Flavonoids are a class of water-soluble plant pigments, that, while not essential nutrients, have been shown to have beneficial actions on the heart and other bodily systems.

Flavonoids all show a similar chemical construction including two benzene rings that lie on either side of a 3-carbon ring. The various classes of flavonoids are created by multiple combinations of hydroxyl groups, sugars, oxygens, and methyl groups that attach to the common structure. The classes are made up of flavanols, flavanones, flavones, flavan-3-ols (catechins), anthocyanins, and isoflavones.

Due to the chemical structure of these substances, many flavoniods, when present in the body, are extremely antioxidant. They combine easily with free radicals in the body such as hydroxyl radicals, superoxide anions, and lipid peroxy radicals. These radicals are often antagonists to certain diseases, cellular and chromosomal damage, and aging, and therefore, the presence of flavoniods counteracts the damage done by these chemicals.

Flavonoids are found in all plants to some degree, and are major contributors to the actions of some herbal medications. They are purported by many herbalists to have antibacterial, anti-inflammatory, antiallergic, antimutagenic, antiviral, antineoplastic, anti-thrombotic, and vasodilatory actions.

While not all the claims associated with these chemicals are proven, studies have shown that certain flavonoids do, in fact, have medicinal properties. For example, the quercetin in onions have been shown to protect the heart from damage and disease. An inverse relationship between flavonoid intake and coronary disease has been shown to exist.

Some potent sources of flavonoids include green tea, black tea, soy beans, citrus fruits, wine, and colored berries.