The variable metabolic response to dietary isoflavones in humans

Gut bacterial metabolism of the soy isoflavone daidzein: exploring the relevance to human health

The indigenous intestinal microflora are involved in a variety of processes within the human body, and are important for maintaining host health. As such, interindividual differences in the ability to harbor certain intestinal bacteria might be associated with interindividual differences in health and/or disease susceptibility. In the last decade there has been considerable interest in phytoestrogen intakes in relation to human health. Daidzein, an isoflavone phytoestrogen found in soy, is metabolized to equol and O-desmethylangolensin (O-DMA) by intestinal bacteria. The specific bacterium/bacteria responsible for equol and O-DMA production in humans have yet to be identified definitively, but in vitro and animal studies have suggested that equol and O-DMA are more biologically active than their precursor daidzein. Interestingly, substantial interindividual differences in daidzein metabolism exist; following soy or daidzein consumption, approximately 30%-50% of the human population produce equol, and approximately 80%-90% produce O-DMA. Observational and intervention studies in humans have suggested that the ability to produce equol and O-DMA may be associated with reduced risk of certain diseases including breast and prostate cancers. However, relatively few studies have been conducted to date. In this review, we discuss the available evidence for a relationship between daidzeinmetabolizing phenotypes and human health, and suggest potential mechanisms for some of the reported relationships.

Isolation and characterisation of an equol-producing mixed microbial culture from a human faecal sample and its activity under gastrointestinal conditions

Only about one third of humans possess a microbiota capable of transforming the dietary isoflavone daidzein into equol. Little is known about the dietary and physiological factors determining this ecological feature. In this study, the in vitro metabolism of daidzein by faecal samples from four human individuals was investigated. One culture produced the metabolites dihydrodaidzein and O-desmethylangolensin, another produced dihydrodaidzein and equol. From the latter, a stable and transferable mixed culture transforming daidzein into equol was obtained. Molecular fingerprinting analysis (denaturing gradient gel electrophoresis) showed the presence of four bacterial species of which only the first three strains could be brought into pure culture. These strains were identified as Lactobacillus mucosae EPI2, Enterococcus faecium EPI1 and Finegoldia magna EPI3, and did not produce equol in pure culture. The fourth species was tentatively identified as Veillonella sp strain EP. It was found that hydrogen gas in particular, but also butyrate and propionate, which are all colonic fermentation products from poorly digestible carbohydrates, stimulated equol production by the mixed culture. However, when fructo-oligosaccharides were added, equol production was inhibited. Furthermore, the equol-producing capacity of the isolated culture was maintained upon its addition to a faecal culture originating from a non-equol-producing individual.

Urinary isoflavonoid excretion is inversely associated with the ratio of protein to dietary fibre intake in young women

OBJECTIVE

To examine the prevalence of excretion of urinary isoflavonoids in women and determine any relationships with accustomed macronutrient intake.

DESIGN

Volunteers in one of two 4-month studies. Study 1 was a randomised crossover study whereby subjects consumed a placebo or isoflavone supplement for 2 months and crossed over. Study 2 was a parallel design in which subjects consumed a placebo for 1 month and an isoflavone supplement for 3 months.

SETTING

All subjects were free-living, healthy volunteers.

SUBJECTS

A total of 25 (study 1, n=14; study 2, n=11) premenopausal women were recruited through advertisements.

INTERVENTIONS

Volunteers were supplemented for 2 months (study 1) or 3 months (study 2) with purified isoflavones (86 mg/day) derived from red clover. Urinary isoflavonoids were measured during the placebo and the second month of isoflavone treatment. Macronutrient intakes were determined from weighed food records.

RESULTS

During isoflavone supplementation, the concentration of urinary total isoflavonoids increased by 15-fold (P<0.0001), with 5.4-fold variation between individuals. Multiple linear regression analysis showed that 24% of this variation could be explained by an interaction between dietary fibre and protein (P=0.047), with a highly significant inverse association between total isoflavonoid concentration and the protein to fibre ratio (r=-0.51, P=0.009).

CONCLUSIONS

Supplementation with purified isoflavones results in an increase in urinary isoflavonoid excretion and part of the individual variation in response is associated with an interaction between intakes of protein and dietary fibre. Whether manipulation of these macronutrients could enhance efficacy of isoflavone supplements remains to be determined.

Biotransformation of soy isoflavone-glycosides in laying hens: intestinal absorption and preferential accumulation into egg yolk of equol, a more estrogenic metabolite of daidzein

Dietary soy-isoflavones have recently been noted as phytoestrogens with potentially beneficial effects on human health, and they are biologically transformed in the intestinal tract into aglycones and further into several specific metabolites. Here we report that in laying hens daidzin, a soy isoflavone-glycoside, in the diet was transformed into equol, absorbed, transported in circulating peripheral blood, and preferentially accumulated into egg yolk in its conjugated form. Laying hens were fed experimental diets containing two levels of soy isoflavone-glycosides (177 or 528 mg per 100 g diet) for 21 or 42 days, and blood and eggs were collected at 1- to 9-day intervals. HPLC analyses revealed that most of the isoflavones (daidzein, glycitein, and genistein) and a metabolite, equol, were present in blood and egg yolk in conjugated form. The concentration of equol-conjugates in blood plasma and egg yolk was higher than any of the other three isoflavone-conjugates analyzed and, especially in egg yolk, the equol-conjugates comprised no less than 60% of the total isoflavone-conjugates. The isoflavones, including equol, distributed mostly (95%) in the high-density fraction of blood serum, and more (65%) in the granule fraction of egg yolk. These results raise the possibility that feeding domestic animals soy-based fodder produces animal-based foods rich in a more active form of phytoestrogens.

Disposition of flavonoids via enteric recycling: enzyme-transporter coupling affects metabolism of biochanin A and formononetin and excretion of their phase II conjugates

The purpose of this study was to continue our effort to determine how enzyme-transporter coupling affect disposition of flavonoids. The rat intestinal perfusion and Caco-2 cell models were used together with relevant microsomes. In perfusion model, isoflavone (i.e., formononetin and biochanin A) absorption and subsequent excretion of its metabolites were always site-dependent. Maximal amounts of intestinal and biliary conjugates excreted per 30 min were 31 and 51 nmol for formononetin, more than that for pure biochanin A (12 and 20 nmol). When a standardized red clover extract (biochanin A/formononetin = 10:7) was used, the results indicated that more metabolites of biochanin A than formononetin were found in the perfusate (36.9 versus 22.8 nmol) and bile (78 versus 51 nmol). In metabolism studies, rat intestinal and liver microsomes always glucuronidated biochanin A faster (p < 0.05) than formononetin, whereas intestinal microsomes glucuronidated both isoflavones faster (p < 0.05) than liver microsomes. However, rapid metabolism in the microsomes did not translate into more efficient excretion in either the rat perfusion model as shown previously or in the Caco-2 model. In the Caco-2 model, both isoflavones were rapidly absorbed, efficiently conjugated, and the conjugates excreted apically and basolaterally. More formononetin conjugates were excreted than biochanin A when used alone, but much more biochanin A conjugates were found when using the isoflavone mixture. In conclusion, efficiency of enzyme-transporter coupling controls the amounts of metabolites excreted by the intestine and liver and determines the relative contribution of enteric and enterohepatic recycling to the in vivo disposition of isoflavones.

Estrogenicity of the Isoflavone Metabolite Equol on Reproductive and Non-Reproductive Organs in Mice1

Equol, a metabolite of the phytoestrogen daidzein, is present at significant levels in some humans who consume soy and in rodents fed soy-based diets. Equol is estrogenic in vitro, but there have been limited studies of its activity in vivo. We evaluated equol effects on reproductive and non-reproductive endpoints in mice. Ovariectomized age-matched (30-day-old) female C57BL/6 mice were fed phytoestrogen-free diets and given a racemic mixture of equol by daily injections (0, 4, 8, 12, or 20 mg [kg body weight](-1) day(-1)) or in the diet (0, 500, or 1,000 ppm) for 12 days. Mice were killed, and serum concentrations of total and aglycone equol were measured. Total serum equol concentrations ranged from 1.4 to 7.5 microM with increasing doses of injected equol, but uterine weight increased significantly only at 12 and 20 mg (kg body weight)(-1) day(-1). Dietary equol at 500 or 1,000 ppm produced total serum equol concentrations of 5.9 and 8.1 microM, respectively, comparable with those in rodents consuming certain high-soy chows; the proportion of equol present as the free aglycone was much lower with dietary administration than injections, which may be a factor in the greater biological effects induced by injections. Dietary equol did not significantly increase uterine weight. Increasing dietary and injected equol doses caused a dose-dependent increase in vaginal epithelial thickness. Uterine epithelial proliferation was increased by equol injections at 8-20 mg (kg body weight)(-1) day(-1) and 1,000 ppm dietary equol. Neither dietary nor injected equol decreased thymic or adipose weights. In conclusion, equol is a weak estrogen with modest effects on endpoints regulated by estrogen receptor alpha when present at serum levels seen in rodents fed soy-based diets, but quantities present in humans may not be sufficient to induce estrogenic effects, although additive effects of equol with other phytoestrogens may occur.

Genotoxicity of the isoflavones genistein, daidzein and equol in V79 cells

Hormonally active chemicals in the human diet, such as man-made estrogenic chemicals or plant-derived compounds (phytoestrogens), have become a matter of public concern. A significant part of human exposure to phytoestrogens is attributable to soy isoflavones. Besides their estrogenic properties, soy isoflavones also exert genotoxic actions. In this paper, the micronucleus (MN) assay in V79 cells was used to study chromosomal genotoxicity. Genistein caused a clear dose-related induction of MN within the range of 5-25 microM; MN rates were declining at higher genistein concentrations. This was probably due to cytotoxicity of genistein since reduced neutral red uptake and MTT formation with an IC(50) of about 75 microM occurred. Daidzein induced a comparatively shallow increase in the number of MN between 25 and 100 microM. In contrast, the daidzein metabolite equol caused an increase in the number of MN up to 25 microM with no further increase at higher concentrations. Additional staining with anti-kinetochore (CREST) antibodies served to determine if the micronuclei contain whole chromosomes or acentric fragments. Genistein induced mostly CREST(-) micronuclei, i.e. MN with chromosomal fragments, thus indicative of a clastogenic mode of action. MN induced by high concentrations of daidzein were partly CREST(+) and CREST(-), whilst equol induced mostly CREST(+) micronuclei indicative of an aneugenic action. These results point to a differential genotoxicity of phytoestrogens.

Plasma phytoestrogens are not altered by probiotic consumption in postmenopausal women with and without a history of breast cancer

Soy phytoestrogens were suggested to reduce the risk of a number of diseases including breast cancer. Given that these compounds are metabolized by bacteria, alteration of intestinal bacteria and enzymes may affect phytoestrogen metabolism. We hypothesized that probiotics, when consumed with soy protein, would increase plasma isoflavones, as well as equol producer frequency, in postmenopausal women. We further hypothesized that these effects would differ between women who have had breast cancer and women who have not. To test these hypotheses, 20 breast cancer survivors and 20 controls completed four 6-wk treatments in a randomized, crossover design: supplementation with soy protein (S) (26.6 +/- 4.5 g protein, 44.4 +/- 7.5 mg isoflavones/d); soy + probiotics (S+P) (10(9) colony-forming units Lactobacillus acidophilus DDS+1 and Bifidobacterium longum, 15-30 mg fructooligosaccharide/d); milk protein (M) (26.6 +/- 4.5 g protein/d); and milk + probiotics (M+P). Plasma phytoestrogen concentrations did not differ between controls and survivors, although genistein tended to be lower in survivors at baseline (P = 0.15), and during soy (P = 0.16) and milk protein (P = 0.16) consumption. As expected, soy consumption increased plasma phytoestrogen concentrations (P < 0.0001). Plasma phytoestrogen concentrations and the number of equol producers did not differ between the S and S+P diets. At the same time, plasma equol concentrations as well as urinary equol excretion in 2 subjects were more than 7-fold different between the 2 diets. These results indicate that this particular probiotic supplement does not generally affect plasma isoflavones, although the large differences between plasma and urinary equol in some subjects suggest that equol producer status may be modifiable in some individuals.

Mammographic Density in Relation to Daidzein-Metabolizing Phenotypes in Overweight, Postmenopausal Women

Circulating hormones are associated with mammographic density, an intermediate marker of breast cancer risk. Differences in circulating hormones, including estrone and testosterone, have been observed in premenopausal women based on their capacity to metabolize daidzein, an isoflavone found predominantly in soybeans. Equol and O-desmethylangolensin (O-DMA) are products of intestinal bacterial metabolism of daidzein. There is interindividual variability in the capacity to produce daidzein metabolites; individuals can be equol producers or non-producers and O-DMA producers or non-producers. We tested the hypothesis that daidzein-metabolizing phenotypes are associated with mammographic density. Participants were recruited from among 92 sedentary, postmenopausal women, ages 50 to 75 years, who participated in a 1-year physical activity intervention. Pre-intervention mammographic density was determined using a computer-assisted, gray-scale thresholding technique. Fifty-five of these women consumed supplemental soy protein (&gt;10 mg daidzein/d) for 3 days and collected a first-void urine sample on the fourth day to determine daidzein-metabolizing phenotypes. Equol and O-DMA concentrations were measured using gas chromatography-mass spectrometry. Associations between daidzein-metabolizing phenotypes and percent mammographic density were adjusted for age, maximum adult weight, gravidity, family history of breast cancer, and serum follicle-stimulating hormone and free testosterone concentrations. Mammographic density was 39% lower in equol producers compared with non-producers (P = 0.04). O-DMA producers had mammographic density 69% greater than non-producers (P = 0.05). These results suggest that particular intestinal bacterial profiles are associated with postmenopausal mammographic density, and these associations are not entirely explained by differences in reproductive or anthropometric characteristics or circulating hormones.

Soya food intake and risk of endometrial cancer among Chinese women in Shanghai: population based case-control study

OBJECTIVE

To evaluate the association of intake of soya food, a rich source of phytoestrogens, with the risk of endometrial cancer.

DESIGN

Population based case-control study, with detailed information on usual soya food intake over the past five years collected by face to face interview using a food frequency questionnaire.

SETTING

Urban Shanghai, China.

PARTICIPANTS

832 incident cases of endometrial cancer in women aged of 30 to 69 years diagnosed during 1997-2001 and identified from the Shanghai Cancer Registry; 846 control women frequency matched to cases on age and randomly selected from the Shanghai Residential Registry.

MAIN OUTCOME MEASURES

Odds ratios for risk of endometrial cancer in women with different intakes of soya foods.

RESULTS

Regular consumption of soya foods, measured as amount of either soya protein or soya isoflavones, was inversely associated with the risk of endometrial cancer. Compared with women with the lowest quarter of intake, the adjusted odds ratio of endometrial cancer was reduced from 0.93 to 0.85 and 0.67 with increasing quarter of soya protein intake (P for trend 0.01). A similar inverse association was observed for soya isoflavones and soya fibre intake. The inverse association seemed to be more pronounced among women with high body mass index and waist:hip ratio.

CONCLUSION

Regular intake of soya foods is associated with a reduced risk of endometrial cancer.

Phytoestrogen Concentrations in Serum and Spot Urine as Biomarkers for Dietary Phytoestrogen Intake and Their Relation to Breast Cancer Risk in European Prospective Investigation of Cancer and Nutrition-Norfolk

Subjects of this study consisted of 333 women (aged 45–75 years) drawn from a large United Kingdom prospective study of diet and cancer, the European Prospective Investigation of Cancer and Nutrition-Norfolk study. Using newly developed gas chromatography/mass spectrometry and liquid chromatography/mass spectrometry methods incorporating triply 13C-labeled standards, seven phytoestrogens (daidzein, genistein, glycitein, O-desmethylangolensin, equol, enterodiol, and enterolactone) were measured in 114 spot urines and 97 available serum samples from women who later developed breast cancer. Results were compared with those from 219 urines and 187 serum samples from healthy controls matched by age and date of recruitment. Dietary levels were low, but even so, mean serum levels of phytoestrogens were up to 600 times greater than postmenopausal estradiol levels. Phytoestrogen concentrations in spot urine (adjusted for urinary creatinine) correlated strongly with that in serum, with Pearson correlation coefficients &gt; 0.8. There were significant relationships (P &lt; 0.02) between both urinary and serum concentrations of isoflavones across increasing tertiles of dietary intakes. Urinary enterodiol and enterolactone and serum enterolactone were significantly correlated with dietary fiber intake (r = 0.13–0.29). Exposure to all isoflavones was associated with increased breast cancer risk, significantly so for equol and daidzein. For a doubling of levels, odds ratios increased by 20–45% [log2 odds ratio = 1.34 (1.06–1.70; P = 0.013) for urine equol, 1.46 (1.05–2.02; P = 0.024) for serum equol, and 1.22 (1.01–1.48; P = 0.044) for serum daidzein]. These estimates of risk are similar to those established for estrogens and androgens in postmenopausal breast cancer but need confirmation in larger studies.

Serum steroid hormones, sex hormone-binding globulin concentrations, and urinary hydroxylated estrogen metabolites in post-menopausal women in relation to daidzein-metabolizing phenotypes

Equol and O-desmethylangolensin (O-DMA) are products of bacterial metabolism of daidzein, an isoflavone in soybeans; thus, the presence or absence of equol and/or O-DMA in urine is a marker of particular intestinal bacteria profiles. Plasma hormone concentrations may be lower in pre-menopausal women who harbor the bacteria capable of producing equol (equol producers) compared to women who do not (equol non-producers). We evaluated concentrations of serum hormones, sex hormone-binding globulin (SHBG), and urinary 2-hydroxyestrone (2-OH E(1)) and 16alpha-hydroxyestrone (16alpha-OH E(1)) in relation to equol-producer and O-DMA-producer phenotypes in 89 post-menopausal women. Follicle stimulating hormone (FSH) was 23% greater in O-DMA-producers compared to non-producers (P = 0.04). No significant differences in serum estrogens, androgens, metabolic hormones, or SHBG were observed in relation to either daidzein-metabolizing phenotype. Compared with non-producers within each phenotype, age-adjusted 2-OH E(1):16alpha-OH E(1) was 27% greater (P = 0.06) in equol-producers and 9% greater (P > 0.10) in O-DMA-producers, and 2-OH E(1) concentrations were 24% greater in equol producers (P = 0.07) and 42% greater in O-DMA producers (P = 0.02). No significant differences in 16alpha-OH E(1) were observed in relation to either phenotype. These results suggest that interindividual variability in intestinal bacteria may be related to differences in products of hormone metabolism in post-menopausal women.

Dietary soy containing phytoestrogens does not have detectable estrogenic effects on hepatic protein synthesis in postmenopausal women

BACKGROUND

Dietary phytoestrogens are ligands for the estrogen receptor and may mimic estrogenic effects in vivo.

OBJECTIVE

To assess the biological activity of isoflavone phytoestrogens, we analyzed the effect of dietary soy isoflavone supplementation on in vivo bioassays of estrogenicity.

DESIGN

Fifty healthy postmenopausal women aged 50-75 y participated in a double-blind, placebo-controlled trial in which they received either soy protein isolate (40 g soy protein, 118 mg isoflavones) or casein placebo. Measurements were made at baseline and at 3 mo. Urinary isoflavone excretion was measured to reflect compliance. The bioassays of estrogenicity included measurement of hepatic proteins and gonadotropin concentrations.

RESULTS

Baseline characteristics were not significantly different between the soy and placebo groups. Urinary isoflavone excretion increased in the soy group and at the end of 3 mo was higher in the soy group than in the placebo group. In plasma samples from both groups, C-reactive protein increased significantly over the 3-mo treatment period, whereas sex hormone-binding globulin and thyroid-binding globulin decreased significantly. However, there were no significant differences between the groups in hepatic protein synthesis (change over 3 mo +/- SEM in the soy and placebo groups, respectively): C-reactive protein, 0.42 +/- 0.2 and 0.48 +/- 0.2 U/mL; sex hormone-binding globulin, -6.9 +/- 1.5 and -10.0 +/- 2.1 micro g/mL; thyroid-binding globulin, -16 +/- 8 and -26 +/- 7 nmol/L. Furthermore, gonadotropin and dehydroepiandrosterone sulfate concentrations did not change significantly in either group.

CONCLUSIONS

In healthy postmenopausal women, dietary soy isoflavones do not affect in vivo biological indicators of estrogenicity, including hepatic protein synthesis and gonadotropin concentrations. This suggests that soy isoflavones have little biologically relevant estrogenic effect in vivo in postmenopausal women.

In vitro incubation of human feces with daidzein and antibiotics suggests interindividual differences in the bacteria responsible for equol production

Daidzein can be metabolized to equol, dihydrodaidzein (DHD), and O-desmethylangolensin (ODMA) by intestinal bacteria. Only one third to one half of individuals produce equol, and evidence exists to support potential cancer-protective effects of equol production. We investigated the in vitro metabolism of daidzein by fecal bacteria and assessed the effect of several antibiotics on metabolism. Fresh or previously frozen feces from 7 equol producers and 6 nonproducers were incubated with daidzein, with or without antibiotics, for 5 d at 37 degrees C. With the exception of one previously frozen sample, fecal inoculates from equol producers converted daidzein to equol. Conversion occurred under anaerobic, but not aerobic conditions. Fecal inoculates from equol nonproducers did not produce equol, but some produced ODMA and DHD. Between-subject differences in the effects of antibiotics on daidzein metabolism were apparent. Some antibiotics inhibited the production of equol but had no effect on DHD production. These results suggest that several bacteria may be involved in daidzein metabolism, and that they may differ among subjects. This simple in vitro system can facilitate the study of factors influencing equol production and minimize the need for animal models or human interventions. Furthermore, these analyses can be conducted on fecal samples that have been frozen and stored.

Risks and benefits of soy isoflavones for breast cancer survivors

PURPOSE/OBJECTIVES

To present state-of-the-art information about the risks and benefits of soy isoflavones for breast cancer survivors.

DATA SOURCES

Published research articles, pertinent articles and books, and computerized databases.

DATA SYNTHESIS

Some epidemiologic data suggest that soy isoflavones play an important role in preventing breast cancer in Asian women and promoting women's health in a variety of ways. However, the use of soy isoflavones in women with breast cancer is controversial. Risks and benefits exist regarding the use of soy isoflavones by breast cancer survivors.

CONCLUSIONS

The use of soy isoflavones to promote health in breast cancer survivors remains controversial because of scant scientific data.

IMPLICATIONS FOR NURSING

Nurses should not only provide updated information to the public but also interpret research results carefully. More clinical trials need to be conducted on a longitudinal basis with the enrollment of breast cancer survivors.

Phytoestrogens: a review of the present state of research

Phytoestrogens are a diverse group of plant-derived compounds that structurally or functionally mimic mammalian estrogens and show potential benefits for human health. The number of articles published on phytoestrogens has risen dramatically in the past couple decades. Further research continues to demonstrate the biological complexity of phytoestrogens, which belong to several different chemical classes and act through diverse mechanisms. This paper discusses the classification of phytoestrogens, methods of identification, their proposed mechanisms of action and botanical sources for phytoestrogens. The effects of phytoestrogens on breast and prostate cancers, cardiovascular disease, menopausal symptoms and osteoporosis will also be examined including research on benefits and risks.

Phytoestrogens: Potential Benefits and Implications for Breast Cancer Survivors

Phytoestrogens are a group of plant-derived substances that are structurally or functionally similar to estradiol. There has been much interest in the potential role of phytoestrogens in cancer prevention and treatment of estrogen-deficient states. This review summarizes the evidence for phytoestrogen risks and benefits relevant to the breast cancer survivor, including prevention of a second primary breast cancer or metastatic disease, reduction in menopausal symptoms, and interactions with tamoxifen. Epidemiological data suggest a breast cancer protective role for phytoestrogens, and there is some supporting clinical data, but they are far from conclusive. In addition, there is some evidence that genistein, the most prevalent isoflavone in soy, can stimulate estrogen receptor-positive (ER+) breast cancer growth and interfere with the antitumor activity of tamoxifen at low levels. Given current knowledge, women who have ER+ tumors should not increase their phytoestrogen intake. Several studies suggest an inhibitory effect on ER- breast cancer cell growth, and it may be reasonable for women with ER- tumors to safely consume soy and possibly other phytoestrogens. However, the optimal amount and source are not clear. More research is needed to clarify the role of phytoestrogens in breast cancer prevention and in treating estrogen-deficient diseases in women who have had breast cancer.

Types of dietary fat and soy minimally affect hormones and biomarkers associated with breast cancer risk in premenopausal women

Fourteen premenopausal women participated in a randomized, crossover controlled feeding study of three diets, each two menstrual cycles long. We compared a high saturated fat Western diet (control diet) with two other diets: the control diet plus soy protein (soy diet) and the control diet with polyunsaturated fat (PUFA diet) replacing most of the saturated fat. We measured reproductive and serum hormones, urinary estrogen metabolites and isoflavonoids, and menstrual cycle length. In the follicular phase, prolactin concentrations significantly decreased by 3.6 micrograms/dl (P = 0.047), follicle-stimulating hormone concentrations slightly increased by 0.1 IU/l (P = 0.076), and cortisol concentrations slightly decreased by 81.8 nmol/l (P = 0.088) with the PUFA diet vs. the control diet. The soy diet slightly increased menstrual cycle length by 1.8 +/- 0.7 days (P = 0.088) and significantly increased (P < 0.0001) urinary isoflavonoid excretion. These well-controlled diets did not affect serum estrogens or urinary estrogen metabolites, suggesting that type of fat or consumption of soy with a high saturated fat diet may not alter breast cancer risk by these mechanisms.

Treatment with antibiotics reduces plasma equol concentration in cynomolgus monkeys (Macaca fascicularis)

To explore the importance of equol on health outcomes in future studies, it was necessary to develop a method to reduce equol production. Female monkeys (n = 22) fed a soy diet were treated twice daily with vehicle (control; n = 4), doxycycline (2.5 mg/kg; n = 4), metronidazole (125 mg/d; n = 3), kanamycin (1000 mg/d; n = 4), vancomycin (100 mg/d; n = 3) or kanamycin+vancomycin (n = 4). Plasma samples were collected 4 h postfeeding at baseline, after 4 wk of treatment and 8 wk after the end of treatment and analyzed for isoflavonoid concentrations. Fecal swabs were collected at baseline and at the end of antibiotic treatment for analysis of Gram(+) and Gram(-) bacterial growth. Equol concentrations were reduced (P < 0.05) compared with baseline by 80, 93, 98 and 99% after treatment with metronidazole (955 +/- 164 vs. 193 +/- 53 nmol/L), kanamycin (545 +/- 211 vs. 37.1 +/- 17.6 nmol/L), vancomycin (607 +/- 163 vs. 8.9 +/- 8.2 nmol/L) and kanamycin+vancomycin (721 +/- 169 vs. 17.4 +/- 17.3 nmol/L), respectively. Daidzein concentrations were increased (P < 0.05) compared with baseline by treatment with doxycycline (336 +/- 87 vs. 576 +/- 76 nmol/L), kanamycin (168 +/- 67 vs. 374 +/- 15 nmol/L), and kanamycin+vancomycin (166 +/- 35 vs. 384 +/- 78 nmol/L). Similar increases (P < 0.05) in dihydrodaidzein were observed after treatment with kanamycin (31.2 +/- 6.2 vs. 479 +/- 188 nmol/L) and metronidazole (56.0 +/- 27.9 vs. 414 +/- 212 nmol/L). Isoflavonoid concentrations returned to baseline values after antibiotic treatment was terminated. Gram(+) bacterial growth was reduced by all treatments, including Control, compared with baseline. In conclusion, treatment with antibiotics resulted in a marked reduction in plasma equol concentrations and altered plasma isoflavonoid patterns in cynomolgus monkeys.

Bioavailability of soybean isoflavones from aglycone and glucoside forms in American women

BACKGROUND

Test results on the bioavailability of isoflavones in the aglycone or glucoside form in Eastern and Western human subjects are contradictory.

OBJECTIVE

The objective was to investigate the bioavailability of the soy isoflavones daidzein and genistein in American women with typical American dietary habits after ingestion of the aglycone or glucoside form of isoflavones.

DESIGN

Fifteen American women aged 46 +/- 6 y participated in a randomized, double-blind study. Blood samples were collected 0, 1, 2, 4, 8, 12, 24, and 48 h after consumption of aglycone or glucoside tablets with breakfast. The plasma curves for daidzein, genistein, and equol were constructed and the postprandial maximum concentration (C(max)), time to the maximum concentration (t(max)), and area under the curve (AUC) were determined.

RESULTS

Isoflavone concentrations peaked early (1-2 h) in plasma and peaked again at 4-8 h. Mean C(max), t(max), and AUC values for genistein were not significantly different after ingestion of aglycone or glucoside. However, C(max) and AUC values, but not t(max), were significantly higher for daidzein after aglycone ingestion, which was partly due to its higher content in the aglycone tablets. Equol appeared after 4 h and remained elevated after 48 h. Despite a higher content of daidzein in the aglycone tablets, the AUC for equol was significantly higher after ingestion of the glucoside tablets, probably because of the metabolic action of intestinal bacteria during the long intestinal transit time of glucoside.

CONCLUSION

The apparent bioavailability of genistein and daidzein is not different when consumed as either aglycone or glucoside by American women.

Bioavailability of phyto-oestrogens

The term phyto-oestrogen encompasses isoflavone compounds, such as genistein and daidzein, found predominantly in soya products and the lignans, such as matairesinol and secoisolariciresinol, found in many fruits, cereals and in flaxseed. There is evidence that they have potential health benefits in man particularly against hormone-dependent diseases such as breast and prostate cancers and osteoporosis. This has led to intense interest in their absorption and biotransformation in man. The metabolism of isoflavones and lignans in animals and man is complex and involves both mammalian and gut microbial processes. Isoflavones are present predominantly as glucosides in most commercially available soya products; there is evidence that they are not absorbed in this form and that their bioavailability requires initial hydrolysis of the sugar moiety by intestinal beta-glucosidases. After absorption, phyto-oestrogens are reconjugated predominantly to glucuronic acid and to a lesser degree to sulphuric acid. Only a small portion of the free aglycone has been detected in blood, demonstrating that the rate of conjugation is high. There is extensive further metabolism of isoflavones (to equol and O-desmethylangolensin) and lignans (to enterodiol and enterolactone) by gut bacteria. In human subjects, even those on controlled diets, there is large interindividual variation in the metabolism of isoflavones and lignans, particularly in the production of the gut bacterial metabolite equol (from daidzein). Factors influencing absorption and metabolism of phyto-oestrogens include diet and gut microflora.

The phytoestrogen equol increases nitric oxide availability by inhibiting superoxide production: an antioxidant mechanism for cell-mediated LDL modification

Estrogen replacement therapy (ERT) is reported to lower the incidence of cardiovascular disease in postmenopausal women. ERT also lowers the levels of oxidatively modified low-density lipoprotein (LDL). Because modified LDL can mediate the development of atherosclerosis by inflammatory processes, ERT may exert its LDL protective effect through enhanced antioxidant activity in vascular tissues. Plant sources of estrogenic compounds have been used as alternatives for ERT because they avoid a number of negative health effects produced by estrogen. In this study, the antioxidant properties of the soy isoflavone metabolite, equol (an estrogenic metabolite of daidzein) were studied. Equol has a greater antioxidant activity than the parent isoflavone compounds genistein and daidzein, found in high concentration in soy. Equol inhibits LDL oxidation in vitro and LDL oxidative modification by J774 monocyte/macrophages to LDL(-), an electronegative modified LDL found in human plasma. An antioxidant effect of equol was found to be mediated by inhibition of superoxide radical (O(2)(-*)) production and manifested through enhanced levels of free nitric oxide (NO) that prevents LDL modification. Thus, when NO levels were increased by donor agents, generators, or compounds that facilitate nitric oxide synthase activity, LDL(-) formation by J774 cells was strongly inhibited. Conversely, inhibition of NO production enhanced LDL(-) formation, and the combination of reduced NO and increased O(2)(-*) production yielded maximum LDL(-) formation. Pretreatment of cells with equol inhibited production of O(2)(-*) by J774 cells apparently via the inactivation of the reduced nicotinamide adenine dinucleotide phosphate oxidase complex. Decreased O(2)(-*) production resulted in increased free NO levels (but not total NO production) indicating that decreased reactions between O(2)(-*) and NO are an outcome of equol's antioxidant activity in cell culture.

Effects of supplementation with purified red clover (Trifolium pratense) isoflavones on plasma lipids and insulin resistance in healthy premenopausal women

Consumption of isoflavone-rich soyabean protein is reported to reduce total and LDL-cholesterol, but the specific components responsible are undetermined. In a previous crossover trial we showed that purified isoflavones, derived from red clover (Trifolium pratense), raised HDL3-cholesterol in premenopausal women; however, these findings were inconclusive due to period and carryover effects. In an attempt to overcome this problem, we utilised a parallel study designed to re-examine the effects of purified isoflavones on plasma lipoproteins and markers of insulin resistance in premenopausal women. Twenty-five healthy premenopausal women participated in a double-blind, randomised, parallel study. The treatment group (n 12) consumed a placebo for the first menstrual cycle and an isoflavone supplement (86 mg/d, derived from red clover) for three cycles, while the placebo group (n 13) consumed a placebo supplement for four menstrual cycles. Blood samples were collected weekly during cycles 1, 3 and 4. Supplementation with isoflavones resulted in a 15-fold increase in urinary isoflavone excretion (P<0.0001). There were no significant effects on total cholesterol, LDL- and HDL-cholesterol, HDL subfractions, triacylglycerol, lipoprotein(a), glucose or insulin concentrations. Our present results indicate that purified isoflavones derived from red clover have no effect on cholesterol homeostasis or insulin resistance in premenopausal women, a group which is at low risk of CHD.

Time-resolved fluoroimmunoassay for equol in plasma and urine

We present a method for the determination of the isoflavan equol in plasma and urine. This estrogenic isoflavan, which is formed by the action of the intestinal microflora, may have higher biological activity than its precursor daidzein. High urinary excretion of equol has been suggested to be associated with a reduction in breast cancer risk. The method is based on time-resolved fluoroimmunoassay, using a europium chelate as a label. After synthesis of 4'-O-carboxymethylequol the compound is coupled to bovine serum albumin (BSA), then used as antigen to immunize rabbits. The tracer with the europium chelate is synthesized using the same 4'-O-derivative of equol. After enzymatic hydrolysis (urine) or enzymatic hydrolysis and ether extraction (plasma) the immunoassay is carried out. The antiserum cross-reacted to variable extent with some isoflavonoids. For the plasma method the cross-reactivity does not seem to influence the results, which were highly specific. The overestimation of the values using the urine method (164%) compared to the results obtained by a gas chromatography-mass spectrometry (GC-MS) method is probably due to some influence of the matrix on the signal, and interference of structurally related compounds. It is suggested that plasma assays are used but if urine samples are measured a formula has to be used to correct the values making them comparable to the GC-MS results. The correlation coefficients between the time-resolved fluoroimmunoassay (TR-FIA) methods and GC-MS methods were high; r-values for the plasma and urine method, were 0.98 and 0.91, respectively. The intra-assay coefficient of variation (CV%) for the TR-FIA plasma and urine results at three different concentrations vary between 5.5-6.5 and 3.4-6.9, respectively. The inter-assay CV% varies between 5.4-9.7 and 7.4-7.7, respectively. The working ranges of the plasma and urine assay are 1.27-512 and 1.9-512nmol/l, respectively.

Phytoestrogens--mechanism of action and effect on bone markers and bone mineral density

Dietary supplements, especially those containing phytoestrogens, frequently are used to either promote health or prevent disease. An estimated 20 billion dollars was spent on dietary supplements in the year 2000. Approximately 40% to 55% of Americans use supplements on a regular basis and 24% of these supplements contain herbs. Phytoestrogens are defined as any compound that is structurally or functionally related to ovarian or placental estrogens and their active metabolites. These compounds are widely used for various disorders related to women's health.

Isoflavonoid and lignan phytoestrogens as dietary biomarkers

Isoflavones and lignans are biologically active plant-food constituents that have potential chemopreventive properties. Quantitation of isoflavones and lignans in humans is necessary to establish the benefits and risks of exposure to these compounds in populations and to determine which components of a mixed diet contribute to the exposure. Isoflavones and lignans are metabolized by colonic bacteria to more biologically active metabolites; thus both the parent compounds and the metabolites are measured routinely. Isoflavonoids (genistein, daidzein, dihydrodaidzein, O-desmethylangolensin and equol) and lignans (enterolactone, enterodiol, matairesinol and secoisolariciresinol) can be quantified in various body fluids. Typically, high concentrations of isoflavonoids in urine and serum are associated with soy consumption, and high concentrations of lignans are associated primarily with intake of whole grains and other fiber-containing plant foods. Controlled feeding studies and nutritional epidemiologic studies demonstrate a linear dose response between dietary intake and urinary excretion of isoflavones. Lignan excretion is associated positively with dietary fiber intake as well as with diets that are on average higher in fiber and carbohydrate and lower in fat; thus lignans have also been proposed as a marker of healthier dietary patterns. The complex interactions between the colonic environment and the external and internal factors that modulate it contribute to significant variation in serum and urinary phytoestrogen levels among individuals. Understanding these sources of variation is important to be able to use these measures effectively as dietary biomarkers.

Comparing the pharmacokinetics of daidzein and genistein with the use of 13C-labeled tracers in premenopausal women

BACKGROUND

Despite significant interest in the risks and benefits of phytoestrogens to human health, few data exist on their pharmacokinetics in humans.

OBJECTIVE

We investigated the pharmacokinetics of the (13)C isotopic forms of daidzein and genistein in healthy humans, specifically addressing intraindividual variability, effect of increasing intake, and influence of prolonged exposure to a soy food diet.

DESIGN

Premenopausal women (n = 16) were administered 0.4 mg [(13)C]daidzein or [(13)C]genistein/kg body wt orally on 3 occasions, including once after eating soy foods for 7 d. On a further occasion the dose was doubled. Plasma and urinary [(13)C]isoflavone concentrations were measured by mass spectrometry.

RESULTS

Serum concentrations of [(13)C]genistein and [(13)C]daidzein peaked after 5.5 and 7.4 h, respectively. The systemic bioavailability and maximum serum concentration of [(13)C]genistein were significantly greater than those of [(13)C]daidzein. The bioavailability of both isoflavones did not increase linearly when the dietary intake was doubled. The mean volume of distribution normalized to bioavailability (V(d)/F), clearance rate, and half-life of [(13)C]daidzein were 336.25 L, 30.09 L/h, and 7.75 h, respectively; the corresponding values for [(13)C]genistein were 258.76 L, 21.85 L/h, and 7.77 h. The average recovery of [(13)C]daidzein and [(13)C]genistein in urine was 30.1% and 9.0% of the dose ingested, respectively.

CONCLUSIONS

The serum pharmacokinetics of [(13)C]daidzein and [(13)C]genistein were reproducible among healthy women, and genistein was more bioavailable than was daidzein. Pharmacokinetics were unaffected by chronic exposure to soy foods. Urinary isoflavone concentrations correlated poorly with maximal serum concentrations, indicating the limitations of urine measurements as a predictor of systemic bioavailability. The bioavailability of both isoflavones was nonlinear at higher intakes, suggesting that uptake is rate-limiting and saturable.

Quantitation of soy-derived phytoestrogens in human breast tissue and biological fluids by high-performance liquid chromatography

A new and reliable HPLC method for the quantitation of daidzein, equol, and genistein in human breast tissue has been developed. The method was applied to biopsies from women undergoing breast reductions, who, prior to surgery, had ingested either a soy isoflavone preparation or a placebo tablet. The results were compared with data collected for urine and serum of the same subjects using standard methods. The limits of detection in the breast tissue homogenate were 24.7 nmol/l for daidzein, 148.0 nmol/l for equol, and 28.4 nmol/l for genistein (S/N of 3). The chromatographic limits of quantitation were 62.5 nmol/l for daidzein and genistein, and 125.0 nmol/l for equol, for which the accuracies were 86.0%, 83.6%, and 81.8%, respectively. The coefficients of variation of these measurements were all below 20% (11.1% for daidzein, 16.4% for genistein, and 13.2% for equol). The sample preparation comprised a concentration step and the absolute limits of quantitation were, therefore, 4.7 nmol/l, 18.8 nmol/l, and 0.94 nmol/l for daidzein and genistein, and 9.4 nmol/l, 37.5 nmol/l, and 1.9 nmol/l for equol in urine, serum, and breast tissue homogenate, respectively. Recoveries were between 70% (+/-5.6%) in breast tissue homogenate and 100% (+/-14.1%) in urine and serum for all three compounds. Equol (less than 1 micromol/l homogenate) was found to be the predominant phytoestrogen in breast tissue and its concentrations exceeded those in serum. The concentrations of phytoestrogens were at least 100-fold higher in urine than in serum and breast tissue.

Androgen receptor gene alterations in Finnish male breast cancer

Mutations in the androgen receptor (AR) gene have been suggested to predispose to male breast cancer (MBC). Studies on MBC patients have not been based on the mutation screening of the entire coding region of the AR and the number of subjects has been small. Therefore, some AR gene alterations may have remained undetected. In the present study, we have comprehensively screened the entire coding region of the AR gene for mutations and also studied the role of AR CAG and GGC repeat lengths as risk factors for MBC in a cohort of 32 Finnish MBC patients. To estimate the possible involvement of the prostate cancer predisposing AR Arg726Leu germ-line mutation in MBC, this mutation was tested in 117 MBC patients. No germ-line mutations were found and the CAG and GGC repeat lengths were similar among MBC cases as among Scandinavian population. Our data indicate that the AR gene does not substantially contribute to MBC predisposition.

Phytoestrogens and breast cancer risk. Review of the epidemiological evidence

Phytoestrogens are natural plant substances. The three main classes are isoflavones, coumestans, and lignans. Phytoestrogens have anticarcinogenic potential, but they have also significant estrogenic properties. For an evaluation of the effect of phytoestrogens on breast cancer risk we reviewed the analytical epidemiological data. A total of 18 studies were included. Up to now, there are 13 studies that have assessed the direct relation between the individual dietary intake of soy products and the risk of breast cancer. Overall, results do not show protective effects, with the exception maybe for women who consume phytoestrogens at adolescence or at very high doses. Only four of these 13 studies are prospective, and none of them found statistically significant breast cancer reductions. Four studies assessed urinary isoflavones excretion in relation to breast cancer. Three of these are case control studies, where excretion was measured after breast cancer occurrence and thus seriously limiting causal interpretation of the results. The only prospective study with urinary measurements before breast cancer occurrence was done in a Dutch postmenopausal population and showed a non-significant breast cancer risk reduction for high excretion. Three studies measured enterolactone (lignan): two case control studies reported a preventive effect on breast cancer risk, but the only prospective study did not . In conclusion, few prospective studies (n = 5) were done to assess the effects of phytoestrogens on breast cancer risk. None of them found protective effects. However, these prospective studies did not focus on 'age at consumption', which seems to be important based on results from dietary case control studies done so far.

Soy isoflavonoid equol modulates the growth of benign and malignant prostatic epithelial cells in vitro

BACKGROUND

The dietary consumption of high levels of soy has been linked to reduced risks for prostate cancer (PC) in Asians and vegetarians. In vitro studies have focused on the two most abundant isoflavones in soy, genistein and daidzein. However, daidzein is differentially metabolized by gut microflora in humans, yielding compounds with very different bioactivities and half-lives. Asians are significantly more likely to produce the metabolite equol than Caucasians, suggesting its role in the prevention of PC. We hypothesize that equol is a bioactive metabolite that exerts direct antiproliferative effects on prostatic epithelial cells.

METHODS

Benign and malignant prostatic epithelial cells were treated in vitro with equol, genistein, and daidzein by using the range of concentrations found in the prostatic fluids of Asians consuming soy. Growth and cell cycle distribution were analyzed over time.

RESULTS

After 9 days of treatment, equol inhibited growth of benign human prostatic epithelial cells (PrEC) by 37% at 10(-6) M and 80% at 10(-5) M. Although genistein also had profound effects, daidzein appeared only one tenth as potent as equol. Equol and daidzein caused an accumulation of cells in G0/G1, whereas genistein arrested cells in G2/M. The isoflavonoids demonstrated differential effects on the established PC cell lines 22Rv1, LNCaP, LAPC-4, PC-3, and DU 145. PC-3 cells showed the greatest resistance.

CONCLUSION

Equol is a biologically active metabolite of daidzein that has potent antiproliferative effects on benign and malignant prostatic epithelial cells at concentrations that can be obtained naturally through dietary soy consumption.

The clinical importance of the metabolite equol-a clue to the effectiveness of soy and its isoflavones

Equol [7-hydroxy-3-(4'-hydroxyphenyl)-chroman] is a nonsteroidal estrogen of the isoflavone class. It is exclusively a product of intestinal bacterial metabolism of dietary isoflavones and it possesses estrogenic activity, having affinity for both estrogen receptors, ERalpha and ERbeta. Equol is superior to all other isoflavones in its antioxidant activity. It is the end product of the biotransformation of the phytoestrogen daidzein, one of the two main isoflavones found in abundance in soybeans and most soy foods. Once formed, it is relatively stable; however, equol is not produced in all healthy adults in response to dietary challenge with soy or daidzein. Several recent dietary intervention studies examining the health effects of soy isoflavones allude to the potential importance of equol by establishing that maximal clinical responses to soy protein diets are observed in people who are good "equol-producers." It is now apparent that there are two distinct subpopulations of people and that "bacterio-typing" individuals for their ability to make equol may hold the clue to the effectiveness of soy protein diets in the treatment or prevention of hormone-dependent conditions. In reviewing the history of equol, its biological properties, factors influencing its formation and clinical data, we propose a new paradigm. The clinical effectiveness of soy protein in cardiovascular, bone and menopausal health may be a function of the ability to biotransform soy isoflavones to the more potent estrogenic isoflavone, equol. The failure to distinguish those subjects who are "equol-producers" from "nonequol producers" in previous clinical studies could plausibly explain the variance in reported data on the health benefits of soy.

The atheroprotective effect of dietary soy isoflavones in apolipoprotein E-/- mice requires the presence of estrogen receptor-alpha

OBJECTIVE

Although the mechanisms by which dietary soy inhibits atherosclerosis are unclear, one line of evidence implicates an important role for its phytoestrogenic isoflavones. We sought to determine whether soy isoflavones exert atheroprotective effects through estrogen receptor-dependent processes and, if so, which estrogen receptor subtype (ie, alpha or beta) is involved.

METHODS AND RESULTS

We compared the effects of diets rich in soy protein that were either isoflavone depleted (0.04 mg/g protein isolate) or isoflavone-replete, or Soy(+IF) (1.72 mg/g protein isolate) in apolipoprotein E-deficient (ee) mice that had been crossed with estrogen receptor-alpha- and -beta-deficient mice to produce double-knockout alphaalphaee and betabetaee mice and (estrogen receptor) wild-type controls (AAee and BBee). Both male and ovariectomized female mice were studied (n=10 to 17 per treatment group; total n=201). After 16 weeks, atherosclerosis was assessed by quantifying the aortic content of esterified cholesterol. Atherosclerosis was reduced 20% to 27% (P<0.05) by Soy(+IF) in betabetaee, BBee, and AAee mice but was unaffected in alphaalphaee mice. The inhibitory effect of Soy(+IF) was unrelated to sex, total plasma cholesterol, VLDL, LDL, and HDL cholesterol.

CONCLUSIONS

The results indicate a necessary role for estrogen receptor-alpha-dependent processes in mediating the atheroprotective effects of dietary soy isoflavones.

Hydrolysis of isoflavone glycosides to aglycones by beta-glycosidase does not alter plasma and urine isoflavone pharmacokinetics in postmenopausal women

We investigated whether the bioavailability of isoflavones could be enhanced by enzymatic hydrolysis of glycosides to aglycones before consumption of a nonfermented soy food. Two drinks were formulated with an enriched isoflavone extract from soy germ (Fujiflavone P10), one of which was hydrolyzed enzymatically with beta-glucosidase to produce aglycones. In a randomized, double-blinded, cross-over study, six European, postmenopausal women consumed each soy drink at a 1-wk interval at a concentration of 1 mg total isoflavones/kg body. The plasma and urinary pharmacokinetics of daidzein, genistein and glycitein did not differ after consumption of the two beverages. Plasma total isoflavone concentrations reached 4-5 micro mol/L. The pharmacokinetics of glycitein were similar to those of daidzein. The isoflavone secondary metabolites detected were dihydrodaidzein in plasma and O-desmethylangolensin, equol, and dihydrogenistein in urine. The ratios of individual isoflavones to one another were not conserved from food to plasma to urine, indicating that the individual isoflavones do not have the same absorptions and body retentions. In conclusion, previous hydrolysis of glycosides to aglycones does not enhance the bioavailability of isoflavones in humans.

Absorption in humans of isoflavones from soy and red clover is similar

The absorption of isoflavones varies substantially among individuals. It is unknown whether isoflavone absorption differs between those originating from soy and those from red clover, which contain different mixtures of isoflavones. Because both soy and red clover are increasingly used in foods and supplements, these issues were studied in 14 subjects in a single-blind, randomized, placebo-controlled, crossover trial. Soybean isoflavone glycosides and red clover isoflavone aglycones were incorporated into a breakfast cereal and eaten daily for 2 wk each, separated by a 2-wk control or washout period. The 24-h excretions of isoflavones in urine were measured; approximately 25% of each isoflavone was recovered in urine, suggesting that similar amounts were absorbed irrespective of their glycoside/aglycone nature or the differing compositions of their sources (daidzein and genistein in soy and formononetin and biochanin in red clover). Although interindividual variability was high, there was less intraindividual variability; the amounts excreted when subjects consumed the two sources of isoflavone were correlated (r = 0.69; P = 0.007).

Quantitative analysis of urinary daidzein and equol by gas chromatography after solid-phase extraction and high-performance liquid chromatography

Daidzein and its main metabolite equol are isoflavone phytoestrogens. Several studies have suggested that intake of an isoflavone-rich diet may prevent hormone-related cancer and estrogen-related disorders (cardiovascular disease, osteoporosis and menopausal symptoms). To better understand the role of isoflavones in preventing such severe disease, several methods have been developed to measure these compounds in biological fluids. However, the analytical procedures to measure isoflavones are often time-consuming and require highly skilled technicians. In this paper we describe a method for urinary daidzein and equol measurement that combines solid phase extraction and HPLC purification before gas chromatographic determination. The specificity of the method was confirmed by the gas chromatography-mass spectrometry technique. The mean recovery of daidzein and equol was 94.6% and 97.0%, respectively. The repeatability of the method was in the range of 2.0-7.4% for daidzein and 1.3-4.9% for equol. A linear relationship between observed and expected values was found in the dilution (r2=0.9983 for daidzein; r2=0.9982 for equol) and addition (r2=0.9984 for daidzein; r2=0.9989 for equol) assays. The method is suitable to measure changes in the urinary excretion of isoflavones and to investigate urinary isoflavonoids as biomarkers of isoflavone exposure.

The effect of phytoestrogens on the female genital tract

Environmental oestrogens have been implicated in the pathogenesis of hormonally treated cancers (such as breast and prostate cancer), male infertility, and abnormalities of the male and female reproductive tracts. They may be derived from plants (phytoestrogens), pharmaceuticals, or other synthetic compounds not originally intended to have oestrogenic activity (including soy based infant formulas). This review will discuss the evidence from both animal studies and humans for an effect of these ubiquitous compounds on the development of the human female genital tract, in addition to prolonging the menstrual cycle, alleviating symptoms of the menopause, and protecting against the development of endometrial carcinoma.

Phytoestrogens: a review of recent findings

Phytoestrogens have been investigated at the epidemiological, clinical and molecular levels to determine their potential health benefits. The two major groups of phytoestrogens, isoflavones and lignans, are abundant in soy products and flax respectively, but are also present in a variety of other foods. It is thought that these estrogen-like compounds may protect against chronic diseases, such as hormone-dependent cancers, cardiovascular disease and osteoporosis. Furthermore, phytoestrogens are used as a natural alternative to hormone replacement therapy and to reduce menopausal symptoms. Phytoestrogens have been shown to induce both estrogenic and anti-estrogenic effects but their biological relevance and potency have not been well characterized. In children, consumption of soy-based formulas and soy milk can lead to high levels of exposure to phytoestrogens with only limited data available concerning potential benefits or adverse effects. Phytoestrogens are considered good candidates for use in natural therapies and as chemopreventive agents in adults. Safe and efficacious levels have yet to be established.

Estrogens and environmental estrogens

The natural female sex hormone estrogens binds once inside the cell to a protein receptor to form a 'ligand-hormone receptor complex'. The binding activates the hormone receptor, which triggers specific cellular processes. The activated hormone receptor then turns on specific genes, causing cellular changes that lead to responses typical of a ligand-hormone receptor complex. Estrogens (especially estradiol) bring out the feminine characteristics, control reproductive cycles and pregnancy, influence skin, bone, the cardiovascular system and immunity. Natural hormones are more potent than any of the known synthetic environmental estrogens (except drugs such as diethylstilbestrol [DES]). Estrogen production varies according to different factors (gender, age and reproductive cycles). Women produce more estrogen than men and the production is more abundant during fetal development than in the postmenopausal period. Most natural estrogens are short-lived, do not accumulate in tissue and are easily broken down in the liver. In contrast to natural estrogens, estrogenic drugs such as ethynylestradiol diethylstilbestrol (DES), synthetic environmental estrogens such as beta-hexachlorocyclohexane (beta-HCH), polychlorinated biphenyls (PCBs), o, p, p'DDT, 4-nonylphenol (NP) and phytoestrogens such as isoflavones or lignans, are more stable and remain in the body longer than natural estrogens. Because most of these compounds are lipophilic, they tend to accumulate within the fat and tissue of animals and humans. Thus, depending on the natural estrogen levels, environmental estrogens may have different influences (mimicking, blocking or cancelling out estrogen's effects) on estrogen activities.

The inhibitory effect of soy protein isolate on atherosclerosis in mice does not require the presence of LDL receptors or alteration of plasma lipoproteins

The mechanisms by which dietary soy favorably influences lipoprotein metabolism and inhibits atherosclerosis are uncertain. Studies of blood mononuclear cells and cultured hepatocytes have indicated that certain soy peptides (i.e., 7S globulins) stimulate expression of LDL receptors. This pathway represents a hypothetical mechanism by which soy's hypocholesterolemic and antiatherosclerotic effects may be mediated. However, direct evidence supporting this hypothesis is lacking. To address this, we compared effects of dietary soy protein isolate in two genetically engineered mouse models of atherosclerosis. One mouse [LDL receptor -/- + apolipoprotein (apo) B transgenic] is devoid of LDL receptors and overproduces apolipoprotein B, whereas the other (apoE -/-) has a normal complement of LDL receptors but does not produce apolipoprotein E. Male (n = 10-12/group) and ovariectomized female (n = 10-12/group) mice were studied. There were three treatment groups, which differed principally by the source of the protein component of the diet: 1) casein/lactalbumin (no isoflavones), 2) alcohol-washed soy protein isolate (total isoflavones = 0.04 mg/g), and 3) intact soy protein isolate (total isoflavones = 1.72 mg/g). Atherosclerosis was assessed by quantifying the aortic content of esterified cholesterol. Atherosclerosis was inhibited (relative to the casein/lactalbumin group) by both alcohol-washed (45 and 31%) (P < 0.05) and intact (65 and 41%) (P < 0.05) soy protein isolate in LDL receptor -/- and apoE -/- mice, respectively. There was no sex difference. In a two-way analysis, there were significant effects of type of soy isolate and type of mouse. The antiatherosclerosis effect was enhanced in LDL receptor -/- mice (P < 0.001) and diminished in mice fed alcohol-washed soy protein isolate (P < 0.001). Furthermore, inhibitory effects of soy on atherosclerosis were unrelated to plasma LDL, VLDL or HDL cholesterol concentrations. The results represent direct evidence for the existence of LDL receptor- and plasma lipoprotein-independent pathways by which dietary soy protein isolate inhibits atherosclerosis.

Comparative metabolism of genistin by human and rat gut microflora: detection and identification of the end-products of metabolism

1. Biotransformations by gut microflora play a pivotal role in determining the biological activity of isoflavones that occur in soya-based foods predominantly as betaglycosyl conjugates. Microflora prepared from rat caecae and human faeces were used to investigate the metabolic fate of genistein beta-glycosides extracted from soya flour. The end-products of such metabolism were determined by parallel incubations of microflora with [2',3,5',6'-3H] and [4-14C]-labelled genistein. 2. Quantitative analysis by LC-MS/IS indicated very rapid and complete degradation of genistin, which was associated with a transient increase in genistein. Qualitative studies indicated that the malonyl and acetyl glycosides of genistein were also degraded by the microflora. 3. Incubation of caecal and faecal microflora with [3H] and [14C]genistein yielded similar radiolabelled metabolites, which were identified by radio-LC-MS(n) as the intermediates dihydrogenistein and 6'-hydroxy-O-desmethylangolensin and end-product 4-hydroxyphenyl-2-propionic acid. This profile of genistein metabolites indicated selective hydrolysis of 6'-hydroxy-O-desmethylangolensin between carbon atoms 1' and 1 to yield the end-products 4-hydroxyphenyl-2-propionic acid and 1,3,5-trihydroxybenzene. 4. The biological significance of the products of genistein metabolism warrant further investigation since they may play an important role in mediating the beneficial antioxidant health effects associated with the consumption of isoflavones in food.

Assessing the accuracy of a food frequency questionnaire for estimating usual intake of phytoestrogens

The primary objective of this study was to assess the accuracy of a modified Block food frequency questionnaire (FFQ) with respect to its ability to assess usual dietary intakes of daidzein and genistein. Participants were a convenience sample of 51 Japanese and 18 Caucasian women. All interviews were conducted between February 1997 and October 1997. At each of the four study visits, participants provided a 24-hour urine specimen and a 48-hour dietary recall. At the first visit, participants also completed an interviewer-administered modified Block FFQ. The daidzein and genistein intakes estimated using the FFQ were moderately correlated with the mean estimates of daidzein and genistein intake calculated from four 48-hour dietary recalls (correlation for daidzein = 0.49-0.58 and correlation for genistein = 0.45-0.54) and estimates of urinary concentrations of these compounds calculated from four collections (correlations for daidzein and genistein = 0.49 and 0.30, respectively). The accuracy of the modified Block FFQ for assessment of usual daidzein and genistein intakes is supported by this study. These results support the use of this instrument in epidemiological studies as an easy and low-cost method to assess the usual dietary daidzein or genistein intake.

Population-based case-control study of soyfood intake and breast cancer risk in Shanghai

We evaluated the association of soyfood intake and breast cancer risk in a population-based case-control study among Chinese women in Shanghai. Included in the study were 1459 cases and 1556 age-matched controls, with respective response rates of 91.1% and 90.3%. Usual soyfood intake was assessed using a food frequency questionnaire (FFQ). Separate analyses were performed for all subjects and for the subset who reported no recent change in soyfood intake. The intake levels of soyfoods among women in Shanghai are high, with 96.6% women reporting soyfood consumption at least once a week. A statistically non-significant reduced risk (odds ratio (OR) = 0.78 95% CI = 0.52-1.16) of breast cancer was observed among those who reported eating soyfood at least once a week. Compared to those in the lowest decile intake group, women in the highest decile intake group had a 30% reduced risk of breast cancer (OR = 0.66, 95% CI = 0.46-0.95), but no monotonic dose-response relation was observed (P for trend, 0.28). Stratified analyses showed that the inverse association was restricted primarily among women who had a high body mass index (BMI), with an adjusted OR of 0.30 (95% CI = 0.10-0.94) observed for the highest intake group. The reduction in risk was stronger for breast cancer positive for both oestrogen receptor (ER) and progesterone receptor (PR) (OR = 0.44, 95% CI = 0.25-0.78) than those with other ER/PR status. More pronounced inverse associations were observed in analyses among those who reported no recent change in soyfood intake than those conducted in all subjects. A dose-response relation between soyfood intake and breast cancer risk was observed in this subset of women (P for trend, 0.02), with an OR of 0.46 (95%CI = 0.28-0.75) for those in the highest decile intake group. No clear monotonic dose-response relation was found between soyfood intake and breast cancer risk among regular soy eaters, but nevertheless the results suggest that regular soyfood consumption may reduce the risk of breast cancer, particularly for those positive for ER and PR; the effect may be modified by body mass index.

Biomarkers in disease and health

Biomarkers have considerable potential in aiding the understanding of the relationship between diet and disease or health. However, to assess the role, relevance and importance of biomarkers on a case by case basis it is essential to understand and prioritise the principal diet and health issues. In the majority of cases, dietary compounds are only weakly biologically active in the short term, have multiple targets and can be both beneficial and deleterious. This poses particular problems in determining the net effect of types of foods on health. In principle, a biomarker should be able to contribute to this debate by allowing the measurement of exposure and by acting as an indicator either of a deleterious or of an enhanced health effect prior to the final outcome. In this review, the examples chosen - cancer (stomach, colon/rectal, breast); coronary heart disease and osteoporosis - reflect three major diet-related disease issues. In each case the onset of the disease has a genetic determinant which may be exacerbated or delayed by diet. Perhaps the most important factor is that in each case the disease, once manifest, is difficult to influence in a positive way by diet alone. This then suggests that the emphasis for biomarker studies should focus on predictive biomarkers which can be used to help in the development of dietary strategies which will minimise the risk and be of greater benefit.

Wheat bran and soy protein feeding do not alter urinary excretion of the isoflavan equol in premenopausal women

The capacity to convert the soy isoflavone daidzein to equol in vivo is presumably determined by an individual's intestinal microfloral populations; however, diet may also influence this conversion. The objectives of the present study were to determine whether a 1-mo supplementation of dietary fiber as wheat bran increases urinary equol excretion in equol excreters and stimulates equol production in nonexcreters and whether longer-term soy isoflavone intake increases equol production or alters overall urinary isoflavone excretion. First, we screened 74 women, ages 20-40 y, and determined their equol-excreter status. In these women, health and lifestyle patterns and habitual dietary intake did not differ according to equol-excreter status. Next, 26 of the women (13 equol excreters and 13 nonexcreters) were assigned (blocked on equol-excreter status) to either longer-term (1 mo) or short-term (4 d) soy protein supplementation. Within each soy treatment group, women participated in two 1-mo intervention periods (the exact length was determined by each woman's menstrual cycle) during which they consumed their usual diets supplemented daily with either 0 or 16 g dietary fiber in a randomized crossover design. A 1-mo washout period separated the two diet periods. Among the 19 women who completed both periods, fiber supplementation did not increase equol production in equol excreters or nonexcreters. In addition, isoflavonoid excretion did not differ by fiber dose or length of soy intervention. These results suggest that a daily 16 g-fiber dose as wheat bran and the addition of soy protein do not alter significantly the capacity of colonic microflora to produce equol.

Soy isoflavones: are they useful in menopause?

In October 1999, the US Food and Drug Administration authorized the use on food labels of health claims associated with soy protein and the reduced risk of coronary heart disease. Several studies have indicated that a total daily intake of 25 g of soy protein paired with a low-fat diet resulted in clinically important reductions of total cholesterol and low-density lipoprotein (LDL) cholesterol levels. Soybeans are a rich source of isoflavones, a class of phytoestrogens found predominantly in legumes and beans. Soy isoflavones are heterocyclic phenols with structural similarity to estradiol-17beta and selective estrogen receptor modulators. Actions at the cellular level depend on the target tissue, receptor status of the tissue, and the level of endogenous estrogen. Studies of soy-based diets evaluating the relation between soy consumption and serum lipid concentrations revealed that soy consumption significantly decreased total cholesterol, LDL cholesterol, and triglyceride levels. However, the soy isoflavones do not increase high-density lipoprotein cholesterol or triglyceride levels. The effects of soy protein on other target tissues reflect estrogenlike agonist and antagonist effects. Epidemiological studies suggest a protective effect of soy protein on breast tissue as evidenced by the lower rates of breast cancer in East Asian countries where soy is a predominant part of the diet. Data available from human studies on the effect of isoflavones on osteoporosis are limited, and additional studies are needed to support a role in osteoporosis prevention. Thus far, there is no evidence for a stimulatory effect of isoflavones on the endometrium. A few studies reveal a minimal effect of soy on hot flashes, with soy reducing hot flashes 45% and placebo causing a 30% reduction compared with an approximate 70% reduction in hot flashes with estrogen replacement therapy. Evidence from laboratory studies reveals neither a positive nor a negative effect of soy isoflavones on cognition. To date, no adverse effects of short- or long-term use of soy proteins are known in humans. The only adverse effects known are those reported in animals (infertility in sheep and quails grazing on phytoestrogen-rich pastures). In conclusion, soy isoflavones are biologically active compounds. Current data are insufficient to draw definitive conclusions regarding the use of isoflavones as an alternative to estrogen for hormone replacement in postmenopausal women. Although epidemiological and basic laboratory studies allude to the possible protective effects of soy isoflavones at specific target tissues, randomized, placebo-controlled clinical trials are necessary to address these important issues.