Flavonoids are a group of polyphenols that include flavones, flavan-3-ols, flavanones, proanthocyanidins, anthocyanins and flavonols. As a group, flavonoids may have particular health effects in humans. It Is unclear how flavonoids exert their health effects but this may relate to their antioxidant properties. The free radical theory of disease suggests that the generation of free radicals is able to induce oxidative stress in cells and tissues and that this is a cause of disease. Such oxidative stress is implicated as a causative factor in the damage to the genetic information (DNA) in cells and this may in turn initiate the formation of cancer. One way that flavonoids may protect from disease may therefore be through the protection of DNA from free radical damage. Epidemiological evidence suggests that flavonoids may protect from cancer, however it is difficult to establish a cause and effect due to the confounding variables that are inherent in epidemiology.
For example, flavonoid intakes may simply be a marker for a healthy diet. Those with high quality diets rich in plant foods will have higher intakes of flavonoids, and it may be the diet rather than the flavonoids that are the protective factor. The association between flavonoids and cancer have been investigated by researchers, for example in one study by using responses from dietary questionnaires completed by subjects1. From this data, no overall associations were found between cancer and flavonoid intake. However, there was an inverse association between flavone (e.g. apigenin and luteolin in onions, peppers, parsley and thyme) intake and overall breast cancer risk, and an inverse association between flavan-3-ol intake (e.g. catechins in tea, chocolate and apples) and estrogen negative breast cancer risk. The weak association is typical of phytonutrients, and may indicate a weak protective effect or may signify that factors other than the flavonoids are actually responsible for the protective effects.
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