Long-term kinetics of daidzein and its main metabolites in human equol-producers after soymilk intake: identification of equol-conjugates by UPLC-orbitrap-MS and influence of the number of transforming bacteria on plasma kinetics

The main aim of the study was to establish in vivo a correlation between equol (EQU) production and a number of intestinal bacteria able to perform the transformation. Thus, healthy female volunteers were selected for their ability to convert slowly (n = 6, 10⁵–10⁹ cells/g wet feces) or quickly (n = 6, 10¹⁰–10¹² cells/g wet feces) daidzein (DAI) in EQU. After oral administration of 100 mg DAI in soymilk, plasma (0–99 h) and urine (0–96 h) samples were collected. DAI and its metabolites were determined by LC-MS/MS and EQU -conjugates by UPLC-High Resolution-MS. Only for EQU a direct correlation was found between the number of transforming microorganisms and parameters such as t_max and t_1/2 (p = 0.027). Peak serum concentration time, C_max, AUC_0–72 h and t_1/2 for total EQU (n = 12) were 36 ± 10 h, 89 ± 78 nM, 2.4 ± 1.7 (μmol × h/L) and 15.6 ± 3.3 h, respectively. In plasma and urine EQU was found mainly as 7-O-glucuronide.