Flavonoids in Plants

Structure and Types

Flavonoids, named for the yellow color of the first isolated compounds, also include red and blue anthocyanins. They share a basic structure of fifteen carbon atoms, differing in oxidation levels, hydroxyl and methoxyl group arrangements, and bound sugars. This group encompasses chalcones, flavanones, flavonols, isoflavones, anthocyanidins, leukoanthocyanidins, and procyanidins.

Flavones, flavonols, and anthocyanidins exist in cell sap as aglycones or sugar-bound glycosides. Anthocyanins color not only blue and red flowers and fruits but also leaves and stems. Proanthocyanidins, including polymerized catechins, are abundant in fruits, roots, rhizomes, and wood. Flavanones are common in citrus species and conifers, while isoflavones are found in legumes and irises.

Biological Functions and Health Benefits

Flavonoids are among the most common secondary metabolites in plants, serving diverse biological roles. They attract pollinating insects, protect against harmful insects, viruses, and fungi, inhibit certain enzymes, and influence cellular oxidation and reduction processes. Historically, some flavonoids were used to dye wool, and today, they act as antioxidants in preserving fats and juices. Bitter flavonoid glycosides from orange peel are processed into a very sweet artificial sweetener.

The varied chemical structures of flavonoids result in diverse health benefits. Their effects on blood capillaries were first recognized, leading to their classification as vitamin P (permeability) alongside orange flavonoids and rutin, though the term "bioflavonoids" is now preferred. Certain flavonoids dilate coronary arteries and improve heart function (e.g., hawthorn, arnica), relieve cramps (e.g., chamomile), protect the liver and enhance bile production (e.g., milk thistle, immortelle), promote sweating (e.g., linden, meadowsweet, elderberry), and increase urine output (e.g., silver birch, restharrow, woundwort, sweet violet). They also exhibit significant anti-inflammatory and anti-allergic effects by inhibiting prostaglandin production (e.g., rutin) and reducing fluid accumulation in tissues.