Flavonoids
Flavonoids are polyphenolic compounds, all built upon the same skeleton, but with variations among the many different types. This skeleton is always made of 15 carbon atoms, in the form of three aromatic rings, called phenols; a double ring attached to the third ring by a single bond. Then, the different variations of flavonoids are made by adding or removing hydroxyl groups, oxygen and hydrogen atoms bonded to the rings, or changing the position of the third ring. The Linus Pauling Institute shows an illustration of this basic structure, as well as some of the variations. These variations include flavanols, flavanones, anthocyanins, flavones, isoflavones, and more.
All flavonoids are biological pigments, meaning they only absorb certain parts of the light spectrum and they add either yellow, red, blue or purple coloring to different plants. A small change in the chemical structure of a flavonoid can produce a completely different color. These colors are functional for the plants because they attract animals, bird and bees, which can scatter seeds and spread pollen. Flavones and flavonols can also absorb harmful UV light, protecting the plant from the sun.
Most flavonoids found naturally in plants are attached to a sugar molecule. Flavonoids are water soluble. They are absorbed in the small intestine, and the bioavailability depends on how much good bacteria you have in your intestines, as this helps to metabolise the flavonoids. In general, they have a low bioavailability as they are absorbed and eliminated quickly.
Over 4000 different flavonoids have been identified in nature. For humans, these may have many health benefits, such as anti-viral, anti-inflammatory, antiallergic, and antioxidant properties. The antioxidant properties are attracting the most interest. These compounds can protect against damaging effects of some oxygen compounds such as free radicals. Of course some flavonoids have stronger properties than others. Particularly high antioxidants are the flavonol quercetin, found vegetables such as onions, xanthohumol which is found in hops and beer, and genistein, which is an isoflavone found in soy.
A lot of the biological activity of flavonoids may be due to their effects on cell signalling pathways. These are complex sequences of events that regulate the expression of certain genes. Some of the processes that these pathways regulate are cell growth, cell reproduction and apoptosis, or cell death. By affecting these pathways, flavonoids can help protect against cancers and heart disease.
Flavonoids are being studied for their effects on neurodegenerative disease such as Parkinsons and Alzheimers. Inflammation, accumulations of metal, and oxidative stress (free radicals) appear to play a role in the development of these diseases, and flavonoids protect against these conditions. Some studies like this one have shown that people with a higher intake of flavonoids have better cognitive function.
Taking high doses of flavonoids through the diet does not seem to have any adverse side effects. This may be because they are metabolised and eliminated so quickly - they don't build up in the body at all. A study where men were given quercetin by IV did show side effects of nausea, vomiting, sweating, breathing difficulty and toxic kidney. Flavonoids given in tea supplements also showed side effects of the gastrointestinal and central nervous system, but these were probably due to something in the tea, such as caffeine.
See the phytonutrient page for more about different flavonoids and how to get them in your diet.
The information on this page is not meant to be used in treatment of medical conditions. Please seek the advice of a physician about any medical condition or symptom. Those with medical conditions should consult a medical professional about the appropriateness of taking dietary supplements or diet therapy, and how these methods will interact with their medications.
Antioxidant Activities of Flavonoids
Linus Pauling Institute
For detailed information about the chemical structure, formula, and molecular weight of various phytonutrients, visit
The US Dept of Agriculture.
Phytochemicals also gives detailed information on specific phytochemicals.
Search for herbs at the Sloan Kettering Memorial Cancer Center website.