Flavonoids are a group of dietary polyphenols present in many plant foods. Fruits and vegetables are rich sources of flavonoids, as too are tea and wine. Flavonoids have been researched with regard to their anti-cancer effects. Cell culture experiments have provided evidence that flavonoids may functions as cellular antioxidants in mammals, and this may explain their anti-cancer effects. Protection of the DNA molecules from free radicals could theoretically reduce the rate of cancer initiation or allow DNA repair. Reductions in the levels of oxidative stress may also explain some of the other health effects of flavonoids. Evidence from epidemiology support a role for flavonoids in protection from cancer. However, clinical trials involving humans have provided results that are less clear. Taken as a whole the data indicates that flavonoids may be protective of cancer, but the protective effects are likely modified by the dose of flavonoid, the type of flavonoid and the duration of consumption.
Epidemiological studies have shown for example that particular foods and particular flavonoids are associated with a reduced risk of particular types of cancer. For example, in one study1 researchers assessed the association between a number of flavonoid types and the risk of cancer in women. Researchers assessed the association between cancer and the flavonoid type flavonols, flavones, flavanones, anthocyanins, flavan-3-ols and polymeric flavonoids. During the follow-up period 723 cases of ovarian cancer were indicated by medical records. Analysis of the diets of subjects using a food-frequency questionnaire showed that total flavonoid intake was not associated with cancer risk. However, those with the highest intake of flavonols (e.g. quercetin, kaempferol and galangin) and flavanones (e.g. hesperidin and naringin) had a lower risk of ovarian cancer compared to those with the lowest intake. The association for flavanone intake was stronger for cases of serious invasive and poorly differentiated tumours.
These results suggest show that flavonol and flavanone intakes are associated with a reduced risk of ovarian cancer. However it is unclear if the flavonoids themselves are protective or whether consumption of the food they contain may be the protective factor. In addition, it could be that these foods are markers for the consumption of other foods that may be the protective factor. Apples, wine, tea are rich sources of flavonols. In fact the researchers investigated the effects of foods, and found that more than one cup of black tea per day was protective of ovarian cancer. Mechanisms are known by which flavonoid could protect from cancer, and so flavonoids have the potential to be protective. However, flavonoid absorption, metabolism and excretion is highly complex, and the flavonoids can not only interact with other nutrients and xenobiotics, but also genetic factor may have a modifying influence. These modifying effects explain the inconsistent result of studies with regard protective effects of flavonoids on disease.
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