Flavonoids can act as reducing agent in vitro, and from these cell studies, it is often assumed that flavonoids have an antioxidant activity in vivo. However, this line of reasoning is flawed for a number of reasons. Firstly, it is almost certain now, that flavonoids do not reach the blood in their aglycone or glycoside form, save for a very limited number of exception including some anthocyanins. Instead, most flavonoids are extensively metabolised in the gut, forming methylated, glucuronidated and sulphated forms1, which are subsequently absorbed and contribute the majority of flavonoids in the plasma. Secondly, flavonoids are known to regulate gene expression, and this may be their primary mode of action over and above their role as antioxidants. In fact there is good evidence that the beneficial effects of flavonoids are as a result of gene expression. Lastly, the cancer preventative mechanism of flavonoids may require them to activate protein kinases in their oxidised form. Therefore their role as in vivo antioxidants is controversial.
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