Quercetin

What is quercetin?

Quercetin is a flavonol, one of the plant pigments that colour fruit and vegetables, and it has been part of what people eat for centuries. The richest everyday sources are onions, the skins above all, along with apples, berries, grapes, broccoli, tea and red wine. Isolated from those plants it is sold on its own as a capsule, usually as the free aglycone and often at 500 mg a day or more, well above anything a plate of food provides. It is popular because several different ideas meet in it at once: an antioxidant, a damper on inflammation, and the flavonol most often named in writing about allergy.

Why do people take quercetin?

What does quercetin do in the body?

Quercetin is not a nutrient the body needs, and it has no job of its own to do. What it appears to do is interfere. It mops up the reactive molecules that damage cells, and it gets in the way of the signalling that switches inflammation on, so fewer of the messenger proteins and enzymes are made. In allergy the same interference turns up earlier in the chain, at the point where a cell would let go of its histamine.

Getting it there is the difficult part. In food, quercetin is nearly always attached to a sugar, and those sugar-attached forms, the ones in onions above all, are absorbed far better than the free aglycone that most capsules contain. What is absorbed is worked on quickly by the liver and travels round as methylated, glucuronidated and sulfated versions of itself rather than as quercetin. Pooled human studies put a lecithin-based phytosome at roughly twenty times the absorption of plain quercetin, and a meal with fat in it at about double.

What do people say about taking quercetin?

These are anecdotes people have shared, not evidence.

No community discussions have been summarised for this page yet.

What forms of quercetin are there?

What else is worth knowing about quercetin?

• Unwanted effects have rarely been reported in the human trials, and the ones that were reported were mild. A dose-escalation trial in adults with a long-term lung condition found 2000 mg a day tolerated over a week, but a safety review notes there is no adequate published data on taking 1000 mg a day or more for longer than twelve weeks.
• That same safety review reports that short courses of quercetin changed how much of certain medicines reached the blood, and it names people with existing kidney damage among the groups it could not clear.
• Labels give the amount per serving, which may be more than one capsule. Plain quercetin and the lecithin-based form are listed separately here, because a milligram of one is not the same thing as a milligram of the other.
• Where trials looked at men and women separately the answers sometimes differed: one trial around heart surgery found its improvement in the men and not in the women, and the endurance pooling found its small gain in untrained people and not in trained ones.

If you take medication, are pregnant, or have a health condition, check with a pharmacist or GP first.