Epidemiological evidence suggests that flavonoids are beneficial to the health and may protect from cancer and cardiovascular disease, in particular. However, controversy surrounds their mechanism of action, because the route of absorption, in vivo chemistry and cellular interactions are not fully understood. Evidence suggests that flavonoids are not generally absorbed in their glycoside forms, but instead are extensively metabolised by enterocytes prior to absorption to form more water soluble glucuronide, sulfate and methylated forms. Once in the circulation, flavonoids are metabolised further by the liver, producing an array of new metabolites that may have biological actions. Originally it was thought that flavonoids exert their beneficial effects through their antioxidant activity, with good evidence for this coming from in vitro studies. However, evidence is accumulating that suggests flavonoids may exert their effects, at least in part, through gene regulation.
Anthocyanins are a sub-class of flavonoid that are found in purple, blue and red fruits and vegetables, with their exact colour depending on the pH. Anthocyanins are derived from anthocyanindin aglycones and differ only in the addition of a glycoside residue, most commonly in the 3-position. Anthocyanidin aglycones include pelargonidin, cyanidin, delphinidin, peonidin, petunidin and malvidin, with their anthocyanin glycoside forms being common in the human diet. Strawberries are a rich source of the anthocyanin pelargonidin glycoside and their intakes in the summer months in many Western nations makes them nutritionally important. Pelargonidin glycosides are also present in high concentrations in radishes and their total intake is equal to around 3% of all anthocyanins. Some evidence suggests that anthocyanin may reach the plasma intact in their glycoside form, although this appears dependent on the type of anthocyanin ingested.
Researchers1 have investigated the dose response and metabolism of strawberry anthocyanins, using a cross-over design study on 12 healthy individuals. Subjects consumed 100, 200 or 400g of pureed strawberries which were estimated to contain 15, 30, and 60 µmol of anthocyanins, respectively. Analysis revealed that pelargonidin 3-glucoside was the main anthocyanin present in the strawberries, as would be expected. The urine of the subjects contained 3 monoglucuronides of pelargonidin 3-glucoside, suggesting that the flavonoid was absorbed and metabolised by the subjects. Increases in the quantity of strawberry puree resulted in increasing quantities of pelargonidin 3-glucoside metabolites in the urine, indicating that even with intakes of 60 µmol, absorption and metabolism of strawberry anthocyanins are not saturated. Interestingly, 98% of the anthocyanin dose was not recovered in the urine, suggesting that it was excreted in the faeces or stored in tissues.
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