Flavonoids are molecular compounds found only in plants which serve as a defense mechanism. They make the plant tissue unappetizing to fungi, insects, and other pests. .
While every plant makes flavonoids, they tend to be concentrated in the leaves and fruit. For that reason, fruits tend to be a richer source of flavonoids than many vegetables.
In 2007, research conducted at the Linus Pauling Institute and published in Free Radical Biology and Medicine indicates that inside the human body, flavonoids themselves are of little or no direct antioxidant value. Unlike in the controlled conditions of a test tube, flavonoids are poorly absorbed by the human body (less than 5%), and most of what is absorbed is quickly metabolized and excreted from the body.
The huge increase in antioxidant capacity of blood seen after the consumption of flavonoid-rich foods is not caused directly by the flavonoids themselves, but most likely is due to increased uric acid levels that result from expelling flavonoids from the body. According to Frei, “we can now follow the activity of flavonoids in the body, and one thing that is clear is that the body sees them as foreign compounds and is trying to get rid of them. But this process of gearing up to get rid of unwanted compounds is inducing so-called Phase 2 enzymes that also help eliminate mutagens and carcinogens, and therefore may be of value in cancer prevention…”