Anthocyanidins

The red to purple colored anthocyanidins are responsible for a good portion of color in fruits and flowers. They are only present as glycosides or anthocyanins and their color is pH dependent. In the human diet, anthocyanidins are present in red wine, certain varieties of cereals, certain leafy and root vegetables (e.g., aubergines, cabbage, beans, onions, and radishes) and most abundantly in fruit. The content is generally proportional to the color intensity and may reach values of 2-4 g/kg fresh weight in blackberries and black currants. They are found mainly in the skin, except where the flesh is also colored.

Anthocyanidins and anthocyanins have been reported previously as having several positive effects on health (35,65-72). Much of this evidence has been derived in vitro and very little is known about their bioavailability in vivo. Previous human and rat studies have reported very low recoveries of intact anthocyanins in urine (73). Very little is known of the specific fate of the balance of these compounds. Given their structure, it is likely that they will undergo substantial metabolism by the human gut microbiota in much the same way as any other flavonoid structure. And yet, studies performed in the 1970s indicated that degradation of anthocyanins by the microbiota occurs to a much more limited extent than with other flavonoid structures (61). However recent studies investigated in vitro whether the anthocyanin glycosides, cyanidin-3-glucoside, and cyanidin-3-rutinoside were deglycosylated and whether the resulting aglycones were degraded further to smaller phenolic compounds by colonic bacteria (74). Cyanidin-3-glucoside and cyanidin aglycone were identified as intermediary metabolites of cyanidin-3-rutinoside. Proto-catechuic acid was identified as a major metabolite at early stages of the fermentations along with a variety of other low molecular weight metabolites suggesting that the anthocyanins were converted by the gut microbiota. However, protocatechuic acid was also formed in vitro with the simple incubation of cyanidin with rat plasma in the absence of colonic microbiota (75). These experiments, although far from conclusive indicate that bacterial metabolism of anthocyanins can occur and is likely to involve the cleavage of glycosidic links and the breakdown of the anthocyanidin heterocycle—thus having a potential impact on the bioavailability of these compounds in vivo. However, significantly more investigation is needed before the real extent of the involvement of the microbiota is uncovered in terms of metabolism and the bioavailability of anthocyanins.

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