Endothelial protective effects of anthocyanins: the underestimated role of their metabolites

A very animated debate is still in prog- ress in the scientific community regarding the beneficial effects of anthocyanins (ACN) on endothelial function.We read with great interest the article by Zapolska-Downar et al. [1] who demon- strated the beneficial effects of an ACN- rich extract from Aronia melanocarpa E on human umbilical vein endothelial cells.It is not our intention to question the experimental findings of the above article. However, ACN have until recently been believed to be poorly absorbed in the native form. This notion was supported by abundant literature data reporting the recovery in urine and plasma of a few known metabolites (i.e. glucuroni- dated and methylated compounds) ac- counting for less than 1% of the ingested ACN [2, 3].Thus, the physiological effects of di- etary ACN (i.e. the huge increase of plas- ma antioxidant activity after ingestion of ACN-rich foods) appeared to be in strik- ing contrast with their poor bioavailabili- ty, unless attributable to yet unidentified ACN metabolites. In this regard, in a recent intervention study by Vitaglione et al. [4], 6 healthy vol- unteers consumed 1 liter of a commercial Sicilian red orange juice containing 71 mg of total cyanidine-3-glucoside (CyG) as major ACN. For the first time, the authors identified protocatechuic acid (PCA) as the major CyG metabolite, accounting for about 72% of ingested CyG, as the sum of44% in the serum and 28% in the feces. Moreover, considering its marked antioxi- dant properties [5], it is realistic to assume that PCA is the missing human CyG me- tabolite that effectively reaches tissues in a dose sufficient to exert any biological ef- fects. Although further confirmations are obviously needed, the finding reported by Vitaglione et al. [4] gives rise to important concerns on the scientific value of in vitro culture cell experiments to test the biolog- ical effects of ACN exclusively considered in their native form. In our opinion, con- sidering that (1) ACN are stable at acidic pH (such as in the stomach) but at physi- ological pH (such as in the bloodstream) easily convert to PCA and other hydroxy- benzoic acids [3], (2) following their inges- tion in humans, ACN undergo massive biotransformation [4], (3) cells and tissues are exposed in vivo to amounts of ACN in their native form probably too low to exert significant biological properties, and (4) because ACN metabolites, as in the case of PCA, can retain important biological properties [5], it is not adequate to predict any ACN health properties by exclusively assaying their native structures in cell cul- tures [6, 7]. Thus, the abundant literature data from in vitro cell culture experiments on these dietary compounds have to be thoroughly reconsidered. We believe that all future cell culture studies aimed to es- tablish the health properties of ACN, as well as of other polyphenols, will necessar- ily have to pay adequate attention to the biological properties of known metabo- lites instead of exclusively those of native dietary compounds, and the study by Za- polska-Downar et al. [1] gave us the oppor- tunity to emphasize this.