What is quercetin, and which foods contain it?

Quercetin turns up on supplement labels, but it is a compound that has been in everyday food all along.

What is a flavonoid?

Flavonoids are a group of compounds plants make. They occur in vegetables, fruit, tea and seeds. One subgroup, the anthocyanins, gives red and blue colour; others contribute bitterness and astringency.

The group has thousands of members, divided into subgroups by chemical structure. Quercetin belongs to the subgroup called flavonols.

Which foods contain it?

Amounts vary a great deal by plant and by which part is eaten.

• Shallots and onions, particularly the darker outer layers
• Apples, particularly the skin
• Tea, both green and black
• Capers, which are very high per unit weight
• Broccoli, leafy greens and some berries

In plants, most quercetin is not free but bound to sugars, in forms called glycosides.

Does cooking matter?

It does. Boiling leaches it into the water, and peeling removes a lot, because concentration is usually highest at the outer surface.

Eating an apple with the skin gives a different amount from eating the flesh alone.

How is it absorbed?

Sugar-bound forms have to be broken down first, so absorption depends on the form taken in and on what is eaten with it.

The research on this has several limitations, and reported figures vary with study method.

What should you watch out for?

If you take prescription medication, speak to a doctor or pharmacist before taking a supplement containing a concentrated extract.

Dietary supplements are not intended to prevent, treat or cure any disease.