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

Scope: After intrinsic labeling of spinach (Spinacia oleracea L., Chenopodiaceae) with ¹³ CO₂ , we investigated if labeled polyphenol metabolites were detectable in human plasma.

Methods and result: In a pilot intervention trial, five healthy men consumed 5 g freeze-dried ¹³ C labeled spinach, including a total amount of 160 μmol methoxyflavonols, including 70 μmol 5,3',4'-trihydroxy-3-methoxy-6,7-methylendioxyflavone-4'-glucuronide. Plasma samples of all subjects were analyzed with regard to their ¹³ C/¹² C ratio. Additionally, ¹³ C 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 ¹² C₁¹³ C₁₆ and ¹³ C₁₇ 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 ¹³ C 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 ¹³ C labeled polyphenol metabolites are detectable and quantifiable in human plasma.