Polyphenol Phase-II Metabolites are Detectable in Human Plasma after Ingestion of C-13 Labeled Spinach-a Pilot Intervention Trial in Young Healthy Adults

Scope: After intrinsic labeling of spinach (Spinacia oleracea L., Chenopodiaceae) with (CO2)-C-13, we investigated if labeled polyphenol metabolites were detectable in human plasma. Methods and result: In a pilot intervention trial, five healthy men consumed 5g freeze-dried C-13 labeled spinach, including a total amount of 160mol methoxyflavonols, including 70 mu mol 5,3,4-trihydroxy-3-methoxy-6,7-methylendioxyflavone-4-glucuronide. Plasma samples of all subjects were analyzed with regard to their C-13/C-12 ratio. Additionally, C-13 labeled metabolites of patuletin, spinacetin, and 5,3,4-trihydroxy-3-methoxy-6,7-methylendioxyflavone (TMM) were analyzed in plasma samples in a subgroup of three subjects. TMM-glucuronide, TMM-sulfate, and spinacetin-glucuronide-sulfate, the latter as (C1C16)-C-12-C-13 and C-13(17) isotopologs, were tentatively identified. Plasma concentration of TMM-glucuronide and TMM-sulfate reached c(max) from 19.1-54.3 and 22.5-125.5 nmol L-1, respectively, 7-9 h post-ingestion. Conclusion: It seems likely that C-13 labeled TMM-glucuronide and TMM-sulfate are phase-II metabolites which were converted after colonic transformation. Variations in plasma kinetics were observed for these two metabolites and may be attributed to the individual composition of the microbiota. We conclude that C-13 labeled polyphenol metabolites are detectable and quantifiable in human plasma.