Quercetin
Quercetin is a flavonol, a yellow plant pigment found in onions, apples, capers, kale and tea. It is not an essential nutrient. Most quercetin in food is bound to a sugar, and which sugar decides how much you absorb.
0 mgdaily need: quercetin is not an essential nutrient
Onions are a main source in many diets, along with tea and apples, because people eat them so often
EU health claims · none authorized
- Richest foods Capers then red onions and kale
- In a fruit or bulb Skin and outer layers where sunlight falls
- Best absorbed from Onions bound to glucose
- Evidence in people Weak mostly lab studies
From published reviews of quercetin in food and in people. No food figures: the food library does not measure flavonoids.
Foods With Quercetin
Quercetin builds up where a plant meets the sun, so the skin and outer layers hold the most. Peeling removes much of it.
- Capersamong the richest foods known, but eaten a teaspoon at a time
- Onions, especially red and yellowthe outer fleshy rings hold more than the center; white onions have much less
- Kale and other leafy greensa good source among vegetables
- Apples, berries and grapesmostly in the skin
- Tea and buckwheatlargely as rutin, quercetin bound to a two-sugar unit
- Supplementscapsules give hundreds of milligrams, far more than a day of food, in several forms that are absorbed differently
What Is Quercetin?
Quercetin belongs to the flavonols, a group of flavonoids that plants use as a sunscreen: they absorb ultraviolet light before it can damage the cells beneath. Its name comes from Quercus, the Latin name for oak. In food it almost never appears on its own; a sugar is attached, forming a glycoside.
How Well Quercetin Is Absorbed From Onions, Apples, and Tea
Which sugar is attached decides where quercetin is absorbed. In onions it is bound to glucose, and the small intestine takes these glucosides up directly. In buckwheat, and in part in tea, it is rutin, with a two-sugar unit the small intestine cannot split; it has to wait for bacteria in the colon, which take hours and destroy part of it. Apples carry a mix of sugars. In a classic Dutch study, people absorbed quercetin from onions much better than from apples or from pure rutin.
Cooking moves the numbers too. Quercetin glycosides dissolve in water, so boiled onions lose part of theirs into the cooking water. In a soup you eat that water; when vegetables are drained, it goes down the sink.
What Quercetin Does: Lab Studies vs. Human Trials
In cell studies quercetin neutralizes free radicals, calms immune cells that release histamine and blocks enzymes involved in inflammation. In the body, the gut and liver soon convert it into other compounds, and blood levels stay far below those used in lab dishes. Human trials on blood pressure, exercise, allergies and infections are mostly small and short, with mixed results, and no health claim for quercetin is authorized in the EU.
Diets rich in flavonoid foods are linked with better health in large population studies, but those foods bring fiber, potassium and many other compounds, so the link cannot be pinned on quercetin. The same gap between lab and people runs through resveratrol, another plant polyphenol sold as a supplement.
Quercetin Supplement Forms
Supplements are the other source of quercetin, at doses food cannot reach. Makers attach sugars or fats to work around its poor solubility.
| Form | What the evidence says |
|---|---|
| Quercetin (aglycone) | No sugar attached; the most common form in capsules. Dissolves poorly in water, which limits uptake |
| Rutin | The form in tea and buckwheat; absorbed slowly and less completely, after colon bacteria remove the sugar |
| Isoquercitrin and EMIQ | Quercetin bound to glucose, like the forms in onions; absorbed better than rutin in small human studies |
| Quercetin phytosome | Quercetin wrapped in a phospholipid to help it dissolve; few independent human comparisons |
Quercetin From Food or Capsules
A capsule can hold more quercetin than several days of onions and apples. Food gives less, but in the glucose-bound form the gut absorbs well, together with other flavonoids.
Quercetin Safety
- Side effectsfew in short trials; mostly headache and stomach upset
- Medicinesit can change how the liver clears some drugs; ask a pharmacist
- Pregnancysupplement safety has not been studied
Sources are listed at the end of this page. Not medical advice.
Tracking Quercetin in Caltide
- Not totaled yetCaltide does not add up quercetin yet. The onions, apples and other produce you log still count toward fiber, vitamins and minerals, with each food's breakdown in the food library.
Related Nutrients
Quercetin Sources and References
The figures and claims on this page are checked against these sources.
Questions About Quercetin
A flavonol, one of the most common flavonoids in plants. It is a yellow pigment that protects leaves and fruit skins from ultraviolet light, and it is found in onions, apples, capers, kale, tea and berries.
Capers are among the richest foods, followed by onions, especially red onions, then kale, apples with their skin, berries and tea. Onions, tea and apples are the main sources in many diets because people eat them often.
Yes. In onions quercetin is bound to glucose, a form the small intestine takes up well. Studies found it was absorbed better from onions than from apples or from rutin, the form in tea and buckwheat.
That has not been shown. Lab studies found it can calm immune cells that release histamine, but there are few good human trials, and their results on allergy or infection symptoms are not consistent.
In a test tube, yes. In the body, most of it is quickly changed into other compounds, and blood levels stay far below those used in lab studies, so its antioxidant effect in people is uncertain. EFSA has not accepted any antioxidant claim for it.
Short trials report few side effects, mostly headache and stomach upset. It can change how the liver handles some medicines, so ask a pharmacist first if you take any. Its safety in pregnancy is not known.
No. It is not an essential nutrient and has no daily reference. Eating plenty of fruit, vegetables and tea supplies it along with many other flavonoids.
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