This 2017 review challenged snapshot measurements of biological availability:
“There is a general belief that anthocyanins, flavanones, and other polyphenols are poorly bioavailable with only relatively small amounts of ingested dose entering systemic circulation in the form of metabolites. When lower molecular weight phenolic and aromatic ring-fission catabolites produced primarily by colonic microbiota are taken into account, it is evident that anthocyanins and flavanones are much more bioavailable than previously envisaged.
Although plasma pharmacokinetic measurements provide a snapshot of absorbed circulating metabolites, 0–24-h urinary excretion of both metabolites absorbed in the small intestine and catabolites of distal gastrointestinal (GI) origin that are products of bacterial processing provide a more quantitative reflection of polyphenol absorption. Overall 0–48-h urinary recovery of phenolic compounds – after baseline subtraction – was 43.9 ± 8.0 μmol, which is equivalent to 15% of ingested anthocyanins.
With orders of magnitude higher plasma/serum Cmax levels and significantly longer half-lives, evidence points toward lower molecular weight phenolic and aromatic catabolites being the primary bioavailable products of anthocyanin consumption. Gut-derived catabolites can often exert higher bioactivity than their precursor flavonoid structures.”
https://www.annualreviews.org/doi/full/10.1146/annurev-food-030216-025636 “Anthocyanins and Flavanones Are More Bioavailable than Previously Perceived: A Review of Recent Evidence” (not freely available)
Much of this review’s anthocyanin section was dedicated to a coauthor’s 9-person study where they ate a huge amount of raspberries. Its flavanone section was similarly influenced by another coauthor’s human orange juice studies.
I’d like to see stronger evidence before reviewer statements become faits accomplis, elevated through citations to become indisputable facts. Its underlying point that studies could take more and varied measurements over extended periods seems amenable to evidence.