Quercetin and epicatechin are examples of flavonoids, essential pigments found in many fruits and vegetables.

The abundance of flavonoids coupled with their low toxicity relative to other plant compounds means they can be ingested in large quantities by animals, including humans. Examples of foods that are rich in flavonoids include onions, parsley, blueberries, bananas, dark chocolate and red wine.

Some flavonoids are derived from 2-phenylchromen-4-one (2-phenyl-1,4-benzopyrone) and examples include rutin and quercetin, which are found in citrus fruits. Other forms of flavonoids or isoflavonoids are derived from 3-phenylchromen-4-one and neoflavonoids are derived from 4-phenylcoumarine.

Catechin

Catechins are important flavonoids abundant in the leaves of the tea plant. Examples include epicatechin, epicatechin gallate, and epigallocatechin gallate. When oolong tea or black tea is prepared, the leaves are allowed to oxidize. This causes conversion of some or all of the catechins to larger molecules and reduces the flavonoid content. White tea is tea that has undergone even less processing than green tea and therefore provides the highest catechin content.

Quercetin

Quercetin is the aglycone form of flavonoid glycosides such as rutin and quercitrin which are found in buckwheat, citrus fruit and onions. Quercetin is thought to have anti-inflammatory and antioxidant effects and to protect against some forms of illness such as cancer. However, the clinical evidence to support this claim is not yet available, despite promising initial research findings. Quercetin is found in various types of fruits, vegetables, teas, wine and many other food items.