What is Quercetin?

Quercetin is a flavonoid — one of the plant pigments that colour fruit, flowers and vegetables — and it is one of the most abundant flavonoids in a normal diet. It behaves as an antioxidant in laboratory assays, and human research has looked mainly at immune-related outcomes. The evidence in people is limited and mixed, which is worth knowing before reading anything else about it.

Where quercetin comes from

Onions, particularly red onions, are the densest common source. Capers, apples eaten with the skin, kale, broccoli, asparagus, berries, buckwheat, and black and green tea all contribute. Typical dietary intake across populations is usually estimated in the range of tens of milligrams a day, well below the amounts used in supplement studies, which commonly run from 200 mg to 1,000 mg daily.

In plants, quercetin is mostly bound to sugars as glycosides — quercetin-3-O-glucoside and rutin are the familiar ones. The unbound form, quercetin aglycone, is what supplements usually contain. This matters because absorption differs between forms, and because absorption of quercetin is modest overall and varies considerably between individuals.

What it does in laboratory research

In vitro, quercetin donates electrons to free radicals, the reactive molecules that damage cell membranes and interfere with DNA. It has also been examined for effects on mast cells, the immune cells that store and release histamine, and on several inflammatory signalling pathways. These are cell-culture findings. They describe a mechanism that might operate in people; they do not establish that it does.

What human trials have found

The largest trial to date, published in Pharmacological Research in 2010, randomised 1,002 adults aged 18 to 85 to quercetin or placebo for 12 weeks and tracked upper respiratory tract illness. There was no overall difference between groups. A pre-specified subgroup analysis in physically active participants aged 40 and over reported fewer sick days, which is a hypothesis for further study rather than a conclusion. Smaller trials have looked at seasonal allergic symptoms using validated questionnaires, with modest and inconsistent results. Reviewers of this literature consistently describe it as promising but immature.

How it is usually formulated

Because absorption is limited, quercetin is often paired with vitamin C, with bromelain, or with delivery systems intended to improve solubility. Vitamin C is the most common partner: it has established roles in the normal function of the immune system and in the protection of cells from oxidative stress, and unlike quercetin those roles are settled science rather than an open question.

Safety and practical notes

Quercetin from food is eaten in ordinary amounts every day without concern. Supplemental amounts are generally well tolerated in short-term trials, with headache and digestive discomfort the effects most often reported. Long-term safety at high intakes has not been well characterised. Quercetin can interact with medicines processed by certain liver enzymes, including some antibiotics, anticoagulants and immunosuppressants, so anyone on prescription medication should check with a qualified healthcare professional before starting. It is not recommended for people who are pregnant or breastfeeding, since data are lacking.

The honest summary: quercetin is a well-characterised dietary compound with an active and unfinished research programme behind it. Eating a varied range of plant foods is the reliable way to get it.