Flavonoids are polyphenolic heterocyclic plant compounds that may have beneficial health properties in humans. Although it is not fully understood why consuming plant foods are beneficial to the health, one reason may be the flavonoids that they contain. Flavonoids are in vitro antioxidants, and some evidence suggests that they also perform this role in human tissues. However, flavonoids are extensively metabolised in the gut and liver and so the compounds present in plants are biotransformed before they enter circulation. The glycoside and aglycone forms found in plants are not the predominant forms found in plasma, instead glucuronide and sulphate conjugates are found in tissues. The effects of flavonoids in humans is therefore controversial, despite a large body of research showing health benefits. Despite this, a number of large epidemiological studies have shown inverse association between flavonoids and cardiovascular disease, suggesting that flavonoid metabolites are bioactive.
For example, researchers1 have investigated the association between flavonoid consumption and cardiovascular disease mortality amongst subjects in a large prospective nutritional study. The Cancer Prevention Study II Nutrition Cohort included 38,180 men and 60,289 women with mean ages of 70 and 69 years, respectively. Dietary data was collected by a frequent food questionnaire and cardiovascular mortality risk was calculated by assessing medical record over a 7 year follow-up period. Statistical analysis using Cox proportional hazards modelling showed that men and women with flavonoid intakes in the top quintile had a lower risk (relative risk: 0.82) compared to those in the lowest quintile. Individually, the flavonoid classes anthocyanidins, flavan-3-ols, flavones, flavonols and proanthocyanidins were associated with a lower risk of cardiovascular disease. Interestingly, many of the associations were non-linear, suggesting that optimal intakes may be able to be assigned with further study.
These results support previous studies that show a reduced risk of cardiovascular disease with flavonoid consumption. However, as with all epidemiological data, care should be taken when attempting to assign cause and effect and consideration given to the possibility of confounding variables. The authors noted that inverse associations between flavonoid intake and cardiovascular disease occurred for most flavonoids at intermediate intakes, suggesting that even small intakes of flavonoids may be beneficial at reducing the risk of cardiovascular disease. Also of interest was the fact that in men flavonoid intake was more protective of stroke than heart disease. This study reported an inverse association between both proanthocyanidins and anthocyanidins, and cardiovascular disease, something that has not been previously reported in a large scale epidemiological study. Just how flavonoids are protective of cardiovascular disease is not known, but it may relate to their antioxidant function or their ability to cause gene regulation.
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