Metabolites of dietary (soya) isoflavones in human urine

This study was undertaken to better understand the metabolic fate of dietary isoflavones in humans. Twelve volunteers were challenged with soya flour and urinary diphenol levels were then determined by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). The presence of previously described urinary diphenols was confirmed, i.e. the isoflavones, daidzein and genistein; the isoflavonoid metabolites, equol, dihydrodaidzein (Int-O-D), O-desmethyl-angolensin (O-Dma); the lignan, enterolactone. Diphenols detected for the first time were the isoflavone, glycitein and five novel isoflavonoid metabolites which are tentatively identified as 6'-hydroxy-O-desmethylangolensin (6' OH-O-Dma), dihydrogenistein (Int-O-G), dehydro-O-desmethylangolensin (dehydro-O-Dma) and two isomers of tetrahydrodaidzein. Urinary excretion rates of the three isoflavones (daidzein, genistein, glycitein) over a 3-day period following soya challenge showed moderate variation (4x, 6x and 12x, respectively) between the 12 individuals suggesting some individual variabilities in ability to deconjugate and to absorb dietary isoflavones. However, urinary excretion rates of each of three major isoflavonoid metabolites (equol, O-Dma, 6' OH-O-Dma) showed more marked variation (922x, 17x, 15x, respectively); while some of this variability may reflect varying individual ability to ferment dietary isoflavones per se, an inverse relationship was found between urinary levels of equol and both O-Dma and 6' OH-O-Dma suggesting individual variability in the preferred metabolic pathways of dietary isoflavones.