Flavonoid absorption is complex and not completely understood. However, it is known that on extensive metabolism occurs between the gastrointestinal tract and the plasma, such that the forms of flavonoid ingested are not reflective of the forms detected in plasma. Flavonoids occur in nature mainly as glycosides or in their free form as aglycones. However, the main forms detected in plasma are glucuronide, sulfate and mixed conjugates of the free form or methylated flavonoid. The distal portion of the small intestine contains the β-glucosidase enzyme lactase, that is able to cleave the sugar moiety from the flavonoid, and this allow diffusion of the aglycone across the apical surface of the enterocytes. Once inside the enterocytes the flavonoids are extensively metabolised by phase I and phase II detoxification enzymes, resulting in little of the aglycone remaining to enter the plasma.
However, evidence suggests that the anthocyanin group of flavonoids appear in the plasma of rats and humans in their glycoside form. For example, in rats ≈25% of the anthocyanins administered appear in the plasma as glycosides following absorption from the stomach1. Anthocyanin absorption from the stomach is rapid with intact glycosides and metabolised forms being quickly excreted in the bile. In another study2, blueberry, boysenberry, black raspberry and blackcurrant anthocyanins were detected unmetabolised in the urine of rats and humans following dosing. Differences in the levels of anthocyanins in the urine suggested different absorption characteristics based on structure. Elderly women fed 720 mg anthocyanins3 had glycoside forms detected in their plasma and urine with peak plasma levels occurring at 71.3 min and most anthocyanins being excreted within 4 hours.
Evidence suggests that only the anthocyanin group of flavonoids are absorbed in their glycosylated forms. Other studies support the three cited here and suggests that the effect is common to all mammals. As with all flavonoids, the absorption of anthocyanins is complex and not fully understood, but differences clearly exist between the routes of absorption. Anthocyanins are stable at acidic pH and some evidence from animal studies suggests they may be absorbed from the stomach. Some evidence also suggests that grape anthocyanins can permeate the mammalian stomach mucosa. Aglycones of other flavonoids can be absorbed through the gastric mucosa, but glycosides of other flavonoids are not though to be able to pass via this route to the plasma. However, the lipophilic nature and large molecular weight of some anthocyanins may suggest active transport mechanisms are present, at least in some cases.
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