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ORIGINAL RESEARCH COMMUNICATION |
1 From the Nutrition, Food and Health Research Centre, the Department of Nutrition and Dietetics, King's College London (HW, KC, and RD); TNO-BIBRA International, Surrey, United Kingdom (EAB and MD); the Northern Ireland Centre for Diet and Health, University of Ulster, Coleraine, United Kingdom (IRR); and the DEFRA Central Science Laboratory, York, United Kingdom (ASL, AM, RT, and DBC)
2 Any opinions expressed herein represent the personal views of the authors and are not policy statements of the UK Food Standards Agency. 3 Supported by the UK Food Standards Agency Phytoestrogen Research Programme (project T05011 to HW). 4 Address reprint requests to H Wiseman, Nutrition, Food and Health Research Centre, the Department of Nutrition and Dietetics, King's College London, Franklin-Wilkins Building, 150 Stamford Street, London, SE1 9NN, United Kingdom. E-mail: helen.wiseman{at}kcl.ac.uk.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Objective: We sought to determine whether chronic soy consumption influences isoflavone bioavailability as judged by plasma isoflavone concentrations and modified gut microflora activities [ß-glucoside hydrolysis and equol and O-desmethylangolensin (O-DMA) production]. We also examined whether sex differences in isoflavone metabolism exist.
Design: A randomized, parallel, controlled study design was used to compare a high-soy diet (104 ± 24 mg total isoflavones/d) with a low-soy diet (0.54 ± 0.58 mg total isoflavones/d) in 76 healthy young adults for 10 wk.
Results: Concentrations of isoflavones and their gut microflora metabolites in the plasma, urine, and feces were significantly higher in the subjects who consumed the high-soy diet than in those who consumed the low-soy diet. Concentrations of O-DMA in plasma and urine were higher in the men than in the women. Fecal bacteria from subjects consuming both diets could convert daidzein to equol ex vivo. Fecal ß-glucosidase activity was significantly higher in the subjects who consumed the high-soy diet than in those who consumed the low-soy diet.
Conclusions: Although interindividual variation in isoflavone metabolism was high, intraindividual variation was low. Only concentrations of O-DMA in plasma and urine appeared to be influenced by sex. Chronic soy consumption does not appear to induce many significant changes to the gut metabolism of isoflavones other than higher ß-glucosidase activity.
Key Words: Soy • isoflavone • bioavailability • metabolism • gut microflora • equol
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
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After ingestion, soy isoflavones are hydrolyzed by gut bacterial glucosidases and by mammalian lactase phlorizin hydrolase (18), which release the aglycons, daidzein, genistein, and glycitein. These may be absorbed or further metabolized by the gut microflora to metabolites, including the conversion of daidzein to equol and or O-desmethylangolensin (O-DMA) and of genistein to p-ethyl phenol. More recently, 4-hydroxyphenyl propionic acid was identified from the metabolism of genistein in rats (19). Studies have shown that particular bacterial groups are involved in the metabolism of the isoflavone glycosides (20).
The importance of the gut microflora in the metabolism of soy isoflavones has been shown. Antibiotic administration blocks isoflavone metabolism, and germfree animals do not excrete the metabolites (21). Furthermore, only germfree rats colonized with microflora from a good equol producer excrete equol when fed soy (22). The extent of gut microflora metabolism in humans is variable: 
35% of a Western population can produce equol (5, 14, 23). Dietary modification, such as feeding wheat bran or soy protein, has been unsuccessful at changing equol-producing capability (24), which suggests that the intestinal microflora of an individual is relatively stable and resistant to change. The current study investigates whether chronic soy consumption influences isoflavone bioavailability as judged by plasma isoflavone concentrations and modified gut microflora activities toward isoflavones and investigates sex differences in isoflavone metabolism.
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SUBJECTS AND METHODS |
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TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Subjects
A total of 76 subjects were recruited, started, and successfully completed the study: 38 subjects consumed the high-soy diet and 38 subjects consumed the low-soy diet. The study protocol was reviewed and approved by the Research Ethics Committee at King's College London, and all participants gave their written informed consent. The subjects were recruited from the students and staff at King' College London. They were aged 18–45 y; were nonsmokers; and had fasting serum cholesterol concentrations <6.5 mmol/L, triacylglycerol concentrations <3 mmol/L, body mass index (in kg/m2) <30, normal liver function and hematology, and alcohol intake of <21 units/wk. Potential subjects with gastrointestinal pathology or who had consumed antibiotics within 3 mo of study commencement were excluded. The subjects were asked to keep a prestudy 7-d weighed dietary record to establish usual nutrient and energy intakes. Body weights were recorded on a beam balance.
Dietary intervention
Each subject received 1 of 2 supervised experimental diets (high-soy or low-soy) for 10 wk in the metabolic unit of the Department of Nutrition and Dietetics, King's College London. Meals were prepared and eaten in the unit whenever possible, and takeaway meals were provided as appropriate to ensure maximum compliance. Each diet comprised 20 foods of either high or low soy content from which an 11-d rotating menu was constructed, as reported previously (25).
Food preparation
The commercially prepared high- and low-soy foods were purchased from supermarkets in London. The "home-prepared" high- and low-soy foods were prepared in the metabolic kitchen of the Department of Nutrition and Dietetics, King's College London. The food samples were freeze-dried to a constant weight, powdered, and stored at –70 °C until analyzed for isoflavone (daidzein, genistein, and glycitein) aglycon, and glucoconjugate contents. These results were reported previously (25).
Collection, handling, and storage of biological samples
Venous blood samples were collected into 10-mL evacuated tubes containing lithium heparin (for isoflavone and metabolite analysis) at baseline, midway through the study, and at the end of the study after the subjects had fasted overnight. The blood samples were centrifuged at 1500 x g for 10 min at 4 °C, and the plasma was separated, portioned, and stored at –70 °C until analyzed.
Subjects made 24-h urine collections at baseline, midway through the study, and at the end of the study. Ascorbic acid (0.2 g) was added to the collection vessels as a preservative, the urine volume was measured, and the portions were stored at –70 °C with sodium azide (100 mg/L) as a preservative.
At baseline, midway through the study, and at the end of the study, the subjects collected a fresh fecal sample into a plastic bag within a sealable plastic container. The samples were then frozen at –70 °C until analyzed for enzyme activity. A portion of the sample was freeze-dried to a constant weight and was milled by using a pestle and mortar. The dried, milled fecal sample was analyzed for isoflavone and metabolites. A portion of the fresh midpoint fecal sample was used for the fecal incubations.
Analytic methods
Isoflavone and metabolite analysis
Biological samples were analyzed for their content of isoflavones and metabolites (daidzein, genistein, glycitein, equol, and O-DMA). All liquid chromatography–mass spectrometry (LC-MS) separations were performed on a YMC-Pack ODS-AM HPLC column (Crawford Scientific, Stathaven, United Kingdom) by using a water-acetonitrile gradient containing 0.5% glacial acetic acid according to an adaptation of the method of Wang and Murphy (26). Gas chromatography–mass spectrometry (GC-MS) was conducted according to an adaptation of the method of Adlercreutz et al (27). An HP1 column (0.22 mm x 25 m with a 0.11-µm methyl siloxane bonded phase) was used (Agilent Technologies, Cambridge, United Kingdom). Derivatized samples (2.0 µL) were eluted by using a 75 °C to 300 °C temperature program, with a 45-min cycle time. All results are corrected for procedural losses of each analyte.
Plasma isoflavones and metabolites
Plasma isoflavones were measured by isotope dilution GC-MS. Carbon-13 internal standards ([13C3]daidzein and [13C3]genistein) were added to the plasma samples (0.5 mL), which were deconjugated at pH 5.0 (acetate buffer) by incubating them overnight with excess units (50 000 units/mL) of ß-glucuronidase–sulfatase. Samples were drawn onto Oasis HLB solid-phase extraction cartridges (Waters Corp, Bedford, United Kingdom) and were washed with water, aqueous ammonia, and further water. The columns were dried and the analytes were eluted with methanol (2% acetic acid). N,O-bis-(trimethyl-silyl)trifluoroacetamide derivatives were analyzed by GC-MS in selected ion monitoring (SIM) mode. The mean values and mean intraassay imprecision for analytes in a naturally incurred quality-control sample were as follows: daidzein, 631 nmol/L (CV: 2%); genistein, 1472 nmol/L (CV: 6%); glycitein, 0 nmol/L (no CV); O-DMA, 50 nmol/L (CV: 6%); and equol, 35 nmol/L (CV: 5%).
Urinary isoflavones and metabolites
Urinary isoflavones were measured by isotope dilution LC-MS according to an adaptation of the extraction method of Lu et al (28). Carbon-13 internal standards (5 µg each of [13C3]daidzein and [13C3]genistein from Nigel Botting, University of St Andrews, St Andrews, United Kingdom) were added to urine samples (3 mL), which were deconjugated by incubating them overnight at pH 5.0 (acetate buffer) with ß-glucuronidase–sulfatase (Helix pomatia extract; Sigma Chemical Co, Poole, United Kingdom). Samples were neutralized with ammonium carbonate and were then allowed to absorb onto Chem-elut CE1010 solid-phase extraction cartridges (Chrompack, London). Analytes were eluted with ethylacetate, followed by diethyl ether. Solutions of extracts in 15% aqueous acetonitrile were analyzed by using LC-MS in +APcI mode by SIM. The mean values and mean intraassay imprecision for analytes in a naturally incurred quality-control sample were as follows: daidzein, 3.99 mg/L (CV: 1.0%); genistein, 1.60 mg/L (CV: 2.0%); glycitein, 0.04 mg/L (CV: 11%); O-DMA, 5.11 mg/L (CV: 0.3%); and equol, 2.76 mg/L (CV: 1.2%).
Fecal isoflavones and metabolites
Fecal isoflavones (including those in fecal incubations) were measured by isotope dilution LC-MS according to an adaptation of the extraction method of Xu et al (29). Carbon-13 internal standards ([13C3]daidzein and [13C3]genistein) were added to freeze-dried feces samples (0.5 g), which were extracted into acidic acetonitrile. Isoflavones in fecal incubations were measured as for feces extracts from this point forward. After hydrochloric acid–mediated deconjugation (0.017 mol/L in 17% acetonitrile:water at 45 °C for 1 h), extracts were drawn onto Oasis HLB solid-phase extraction cartridges and were washed with water, aqueous ammonia, and further water. The columns were dried and the analytes were eluted with methanol (2% acetic acid). Ethanol solutions of extracts were analyzed by using LC-MS in +APcI by SIM. The mean intraassay imprecision (CV) for analytes in a dried feces sample spiked with the aglycons at 5 mg/kg were as follows: daidzein, 4%; genistein, 6%; glycitein, 3%; O-DMA, 5%; and equol, 9%.
Fecal enzyme activity
Frozen fecal samples were defrosted and kept on ice while processed. A sample of 1 g of feces was weighed out and diluted 10-fold with phosphate buffer (0.1 mol/L, pH 7.0). Five glass beads were added, and the sample was mixed by vigorous vortexing for 30 s. The samples were then centrifuged to sediment the food debris (1500 x g for 10 min at 4 °C), and the supernatant fluid was decanted and analyzed for enzyme activity.
Fecal bacterial ß-glucuronidase and ß-glucosidase analyses were performed at 37 °C under an anaerobic atmosphere by using 3 mmol 
-nitrophenol-ß-D glucuronide/L and 3 mmol -nitrophenol-ß-D glucopyranoside/L as substrates, respectively. The mean interassay CVs were 19% and 14%, respectively, and the mean intraassay CVs were 5.0% and 4.6%, respectively.
Fecal incubations
Fresh fecal samples were weighed within an anaerobic cabinet maintained at 37 °C. A 10% slurry was made with prereduced anaerobic brain heart infusion medium by adding 0.5 mL of the fecal slurry to 25.5 mL of the medium. At time 0, the substrate daidzein was added to the incubation medium at a final concentration of 125 µmol/L, and the sample was mixed. The samples were centrifuged (1500 x g for 10 min at 4 °C), and the supernatant fluid was decanted and stored at –70 °C until analyzed. The sampling procedure was repeated after 72 h. Additional incubations were carried out with soy flour.
Statistical analysis
The statistical analysis was carried out by using GENSTAT, release 6.1 (30). The data were checked for normality, and histograms, normal P-P, and normal Q-Q plots showed that the data were not normally distributed. Data transformations were therefore investigated. A log10 transformation was found to give a closer approximation to normality; zeros were set equal to the limit of detection because log(0) is undefined. Limit of detection rather than 0.5 limit of detection was used because the former gives a worst case or conservative estimate by setting the zeros equal to the maximum values they could plausibly have. Histograms, fitted-value plots, normal plots, and half-normal plots of residuals after analysis of variance (ANOVA) verified that the transformation was successful in approximating normality. This was approximated sufficiently well as not to violate the assumptions of the ANOVA (which is fairly robust against departures from strict normality). Furthermore, a log scale is multiplicative on the original scale of measurement and seems entirely appropriate for modeling a situation such as this in which the effects are brought about by alterations in metabolic rates. ANOVA was performed for each isoflavonoid (genistein, daidzein, equol, glycitein, and O-DMA) to determine the various effects and interactions of sex, diet, and phase (baseline, midpoint, and endpoint).
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RESULTS |
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; these characteristics did not differ significantly between the 2 groups. The prestudy habitual diets of the 2 groups of subjects also did not differ significantly in macronutrient content (Table 2).
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The plasma isoflavone and gut microflora metabolite concentrations of the subjects in the high-soy and low-soy diet groups at baseline and after 10 wk are shown in Figure 1. At baseline, relatively low concentrations of isoflavones and metabolites were found in the plasma of most of the subjects in both groups. This reflects the soy isoflavone content of the subjects' habitual diet. After 10 wk, the plasma concentrations of genistein, daidzein, equol, and O-DMA in the subjects consuming the high-soy diet were significantly higher than in those consuming the low-soy diet (P < 0.001). There were no significant differences between the midpoint (data not shown) and endpoint plasma concentrations of the isoflavones and metabolites.
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The urinary isoflavone and gut microflora metabolite concentrations in the subjects in both groups at baseline and after 10 wk are shown in Figure 2. At baseline, relatively low concentrations of isoflavones and metabolites were found in the urine of subjects in both groups; this reflects the soy isoflavone content of the subjects' habitual diet. After 10 wk, the urine concentrations of genistein, daidzein, glycitein, equol, and O-DMA in subjects consuming the high-soy diet were significantly higher than in those consuming the low-soy diet (P < 0.001). There were no significant differences between the midpoint (data not shown) and endpoint urinary concentrations of the isoflavones and metabolites.
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Equol was detectable in urine at the endpoint of the study (in amounts >1000 nmol/24 h or 1 µmol/24 h) in 34% of the subjects consuming the high-soy diet, who were classified as good equol producers on this basis. The concentrations of isoflavonoids in plasma, urine, and feces in equol producers and in non-equol-producers, at baseline and after consumption of the high-soy diet for 10 wk, are shown in Table 3. The concentration of equol in urine, plasma, and feces was significantly higher (P < 0.001) in the equol producers than in the non-equol-producers.
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. At baseline, relatively low concentrations of genistein and daidzein, glycitein, equol, and O-DMA were found in the feces of most of the subjects, and this reflects the soy isoflavone content of their habitual diets. After 10 wk, the fecal concentrations of genistein, daidzein, equol, and O-DMA in the subjects consuming the high-soy diet were significantly higher than in those consuming the low-soy diet (P < 0.001), as were concentrations of glycitein (P < 0.039). There were no significant differences between the midpoint (data not shown) and endpoint fecal concentrations of the isoflavones and their metabolites.
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There was a significant main effect of diet (high-soy compared with low-soy) for genistien, daidzein, equol, and O-DMA in plasma, urine, and feces (P < 0.001) and for glycitein in urine (P < 0.001) and feces (P = 0.039). There was also a significant effect of time (baseline compared with the midpoint and endpoint of the study) for genistein, daidzein, and O-DMA in plasma, urine, and feces (P < 0.001); for equol in plasma, urine (P < 0.001), and feces (P = 0.015); and for glycitein in urine (P < 0.001). There were no significant interactions with the exception of sex and diet for O-DMA concentrations in plasma and urine: males had significantly higher concentrations of O-DMA than did females in plasma (
± SD: 165 ± 208 compared with 69 ± 93 nmol/L; P = 0.006) and in urine (11.6 ± 15.9 compared with 8.2 ± 12.2 µmol/24 h; P = 0.039).
The correlation between the isoflavones and metabolites within each matrix (plasma, urine, and feces) and between each of these matrices was calculated. For example, the urinary excretions of daidzein and genistein were strongly correlated (r = 0.92, P < 0.001), and there was a strong positive correlation between urinary equol excretion and plasma equol concentrations (r = 0.79, P < 0.001). It is useful in particular to note the correlations with fecal concentrations, because few previous studies have measured fecal isoflavone and metabolite concentrations. For example, examination of genistein concentrations in the 3 matrices showed that the plasma genistein concentration was strongly correlated with urinary genistein (r = 0.83, P < 0.001) and with fecal genistein excretion (r = 0.40, P < 0.001). Urinary genistein excretion was also strongly correlated with fecal genistein excretion (r = 0.53, P < 0.001).
The recovery of genistein and daidzein (and the daidzein metabolites equol and O-DMA) in urine and feces was calculated. Recovery was 8% for genistein, 15% for daidzein, 4% for equol, and 6% for O-DMA in urine and 1% for genistein, 2% for daidzein, 2% for equol, and 3% for O-DMA in feces.
Results from the fecal incubations (Table 4) showed that daidzein can be converted to equol and O-DMA by fecal bacteria ex vivo. Feces from subjects who had been consuming either the high-soy or the low-soy diet could convert daidzein to equol. It is of particular interest that feces from subjects who had been consuming the low-soy diet and therefore were not soy food consumers in this study were nevertheless able to convert daidzein to equol.
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As shown in Figure 4, after 10 wk, fecal ß-glucosidase activity in subjects consuming the high-soy diet was significantly higher than in subjects consuming the low-soy diet (P < 0.05). There was no significant change in fecal ß-glucuronidase activity after consumption of either the high-soy or the low-soy diet for 10 wk (± SD enzyme activity in µmol · h–1 · g feces–1: high-soy diet, baseline = 34 ± 19, endpoint = 35 ± 17; low-soy diet, baseline = 30 ± 21, endpoint = 30 ± 18).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Generally, there was no significant effect of sex on plasma, urinary, or fecal concentrations of the isoflavones and their metabolites. These observations suggest that the sex of the subjects was generally not important in terms of their response to the diets. It is thus of interest that significant differences were observed between the men and the women in terms of O-DMA concentrations in plasma and urine. The men had significantly higher concentrations of O-DMA in their plasma and urine than did the women. Previous studies suggested that men and women respond differently to chronic exposure to isoflavones (15–17), although considerable inconsistencies in the data exist. Recovery of genistein and daidzein was lower and that of equol was higher in women after the daily ingestion of soymilk (36 oz /d) for 1 mo (16, 17). Furthermore, 3 of the 12 women studied developed the ability to produce equol after 2 wk of soymilk consumption, which suggests that chronic soy ingestion may alter the metabolism of daidzein and thus contribute to interindividual variability (16). This was not observed in men (15).
There was good correlation between plasma, urinary, and fecal concentrations of isoflavonoids. The urinary excretions of daidzein and genistein were strongly correlated, and there was a strong positive correlation between urinary equol excretion and plasma equol concentrations, which is in agreement with previous studies (5).
Considerable interindividual variation in plasma, urinary, and fecal concentrations of isoflavones and metabolites was observed at the endpoint of the study between the subjects in the high-soy diet group, which is in agreement with previous studies (5). Equol was detectable in the urine at the endpoint of the study in 34% of the subjects consuming the high-soy diet, which also agrees with previous studies, in which usually 35% of the population have been found to be good equol producers (5, 14, 23) and are considered to possess the gut bacteria capable of converting daidzein to equol.
After the consumption of genistein (206 ± 40 µmol/d) and daidzein (170 ± 41 µmol/d) from a wide range of soy foods for 10 wk, the recovery of genistein and daidzein in urine was 8% for genistein, 15% for daidzein, 4% for equol, and 6% for O-DMA. This result agrees with a previous study (5) in which lower doses of genistein (129 µmol/d) and daidzein (84 µmol/d) were consumed in a textured soy protein vegetable burger for a shorter duration (2 wk) and the recoveries of genistein, daidzein, equol, and O-DMA in urine were 7%, 21%, 5%, and 5%, respectively (5). These urinary recoveries of genistein and daidzein also agree with a previous study in women that reported urinary recoveries of 9% for genistein and 21% for daidzein after the consumption of isoflavones in a single dose of a powdered soymilk drink (29). However, in a single-dose study in men that reported recoveries of isoflavones (112 µmol genistein and 103 µmol daidzein consumed in kinako, a soybean powder) of 18% for genistein, 36% for daidzein, 7% for equol, and 4% for O-DMA (4), the urinary recovery of daidzein was higher than in the current study. In the current study, the low fecal recoveries of 1% for genistein, 2% for daidzein, 2% for equol, and 3% for O-DMA are comparable with the 2% for genistein, 4% for daidzein, 2% for equol, and 2% for O-DMA reported previously (4).
The fecal incubations with daidzein as a substrate showed the successful ex vivo conversion of daidzein to equol by individual volunteers. This conversion was also seen in volunteers who had not been consuming the soy-containing diet. The ability to convert daidzein to equol by some intestinal bacteria is thus independent of soy food consumption. This suggests that the bacterial enzymes necessary for the conversion are present within the gut in certain individuals and are not induced in response to increased substrate in the gut (eg, from soy foods consumed in the diet). Furthermore, the fecal incubations with soy flour (containing the isoflavone glycoside daidzin as the substrate) showed the efficient conversion of daidzein in the form of daidzin to O-DMA and equol. This finding suggests the efficient hydrolysis of glycosides before further metabolism to equol and indicates the presence of both ß-glucosidase and equol-producing bacterial activity in the fecal microflora population.
The identification of the bacterial species involved in the conversion of daidzein to equol is of considerable importance and is a huge challenge because of the large number of bacteria present in the colon and small intestine. Ueno et al (31) identified equol producers by culturing the fecal flora from healthy Japanese adults after they consumed 70 g tofu. Three stains of bacteria were reported to convert pure daidzein to equol in vitro: the gram-negative Bacteroides ovatus spp. and the gram-positive Strepotococcus intermedius spp. and Ruminococcus productus spp.
In the fecal incubations performed by Xu et al (32), daidzein had completely disappeared by 72 h. Zhang et al (33) identified distinguishable groups with respect to the ability of their feces to degrade daidzein and genistein; these responses were shown to be stable when reexamined 10 mo later (34). Differences in the degradation ability of the gut microflora (high isoflavone degraders and low isoflavone degraders) may be responsible for the observed large differences in bioavailability of soy isoflavones; therefore, these differences are of considerable importance to our further understanding of the health benefits of isoflavones (35).
After 10 wk, fecal ß-glucosidase activity in the subjects consuming the high-soy diet was significantly higher than that in the subjects consuming the low-soy diet. There was, however, no significant change in fecal ß-glucuronidase activity after the consumption of either diet for 10 wk. These enzymes are inducible by their substrates, and the increase in ß-glucosidase activity is likely to be in response to the considerable soy isoflavone glucoside consumption (18). Similarly, after the consumption of isoflavones and their subsequent conjugation and excretion in bile, the higher concentrations of glucuronide conjugates likely to have been present in the colon might have been expected to result in higher ß-glucuronidase activity in the subjects who consumed the high-soy diet than in those who consumed the low-soy diet.
In conclusion, the present data suggest that although interindividual variation in isoflavone metabolism was high, intraindividual variation (comparing the midpoint with the endpoint results) in metabolism was low. Only concentrations of O-DMA in plasma and urine appeared to be influenced by sex, with men having significantly higher concentrations than women. Furthermore, chronic soy consumption does not induce many significant changes to the gut metabolism of isoflavones other than effects on ß-glucosidase activity. This suggests that the bacteria and enzymes responsible for equol or O-DMA production are not inducible.
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ACKNOWLEDGMENTS |
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
; n = 38) or a low-soy diet (
; n = 38). After 10 wk, plasma concentrations of genistein, daidzein, equol, and O-desmethylangolensin (O-DMA) were significantly higher in the subjects who consumed the high-soy diet than in those who consumed the low-soy diet (P < 0.001, ANOVA).
