Recently, I wrote why “super” fruits are no better than regular fruit. Among those reasons, is because constituents in fruit have other discovered roles beside functioning as antioxidants, and in vitro antioxidant activity does not measure these. I do not think antioxidant activity is a major reason why fruits are good for us.
A recent study with cranberry juice by Milbury et al. (1) may further support this.
Cranberries, and especially anthocyanins, which make up a significant portion of polyphenols in cranberry, have been studied on various pathologies such as cardiovascular contexts, cancer, and age-related cognition (see the super fruit article). However, as discussed by the authors, individual anthocyanins have a bioavailability of generally less than 1%, though they do accumulate in tissues. As discovered from other studies however, the low plasma concentrations do alter signal transduction and gene expressions in vivo.
15 subjects, fasted overnight consumed a 480 mL (~2 cups) “double-strength” cranberry juice of 54% juice with 835 mg total polyphenols. 94.47 mg were found to be anthocyanins after analysis. 6 anthocyanins (of 15 detected) with the highest concentration in the juice were measured at baseline and 4 hours after consumption.
All of the anthocyanins were at the highest concentration in plasma within 1.5 hours after consumption. Interestingly, glucose moieties on certain anthocyanins seem to increase bioavailability. The total percent of the original dose recovered in the urine was only 0.79 +/- 0.90% (range 0.078-3.2%). Though low, this is higher than some other studies when anthocyanins are administered alone or from different berries, suggesting, as stated by the authors: “these flavonoids may be absorbed and metabolized differently from various food matrices. Additional studies are needed on the effects of food matrix in absorption efficiency and stability of anthocyanins.”
This of course brings up the issue of supplements; they cannot use claims from studies such as this because it is possibly that small differences in pharmacokinetics between the compounds themselves and in conjunction with food matrices may have significant differences on measured endpoints.
The same group performed a long-term feeding study in pigs that showed accumulation of anthocyanins in tissues and a longer residence time in plasma. More studies will be needed to elucidate mechanisms, as we still don’t have a good understanding on how polyphenols truly contribute to good health in different contexts.
Importantly, the authors note: that the “concentration of cranberry anthocyanins achieved in plasma appear too low to compete effectively with antioxidants like ascorbate and glutathione in quenching free radicals” and “these findings do suggest that anthocyanin concentrations are too low to directly contribute to in vivo quenching of reactive oxygen species but may be adequate to influence signal transduction and gene expression pathways.”
They did not detail why they think this. However, plasma concentrations were between 0.56 and 4.64 nmol/L, and this is very small. In vitro studies use much higher concentrations, and ascorbate and glutathione, for example exist in plasma in the thousands of nmol/L. Clearly cranberries do something per animal and human studies thus far, but scavenging ROS, at least from anthocyanins, is probably not why.
1. Milbury PE, Vita JA, & Blumberg JB (2010). Anthocyanins are Bioavailable in Humans following an Acute Dose of Cranberry Juice. The Journal of nutrition, 140 (6), 1099-104 PMID: 20375263