flavonols vs. Flavonoids: What Is the Difference?

Flavonoid is the broader family.

Flavonol is one specific subclass within that family.

So every flavonol is a flavonoid, but not every flavonoid is a flavonol.

For example:

• Quercetin — flavonol
• Myricetin — flavonol
• Kaempferol — flavonol
• Cyanidin — anthocyanidin, not a flavonol
• Anthocyanins — a different flavonoid subclass
• Catechin — flavan-3-ol, not a flavonol

This hierarchy is important when interpreting research because results involving one flavonoid subclass should not automatically be attributed to another.

Flavonols vs. Flavanols: What Is the Difference?

The names are very similar, but flavonols and flavanols are different chemical subclasses.

Flavonols include compounds such as:

• quercetin;
• kaempferol;
• myricetin; and
• isorhamnetin.

Flavanols, often called flavan-3-ols, include compounds such as:

• catechin;
• epicatechin;
• epigallocatechin; and
• epigallocatechin gallate, or EGCG.

Flavan-3-ols can also join together to form proanthocyanidins.

This distinction is particularly useful when discussing tea. Tea contains flavonols, but its famous catechins are flavan-3-ols, not flavonols.

What Are the Major Dietary Flavonols?

Quercetin

Quercetin is one of the most widespread and extensively studied dietary flavonols.

It occurs in foods such as onions, apples, berries, grapes, tea, leafy vegetables, and numerous other plant foods.

In plants, quercetin commonly occurs as glycosides rather than as free quercetin aglycone.

Different sugars attached to quercetin can substantially affect its absorption and metabolism.

Kaempferol

Kaempferol is another widely distributed flavonol.

Reported food sources include leafy vegetables, kale, broccoli, tea, beans, grapes, berries, and other plant foods.

Like quercetin, kaempferol frequently occurs as glycosides rather than as the free aglycone.

Kaempferol and quercetin differ by the number and position of hydroxyl groups on their molecular structures, which changes their chemical behavior.

Myricetin

Myricetin occurs in several berries, grapes, tea, vegetables, and other plant foods.

Berry-composition studies have identified myricetin or its glycosides in foods including cranberries, black currants, blueberries, bilberries, and related fruits.

Myricetin has been extensively investigated in laboratory and animal research, but direct human clinical evidence remains much more limited.

Isorhamnetin

Isorhamnetin is a methylated flavonol structurally related to quercetin.

It occurs naturally in various plant foods and can also appear as a metabolite formed during quercetin metabolism.

Isorhamnetin glycosides have been identified in berries and numerous other plants.

Rutin

Rutin is often discussed alongside the major flavonols, but chemically it is a quercetin glycoside.

Rutin consists of quercetin attached to the sugar group rutinose and is also called quercetin-3-O-rutinoside.

This attached sugar changes its absorption and metabolism compared with other quercetin forms.

What Are Flavonol Glycosides?

Most flavonols in plant foods do not occur primarily as free aglycones.

Instead, plants attach sugars to the flavonol structure.

Common sugars can include:

• glucose;
• galactose;
• rhamnose;
• arabinose;
• rutinose; and
• more complex sugar combinations.

For example, foods can contain:

• quercetin-3-glucoside;
• quercetin-3-galactoside;
• quercetin-3-rutinoside, or rutin;
• myricetin-3-galactoside;
• kaempferol glycosides; and
• many other flavonol derivatives.

The identity of the attached sugar is not a trivial detail.

Human pharmacokinetic studies show that different quercetin glycosides can have substantially different rates and degrees of absorption.

What Foods Contain Flavonols?

Flavonols occur throughout the plant kingdom.

Common dietary sources include:

• Onions — especially well known for quercetin glycosides.
• Apples — contain quercetin and other flavonoids, with substantial amounts often associated with the peel.
• Kale and leafy vegetables — sources of quercetin and kaempferol compounds.
• Broccoli — contains several flavonol glycosides.
• Cranberries — contain diverse quercetin- and myricetin-related flavonols.
• Aronia berries — contain quercetin-related flavonols alongside anthocyanins and proanthocyanidins.
• Blueberries — contain quercetin, myricetin, and other flavonol glycosides.
• Black currants — contain myricetin-, quercetin-, and other flavonol derivatives.
• Grapes — contain quercetin, myricetin, kaempferol, and related compounds depending on variety and fruit tissue.
• Cherries — contain quercetin- and kaempferol-related glycosides along with anthocyanins and phenolic acids.
• Tea — contains flavonols in addition to its better-known flavan-3-ols.
• Herbs and other vegetables — can provide additional flavonols depending on species and preparation.

The fact that a food contains a flavonol does not establish a particular concentration.

Amounts vary according to species, cultivar, growing conditions, plant tissue, maturity, processing, storage, and analytical method.

Which Foods Are Highest in Flavonols?

There is no single universal ranking.

Food-composition studies commonly identify onions, kale, broccoli, certain berries, and other plant foods as notable sources, but comparisons depend heavily on which flavonols are measured and how results are reported.

Berry studies have shown especially large differences among species.

For example, research comparing edible berries found substantial quercetin concentrations in cranberry and chokeberry, while myricetin was identified in cranberry, black currant, blueberries, bilberry, and several other species.

A separate analysis of 28 berry species identified more than 50 individual flavonol glycosides belonging to quercetin, myricetin, kaempferol, isorhamnetin, syringetin, and laricitrin families.

That diversity is one reason a simple label such as “high in flavonols” provides less information than a properly characterized flavonol profile.

Flavonols in FruitFast-Relevant Fruits

Several fruits used by FruitFast naturally contain flavonols, but their profiles differ substantially.

Cranberries

Cranberries are particularly interesting flavonol-containing fruits.

Analytical studies have identified numerous cranberry flavonol glycosides, including:

• quercetin-3-galactoside;
• quercetin-3-glucoside;
• quercetin arabinosides;
• quercetin-3-rhamnoside;
• myricetin-3-galactoside; and
• other quercetin- and myricetin-related compounds.

In one modern cranberry analysis, quercetin derivatives represented roughly 70% of the flavonols identified in the samples tested.

Cranberries also contain anthocyanins, proanthocyanidins, phenolic acids, and other compounds, so cranberry research should not automatically be attributed to flavonols alone.

Aronia Berries

Aronia, or chokeberry, contains flavonols alongside its better-known anthocyanins and polymeric proanthocyanidins.

Quercetin derivatives make up an important portion of the flavonol profile reported in aronia.

Composition studies also show that aronia can contain substantially different flavonol amounts from other berries.

Whole-aronia findings therefore should not automatically be described as quercetin or flavonol effects.

Explore our Aronia educational guide →
That guide covers aronia as a fruit and juice ingredient; it should not be interpreted as evidence that flavonols alone explain whole-aronia research.

Wild Blueberries

Blueberries contain numerous flavonol glycosides in addition to anthocyanins and phenolic acids.

Analytical work has identified quercetin-, myricetin-, isorhamnetin-, syringetin-, and laricitrin-related flavonols in different blueberry species and cultivars.

One detailed analysis of blueberry varieties identified dozens of individual flavonol compounds, illustrating how much chemical diversity can exist within a single fruit category.

Black Currants

Black currants contain flavonols including quercetin and myricetin derivatives.

Their overall polyphenol profile is also strongly influenced by anthocyanins, so flavonols represent only one portion of black currant chemistry.

Grapes

Grape skins can contain quercetin, myricetin, kaempferol, and related flavonol glycosides.

The relative amounts vary by grape cultivar, growing environment, fruit tissue, maturity, and processing.

Grapes also contain anthocyanins in dark varieties, flavan-3-ols, proanthocyanidins, phenolic acids, and stilbenes such as resveratrol.

Cherries

Cherries contain flavonols in addition to anthocyanins, flavan-3-ols, and phenolic acids.

Analytical studies of cherry fruit have identified quercetin- and kaempferol-based glycosides.

As with other fruits, the exact profile varies according to species, cultivar, maturity, and analytical method.

Are Flavonols Antioxidants?

Flavonols demonstrate antioxidant and redox activity in laboratory chemical systems.

The hydroxyl groups on their molecular structures can participate in reactions involving reactive chemical species, metals, and oxidation processes.

However, laboratory antioxidant activity should not automatically be interpreted as evidence that flavonols circulate through the human body as unchanged molecules and directly neutralize free radicals.

After consumption, flavonols are extensively transformed.

The compounds circulating in blood are commonly glucuronidated, sulfated, or methylated metabolites rather than large amounts of the original free flavonol aglycone.

Gut microorganisms can also break flavonols down into smaller phenolic compounds.

This is why modern flavonol research considers absorption, metabolism, food matrix, and metabolites rather than relying only on test-tube antioxidant measurements.

Learn more in our Antioxidants, Free Radicals & Oxidative Stress guide.

How Are Flavonols Absorbed and Metabolized?

Flavonol absorption depends heavily on chemical form.

Most dietary flavonols are consumed as glycosides.

During digestion, sugar groups can be removed or transformed, after which flavonol structures and their metabolites may be absorbed.

Human studies involving quercetin show that:

• different glycosides can have substantially different bioavailability;
• glucose-containing quercetin glycosides can be absorbed efficiently in the small intestine;
• rutin follows a different absorption pattern because of its rutinose sugar group;
• absorbed quercetin is rapidly converted into conjugated metabolites; and
• gut microorganisms contribute to formation of smaller phenolic metabolites.

Human plasma after dietary quercetin exposure commonly contains glucuronide, sulfate, and methylated metabolites rather than substantial amounts of unchanged free quercetin.

This means that two foods containing the same amount of total quercetin do not necessarily produce identical systemic exposure.

Does the Food Source Affect Flavonol Bioavailability?

Yes.

The food source can affect flavonol absorption because plants contain different glycosides and food matrices.

A classic human crossover study found that quercetin glucosides from onions were absorbed differently from quercetin supplied as rutin.

Research reviews continue to emphasize that the sugar attached to quercetin or kaempferol influences absorption and metabolism.

Therefore:

• quercetin in onion is not necessarily absorbed identically to rutin in buckwheat;
• quercetin glycosides in cranberry may differ from those in apple or blueberry;
• a purified quercetin supplement is not equivalent to a whole food; and
• total flavonol concentration does not by itself determine bioavailability.

Flavonol Research: What Has Human Research Studied?

Flavonols have been studied extensively in laboratory, animal, observational, and human intervention research.

However, there is an important limitation when interpreting the evidence:

Most controlled human intervention research does not administer “flavonols” as one unified class.

Studies instead test specific compounds such as quercetin, particular supplements, onions, berries, tea, or other flavonol-containing foods.

Results from those interventions should be attributed to the material that was actually tested.

Quercetin Supplementation

Quercetin is the flavonol with the largest controlled human supplementation literature.

Meta-analyses of randomized trials have reported modest differences in some blood-pressure measurements following quercetin supplementation.

However, findings vary according to dose, participant population, study duration, and outcome.

A 2023 umbrella review of randomized evidence concluded that quercetin supplementation was associated with a small reduction in systolic blood pressure, while evidence did not show consistent effects across diastolic pressure, lipids, inflammatory markers, glucose, or several other cardiometabolic outcomes.

The certainty of evidence ranged from very low to moderate.

These trials generally involve concentrated quercetin doses and should not automatically be applied to fruit containing much smaller amounts of quercetin glycosides.

Inflammatory Biomarkers

Quercetin supplementation has also been studied in relation to markers such as C-reactive protein, IL-6, and TNF-α.

A meta-analysis of randomized trials did not find significant overall changes across those three markers, although some subgroup findings differed.

This is a useful example of why extensive laboratory research on inflammatory pathways should not automatically be converted into a claim that dietary flavonols “reduce inflammation” in people.

Flavonol-Rich Foods

Human studies have also investigated foods containing flavonols, including onions, apples, berries, tea, and other plant foods.

These foods contain numerous compounds in addition to flavonols.

If a berry intervention changes a vascular or metabolic measurement, the result demonstrates an effect of the berry preparation tested—not necessarily quercetin, myricetin, or flavonols as a class.

Observational Research

Population studies have examined relationships between estimated dietary flavonol intake and long-term health outcomes.

These studies can identify associations but cannot prove that flavonols themselves caused the observed differences.

People who consume more flavonol-rich foods may also differ in vegetable intake, fruit intake, overall dietary pattern, physical activity, smoking, socioeconomic factors, and many other characteristics.

Flavonol Research vs. Whole-Fruit Research

This distinction is central to FruitFast's Health Information approach.

A cranberry, blueberry, aronia, cherry, grape, or black currant contains many compounds in addition to flavonols.

Depending on the fruit, these can include:

• anthocyanins;
• proanthocyanidins;
• phenolic acids;
• flavan-3-ols;
• organic acids;
• vitamins and minerals;
• sugars;
• fiber in whole fruit; and
• many other plant constituents.

Research using a whole fruit therefore cannot automatically isolate flavonols as the reason for an observed outcome.

The reverse is also true: a trial using 500 mg of purified quercetin should not automatically be used to describe the effect of a serving of berries or fruit juice concentrate.

How Processing & Storage Affect Flavonols

The flavonol profile of a food can change during processing and storage.

Important factors include:

• fruit tissue — skins, seeds, and flesh can contain different concentrations;
• pressing and extraction — determine which tissues contribute compounds to juice;
• clarification and filtration — can remove or redistribute plant solids and associated compounds;
• heat — can alter glycosides and other phenolic compounds;
• oxygen exposure — can contribute to oxidation reactions;
• enzymatic activity — can alter phenolic profiles after fruit is crushed;
• fermentation — can transform flavonol glycosides;
• storage — temperature and duration can influence stability; and
• analytical method — determines which free, glycosylated, or total flavonol fractions are reported.

Processing therefore does not simply “preserve” or “destroy” flavonols in one predictable way.

Does Fruit Juice Concentrate Contain Flavonols?

A juice concentrate made from a flavonol-containing fruit can contain flavonol compounds derived from the starting fruit.

However, the amount and profile in the finished concentrate depend on:

• fruit species and cultivar;
• starting-fruit composition;
• skin and seed contact;
• pressing and extraction;
• clarification and filtration;
• processing conditions;
• degree of concentration;
• storage;
• age of the finished product; and
• analytical method.

How Are Flavonols Measured?

Researchers can identify individual flavonols using chromatographic and mass-spectrometry methods.

These methods can distinguish between compounds such as:

• quercetin;
• quercetin-3-glucoside;
• quercetin-3-galactoside;
• rutin;
• myricetin glycosides;
• kaempferol glycosides; and
• other flavonol derivatives.

Some analytical methods hydrolyze sugars before measurement and report flavonols as aglycone equivalents.

Other methods measure the individual glycosides directly.

Those results are not necessarily interchangeable, so the analytical method should always be considered when comparing food-composition values.

Frequently Asked Questions About Flavonols

How to Evaluate Flavonol Research

When reading research about flavonols, first determine exactly what was tested.

A study may involve:

• quercetin aglycone;
• a particular quercetin glycoside;
• rutin;
• kaempferol;
• myricetin;
• a flavonol-rich extract;
• onions or another vegetable;
• berries or whole fruit;
• fruit juice;
• juice concentrate; or
• a supplement containing several polyphenols.

These interventions are not interchangeable.

It is also useful to consider:

• Compound identity — Which flavonol and which glycoside were tested?
• Dose — Was the exposure comparable with food intake or a concentrated supplement?
• Food matrix — Was the compound isolated or consumed within a complex food?
• Population — Were participants healthy or selected for a particular condition?
• Duration — Was the study acute or long term?
• Metabolites — Were researchers measuring the original flavonol or its metabolites?
• Other compounds — Did the intervention contain anthocyanins, proanthocyanidins, phenolic acids, vitamins, minerals, sugars, or other substances?
• Outcome — Was the study measuring absorption, antioxidant chemistry, a biomarker, blood pressure, symptoms, or a clinical health outcome?

A laboratory antioxidant test, detection of a flavonol metabolite, a small change in blood pressure, and prevention of cardiovascular disease represent very different levels of evidence.