The soy isoflavones daidzein and genistein produce several biological activities related to health benefits. A number of isoflavone extracts are commercially available, but there is little information concerning the specific isoflavone content of these products or differences in their bioavailability and pharmacokinetics. This study describes the development and validation of an analytical method to detect and quantify daidzein, genistein, and equol in human plasma using liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). The method was applied in a crossover, randomized, bioavailability study. Twelve healthy volunteers were administered the same total isoflavones dose from two isoflavone supplement preparations (Super-Absorbable Soy Isoflavones (Life Extension, USA) and Fitoladius (Merck, Spain)). The pharmacokinetic parameters (AUC0-24/dose and Cmax/dose) of the isoflavones from the two preparations differed significantly. Such differences in bioavailability and kinetics may have relevant effects on the health benefits derived from their intake.

Funded by grants from the Ministerio de Sanidad (Investigación Clínica Independiente, EC-10-166), Fondo de Investigación Sanitaria-ISCIII-FEDER RTA (RD12/0028/0009), CIBEROBN (CB06/03/0028), Generalitat de Catalunya (AGAUR 2014SGR680, predoctoral fellowship FI-DGR 2013 for JRM) and PN I+D+I 2008-2011 and ISCIII CA11/00215 for NP. CPM and EP are Rio Hortega fellows (ISC-III, CM12/00085 and CM13/00016). The author's work was independent of the sources of funding.