Cited by 473Phytoestrogen effects and researchPhytochemicals and Antioxidant ActivitiesEstrogen and related hormone effects

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S-Equol, a potent ligand for estrogen receptor β, is the exclusive enantiomeric form of the soy isoflavone metabolite produced by human intestinal bacterial flora1–4,

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Setchell, Kenneth DR, Clerici, Carlo, Lephart, Edwin D, Cole, Sidney J, Heenan, Claire, Castellani, Danilo, Wolfe, Brian E, Nechemias-Zimmer, Linda, Brown, Nadine M, Lund, Trent D, Handa, Robert J, Heubi, James E·2005·The American Journal of Clinical Nutrition

DOI 10.1093/ajcn/81.5.1072

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Abstract

Background The discovery of equol in human urine more than 2 decades ago and the finding that it is bacterially derived from daidzin, an isoflavone abundant in soy foods, led to the current nutritional interest in soy foods. Equol, unlike the soy isoflavones daidzein or genistein, has a chiral center and therefore can occur as 2 distinct diastereoisomers.Objective Because it was unclear which enantiomer was present in humans, our objectives were to characterize the exact structure of equol, to examine whether the S- and R-equol enantiomers are bioavailable, and to ascertain whether the differences in their conformational structure translate to significant differences in affinity for estrogen receptors.Design With the use of chiral-phase HPLC and mass spectrometry, equol was isolated from human urine and plasma, and its enantiomeric structure was defined. Human fecal flora were cultured in vitro and incubated with daidzein to ascertain the stereospecificity of the bacterial production of equol. The pharmacokinetics of S- and R- equol were determined in 3 healthy adults after single-bolus oral administration of both enantiomers, and the affinity of each equol enantiomer for estrogen receptors was measured.Results Our studies definitively establish S-equol as the exclusive product of human intestinal bacterial synthesis from soy isoflavones and also show that both enantiomers are bioavailable. S-equol has a high affinity for estrogen receptor beta (K(i) = 0.73 nmol/L), whereas R-equol is relatively inactive.Conclusions Humans have acquired an ability to exclusively synthesize S-equol from the precursor soy isoflavone daidzein, and it is significant that, unlike R-equol, this enantiomer has a relatively high affinity for estrogen receptor beta.

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Authors: Setchell, K. D., Clerici, C., Lephart, E. D., Cole, S. J., Heenan, C., Castellani, D., Wolfe, B. E., Nechemias-Zimmer, L., Brown, N. M., Lund, T. D., Handa, R. J., & Heubi, J. E.
Affiliations: Brigham Young University (US)
Corresponding author: Setchell, Kenneth DR
ORCID: Setchell, Kenneth DR
Clerici, Carlo
Lephart, Edwin D
Castellani, Danilo
Handa, Robert J
Venue: The American Journal of Clinical Nutrition
Publisher: Elsevier BV
Published: 2005-05
Volume: 81
Issue: 5
Pages: 1072-1079
ISSN: 0002-9165 (ISSN-L)
DOI: 10.1093/ajcn/81.5.1072
OpenAlex: W2118089000
Cited by: 471
References: 59
Citations by year: 2026: 21 · 2025: 19 · 2024: 14 · 2023: 24 · 2022: 25 · 2021: 26 · 2020: 22 · 2019: 15 · 2018: 22 · 2017: 16 · 2016: 19 · 2015: 27 · 2014: 21 · 2013: 31 · 2012: 26
FWCI: 13.68
Type: journal article
Language: en
Open access: bronze
doi.org
Licence: www.elsevier.com/tdm/userlicense/1.0/ (tdm)
www.elsevier.com/open-access/userlicense/1.0/ (vor, 6511 d)
Locations: American Journal of Clinical Nutrition (publishedVersion)
PubMed (publishedVersion)
Funders: National Institutes of Health (5r01ca073328-03, RR08084)
National Cancer Institute (R01CA73328)
National Center for Research Resources (M01RR008084, RR08084)
Topics: Health Sciences › Nursing › Nutrition and Dietetics › Phytoestrogen effects and research
Health Sciences › Medicine › Biochemistry (medical) › Phytochemicals and Antioxidant Activities
Life Sciences › Biochemistry, Genetics and Molecular Biology › Endocrinology › Estrogen and related hormone effects
Keywords: S-equol, equol, daidzein, soy isoflavones, estrogen receptor beta, intestinal bacterial metabolism, gut microbiota, enantiomers, stereospecificity, bioavailability, pharmacokinetics, human urine, soy products
MeSH terms: Animals, Biological Availability, Chromatography, High Pressure Liquid, Female, Half-Life, Humans, Intestines, Isoflavones, Male, Gas Chromatography-Mass Spectrometry, Stereoisomerism, Rats, Sprague-Dawley, Area Under Curve, Phytoestrogens, Rats, Equol

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S-Equol, a potent ligand for estrogen receptor β, is the exclusive enantiomeric form of the soy isoflavone metabolite produced by human intestinal bacterial flora1–4,

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