Equol: history, chemistry, and formation

Abstract

Equol, first isolated from equine urine in 1932 and identified 50 years later in human urine as a metabolite of the soy isoflavones, daidzin and daidzein, is produced by intestinal bacteria in some, but not all, adults. This observation led to the term equol-producers to define those adults that could make equol in response to consuming soy isoflavones and the hypothesis that the health benefits of soy-based diets may be greater in equol-producers than in equol nonproducers. By virtue of a chiral center, equol occurs as a diastereoisomer and intestinal bacteria are enantiospecific in synthesizing exclusively the S-(-)equol enantiomer, an enantiomer that has selective affinity for the estrogen receptor-beta. Both enantiomers are of interest from a clinical and pharmacological perspective and are currently being developed as nutraceutical and pharmacological agents. The wide range of biological activities these enantiomers possess warrants their investigation for the treatment of a number of hormone-related conditions involving estrogen-dependent and androgen-related conditions. The following review describes the history, chemistry, and factors governing the intestinal bacterial formation of equol.

FIGURE 1

Cumulative number of publications on equol by year since its first identification in human urine.

FIGURE 2

Principal metabolic biotransformations of the soy isoflavones daidzein and genistein. The structure for daidzein is shown and genistein has the identical structure but with an additional hydroxyl group at position C-5 of the A-ring, highlighted by the wavy arrow.

FIGURE 3

Differences in the type of soy foods consumed by Western and Asian populations may account for differences in the frequency of equol-producers.