Why is bioavailability of anthocyanins so low?

Anthocyanins are water-soluble plant pigments conferring blue to pink colour to plant organs [1]. Nearly 600 different molecules have been identified so far, 97% of which occurs as glycosylated compounds. Their aglycone moiety is a flavonoid named anthocyanidin. The six most common anthocyanidins display various hydroxylation and methoxylation patterns. The repertoire of anthocyanins is so large, because various types of glycosyl moieties are bound to the aglycone core [1]. These pigments occur not only in petals, but also in fruits, vegetables and grains [2] and thus are constituents of the human diet. Their intake is highly variable, depending on the consumption of anthocyanin-rich food; data collected in the United States lead to estimate that anthocyanin daily individual intake may span from 12.5 [3] to 650 mg [4]. Following ingestion, anthocyanins are detected intact in blood [5, 6] in a time lapse considerably shorter than that observed with other dietary flavonoids [7]. However, the anthocyanins concentrations in plasma barely exceed 10-7 M, which translates into less than 0.1% absorption, including anthocyanin metabolites [8]. These features are indicative of various biochemical issues underlying the quite limited bioavailability of anthocyanins in mammalian organisms.