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 daid

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 β (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 β.

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 × 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 μg/L) and 16–1401 nmol/L (4.0–356 μg/L), respectively, whereas in urine the corresponding concentrations ranged from 16–12,574 nmol/L (4–3043 μg/L) and 539–26,834 nmol/L (137–6816 μg/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.

The pharmacokinetics of isoflavones in 10 healthy women were determined from serum appearance/disappearance concentration profiles and urinary excretions after single-bolus ingestion of 10, 20 or 40 g of soy nuts delivering increasing amounts of the conjugated forms of daidzein (6.6, 13.2 and 26.4 mg) and genistein (9.8, 19.6 and 39.2 mg). Peak serum daidzein and genistein concentrations were attained after 4–8 h, and elimination half-lives were 8.0 and 10.1 h, respectively. There were no differences in the pharmacokinetics of daidzein and genistein between pre- and postmenopausal women, indicating absorption and disposition of isoflavones to be independent of age or menopausal status. A curvilinear relationship was observed between the bioavailability of daidzein and genistein, apparent from the area under the curve to infinity (AUC_inf) of the serum concentration-time profiles and the amount of isoflavones ingested. The mean fraction of the isoflavones excreted in urine decreased with increasing intake when expressed as a percentage of the administered dose (63.2 ± 8.0, 54.4 ± 8.1 and 44.0 ± 4.3%, respectively, for daidzein, and correspondingly, 25.2 ± 5.3, 13.4 ± 2.1 and 15.8 ± 2.7% for genistein), underscoring the trend toward nonlinear pharmacokinetics. Equol was identified as a metabolite in 30% of women; it was present consistently in urine and blood from the same subjects. Its delayed appearance was consistent with colonic synthesis. On the basis of the pharmacokinetics, optimum steady-state serum isoflavone concentrations would be expected from modest intakes of soy foods consumed regularly throughout the day rather than from a single highly enriched product.

Equol [7-hydroxy-3-(4′-hydroxyphenyl)-chroman], an isoflavan produced by intestinal bacteria in response to soy isoflavone intake in some but not all

Background & Aims The final step in bile acid synthesis involves conjugation with glycine and taurine, which promotes a high intraluminal micellar concentration to facilitate lipid absorption. We investigated the clinical, biochemical, molecular, and morphologic features of a genetic defect in bile acid conjugation in 10 pediatric patients with fat-soluble vitamin deficiency, some with growth failure or transient neonatal cholestatic hepatitis. Methods We identified the genetic defect that causes this disorder using mass spectrometry analysis of urine, bile, and serum samples and sequence analysis of the genes encoding bile acid-CoA:amino acid N-acyltransferase (BAAT) and bile acid-CoA ligase (SLC27A5). Results Levels of urinary bile acids were increased (432 ± 248 μmol/L) and predominantly excreted in unconjugated forms (79.4% ± 3.9%) and as sulfates and glucuronides. Glycine or taurine conjugates were absent in the urine, bile, and serum. Unconjugated bile acids accounted for 95.7% ± 5.8% of the bile acids in duodenal bile, with cholic acid accounting for 82.4% ± 5.5% of the total. Duodenal bile acid concentrations were 12.1 ± 5.9 mmol/L, which is too low for efficient lipid absorption. The biochemical profile was consistent with defective bile acid amidation. Molecular analysis of BAAT confirmed 4 different homozygous mutations in 8 patients tested. Conclusions Based on a study of 10 pediatric patients, genetic defects that disrupt bile acid amidation cause fat-soluble vitamin deficiency and growth failure, indicating the importance of bile acid conjugation in lipid absorption. Some patients developed liver disease with features of a cholangiopathy. These findings indicate that patients with idiopathic neonatal cholestasis or later onset of unexplained fat-soluble vitamin deficiency should be screened for defects in bile acid conjugation.