Compound

Quercetin

Also known as: 3,3',4',5,7-pentahydroxyflavone

A flavonoid (a class of plant polyphenol) found in significant amounts in onions, apples, capers, kale, berries, tea, red wine, and many other plants. One of the most-studied dietary flavonoids. Demonstrates antioxidant, anti-inflammatory, antihistamine, and antiviral effects in laboratory and clinical studies. Bioavailability from food is modest but real; concentrated supplements achieve higher plasma levels but lose the food-matrix context. Quercetin is one of the clearest examples of a *whole-food bioactive* — a compound that has documented effects when eaten as part of plants but whose isolated form behaves differently.

Where it comes from

Quercetin is found across the plant kingdom; the highest dietary concentrations include:

Onions — especially red and yellow, especially the outer layers
Apples — concentrated in the skin
Capers — among the densest dietary sources
Kale, [[broccoli|broccoli]], [[lettuce|lettuce]], leafy greens
Berries — blueberries, blackberries, cranberries
Tea, particularly green and black
Red wine — modest amounts
[[buckwheat|Buckwheat]], hot peppers, tomatoes

What it does

Antioxidant — donates electrons to neutralize reactive oxygen species
Anti-inflammatory — modulates NF-κB and other inflammatory signaling
Antihistamine — stabilizes mast cells; reduces histamine release; foundational to traditional use of onion preparations for allergic conditions
Antiviral — laboratory studies show activity against several viruses; clinical evidence is mixed but substantial
Cardiovascular — modest blood-pressure-lowering effect at higher intakes

Bioavailability — the food matrix matters

Quercetin in plants is mostly bound to sugars (as glycosides — for example, rutin in [[buckwheat|buckwheat]]). Gut bacteria cleave the sugars, freeing quercetin for absorption. The food matrix matters substantially:

Cooked onions release more bioavailable quercetin than raw
Fat in the meal improves absorption
A diverse [[gut-microbiome|gut microbiome]] improves cleavage of glycosides

Isolated quercetin supplements achieve higher plasma levels but do not replicate the synergistic effects of the whole-food matrix — quercetin in an apple is accompanied by hundreds of other polyphenols that interact with it.

Why this matters

Quercetin is a useful illustration of the whole food versus the isolated compound. The clinical literature on isolated quercetin supplements is mixed; the epidemiological literature on diets high in quercetin-rich foods is much stronger. The compound is part of the answer; the food it comes in is the rest of the answer.

Sources

Multiple peer-reviewed reviews of dietary flavonoids
Linus Pauling Institute Micronutrient Information Center

Rooted in life.

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