Flavonoids are a large family of polyphenolic plant compounds, also known as bioflavonoids, that serve as secondary metabolites responsible for the yellow, red, and blue pigmentation in flowers and fruits. Chemically, they possess a 15-carbon skeleton (C6-C3-C6) consisting of two phenyl rings and a heterocyclic ring, and they fulfill diverse functions in plants such as attracting pollinators, protecting against UV light, and regulating cell growth.

Although commonly consumed in foods like berries, tea, cocoa, citrus fruits, and onions, and often marketed in supplements, flavonoids are not classified as essential nutrients because there is no established evidence that they prevent specific diseases or are required for normal body function. Six major subclasses exist: anthocyanidins, flavan-3-ols, flavonols, flavanones, flavones, and isoflavones, with flavan-3-ols and flavonols being the most widespread in the human diet.

Research highlights potential biological activities, including antioxidant, anti-inflammatory, and anti-carcinogenic effects, though the bioavailability of these compounds in humans is generally low due to extensive metabolism. Common dietary sources include:

• Flavonols: Onions, kale, broccoli, and apples.

• Flavan-3-ols: Green tea, cocoa, grapes, and berries.

• Flavones: Parsley, thyme, celery, and chamomile.

• Flavanones: Oranges, grapefruits, and lemons.

• Isoflavones: Soybeans, tofu, and legumes.