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J Nutr Biochem. 2012 Jun;23(6):573-9. doi: 10.1016/j.jnutbio.2011.03.002. Epub 2011 Jul 19.

Equol production changes over time in postmenopausal women.

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Cancer Research Center of Hawaii, Clinical Sciences Program, University of Hawaii, Honolulu, HI 96813, USA.


Equol (EQ) is produced by intestinal bacteria from the soy isoflavone daidzein (DE) in 30%-60% of the population and is believed to provide benefits from soy intake. A robust EQ status definition is lacking, and it is uncertain whether EQ is formed consistently within an individual and ceases upon oral antibiotic treatment. In a randomized, double-blind, placebo-controlled soy intervention trial with 350 postmenopausal women, DE and EQ were analyzed by liquid chromatography/tandem mass spectrometry at baseline and every 6 months over 2.5 years in overnight urine, spot urine and plasma. Equol production changes and status (remaining an EQ producer or nonproducer or changing towards an EQ producer or nonproducer) were assessed. Equol status was determined most dependably by overnight urine applying as cutoff a ratio of EQ/DE≥0.018 with a DE threshold ≥2 nmol/mg creatinine: the soy and placebo groups had approximately 30% consistent EQ producers during the study, but 14% and 35%, respectively, changed EQ status (mean 1.4-1.7 times), while 27% and 17%, respectively, had antibiotic treatment (P<.01 for inverse association). No significant trend in change of EQ production or status was observed when overnight urine was limited to collections closest to before and after antibiotic treatment. Similarly, antibiotic type or class, duration, dose or time between antibiotic treatment and overnight urine collection showed no consistent influence on EQ production. Equol production can markedly change intraindividually over 2.5 years, and antibiotic treatment impacts it inconsistently. Factors other than antibiotic treatment must be considered as causes for EQ production changes.

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