S-equol, a potent ligand for estrogen receptor beta, is the exclusive enantiomeric form of the soy isoflavone metabolite produced by human intestinal bacterial flora.

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.

Variations in isoflavone levels in soy foods and soy protein isolates and issues related to isoflavone databases and food labeling.

The reliability of databases on the isoflavone composition of foods designed to estimate dietary intakes is contingent on the assumption that soy foods are consistent in their isoflavone content. To validate this, total and individual isoflavone compositions were determined by HPLC for two different soy protein isolates used in the commercial manufacture of soy foods over a 3-year period (n = 30/isolate) and 85 samples of 40 different brands of soy milks. Total isoflavone concentrations differed markedly between the soy protein isolates, varying by 200-300% over 3 years, whereas the protein content varied by only 3%. Total isoflavone content varied by up to 5-fold among different commercial soy milks and was not consistent between repeat purchases. Whole soybean milks had significantly higher isoflavone levels than those made from soy protein isolates (mean +/- SD, 63.6 +/- 21.9 mg/L, n = 43, vs 30.2 +/- 5.8 mg/L, n = 38, respectively, p < 0.0001), although some isolated soy protein-based milks were similar in content to "whole bean" varieties. The ratio of genistein to daidzein isoflavone forms was higher in isolated soy protein-based versus "whole bean" soy milks (2.72 +/- 0.24 vs 1.62 +/- 0.47, respectively, p < 0.0001), and the greatest variability in isoflavone content was observed among brands of whole bean soy milks. These studies illustrate large variability in the isoflavone content of isolated soy proteins used in food manufacture and in commercial soy milks and reinforce the need to accurately determine the isoflavone content of foods used in dietary intervention studies while exposing the limitations of food databases for estimating daily isoflavone intakes.

Method of defining equol-producer status and its frequency among vegetarians.

7-Hydroxy-3-(4'-hydroxyphenyl)-chroman (S-equol) is a specific end-metabolite formed in the biotransformation of the dietary soy isoflavones daidzin and daidzein by intestinal bacteria. The frequency of equol production varies among individuals and populations, and it is suggested that the efficacy of soy foods differs depending on the ability of an individual to produce equol. To develop a standardized approach to define equol-producer status that can be universally adopted to differentiate these 2 distinct populations, we measured isoflavones in serum and urine collected from a cohort of 41 healthy adults, comprising 29 vegetarians and 12 nonvegetarians, after consuming 2 x 250 mL/d soy milk on 3 consecutive days. Serum and urinary daidzein and S-equol concentrations were analyzed by MS. Serum S-equol and daidzein concentrations ranged from 10.3-139 nmol/L (2.5-33.6 microg/L) and 16-1401 nmol/L (4.0-356 microg/L), respectively, whereas in urine the corresponding concentrations ranged from 16-12,574 nmol/L (4-3043 microg/L) and 539-26,834 nmol/L (137-6816 microg/L), respectively. The log10-transformed urinary S-equol:daidzein ratio provided a clearer distinction of equol-producer status than the absolute serum or urinary S-equol concentrations because it is independent of isoflavone intake and minimizes interindividual variation in isoflavone pharmacokinetics or differences in analytical methodologies. A threshold value for the log10-transformed urinary S-equol:daidzein ratio of -1.75 provided a demarcation to define equol-producer status. The frequency of equol producers in the vegetarians was 59%, similar to the reported frequency in Japanese adults consuming soy, and much higher than for nonvegetarian adults (25%), suggesting that dietary components other than soy influence S-equol synthesis by intestinal bacteria.