flavonoid
Meaning of flavonoid: A pigment family coloring much of the plant world, and the clearest example of test tube antioxidant power failing to transfer into a body.
Definition of flavonoid
A flavonoid is any of several thousand plant compounds built on the same three ring carbon skeleton, found in fruit, vegetables, tea, cocoa, wine and many herbs. Flavonoids supply much of the yellow, red and blue color of flowers and fruit, shield the plant from ultraviolet light and grazing, and are among the compounds most often measured in food chemistry.
What are the main groups?
| Subclass | Typical sources |
|---|---|
| Flavanols | Tea, cocoa, apples |
| Flavonols | Onions, kale, capers |
| Flavanones | Citrus fruit |
| Anthocyanins | Berries, red cabbage, black rice |
| Isoflavones | Soybeans and soy foods |
| Flavones | Parsley, celery, chamomile |
The groups differ only in small details of the middle ring, and those details change color, taste and how much survives digestion. Anthocyanins shift color with acidity, which is why red cabbage turns blue in alkaline water. Isoflavones resemble estrogen closely enough to bind weakly at the same receptors, which is why soy is discussed apart from other plant foods.
Are they really antioxidants?
In a test tube, beyond argument. Flavonoids give up electrons readily and neutralize reactive molecules, and that measurement is cheap to make, which is how the label became attached to them. Inside a living body the picture changes. Most are absorbed poorly, and what does cross the gut wall is rapidly modified by the liver and excreted, so blood concentrations stay far below the levels used in laboratory work. Direct scavenging cannot account for much at those concentrations.
What do they appear to do instead?
Attention has shifted to signaling. Small amounts seem to nudge the cell's own stress response pathways, prompting it to raise its defenses, an effect that does not require the compound to be present in bulk. A large share is also broken down by gut bacteria into smaller molecules that are absorbed far more easily, so some of what is credited to a flavonoid may be the work of a bacterial product. Diets rich in flavonoid containing foods are consistently associated with better heart health, yet isolating a single compound as an antioxidant supplement has repeatedly failed to reproduce that benefit. The same gap between food and capsule appears with the plant sterol group and with several vitamins.
Also known as
Used in a sentence
The study estimated total flavonoid intake from a food frequency questionnaire.
Good to know
Antioxidant capacity measured in a test tube does not carry into the body, since flavonoids are poorly absorbed and heavily modified, and part of the effect credited to them belongs to gut bacterial breakdown products.
Word origin
Where “flavonoid” comes from
From the Latin flavus, meaning yellow, with the ending -oid from the Greek eidos, meaning form or likeness.