Polyphenols are a category of compounds found in plants that are defined by their structure (a series of hydroxyl groups attached to aromatic rings). There are several thousand molecules in nature that have this structure. Sources and bioavailability of polyphenols are summarized in a review article published in The American Journal of Clinical Nutrition (2004;79(5):727-747). Polyphenols, as a class, have antioxidant properties, and depending on the specific type, often have other medicinal effects like anti-cancer, anti-inflammatory and anti-allergic actvitiy. Subclasses of polyphenols include phenolic acids, lignans, stilbenes and the popularly known, flavonoids, which can be further subdivided.
Most foods contain a mixture of various polyphenols and can vary based on environmental factors, preparation methods, and food processing. Certain foods have been analyzed to contain high levels of particular polyphenols. Phenolic acids are found in tea, red fruits, black radish, and onions. The richest sources of flavonoids are onions, curly kale, dark berries, leeks, broccoli, blueberries, red wine, tea, tomatoes, grapefruit, the white part of oranges, chocolate, and soy. The highest source of lignans is linseed oil, but wheat, lentils, garlic, asparagus, carrots, pears and prunes are also significant sources. Stilbenes are found in low quantities in the diet, but a commonly known stilbene is resveratrol found in wine.
Depending on the type of polyphenol, absorption in the intestines, transport and metabolism, varies. Microbial flora in the large intestine may also play a role in metabolism and absorption. Once these compounds are absorbed, metabolites can be formed that may have biological activity. Due to the vast number of polyphenols, there is much to be learned about the biological processing and the effects exerted in the body. What we do know is that there are many polyphenols that have been examined that have profound effects on our health. Eating a diet rich in fruits and vegetables is crucial in preventing chronic disease, likely due to the presence of phytochemcials like polyphenols. As well, some quality multivitamins now contain significant amounts of polyphenols, and specific types can also be obtained from various supplemental products in amounts higher than could be obtained through diet alone.
