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        Is equol production beneficial to health?
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Abstract

The health benefits associated with soya food consumption have been widely studied, with soya isoflavones and soya protein implicated in the protection of CVD, osteoporosis and cancers such as those of the breast and prostate. Equol (7-hydroxy-3-(4’-hydroxyphenyl)-chroman), a metabolite of the soya isoflavone daidzein, is produced via the formation of the intermediate dihydrodaidzein, by human intestinal bacteria, with only approximately 30–40% of the adult population having the ability to perform this transformation following a soya challenge. Inter-individual variation in conversion of daidzein to equol has been attributed, in part, to differences in the diet and in gut microflora composition, although the specific bacteria responsible for the colonic biotransformation of daidzein to equol are yet to be identified. Equol is a unique compound in that it can exert oestrogenic effects, but is also a potent antagonist of dihydrotestosterone in vivo. Furthermore, in vitro studies suggest that equol is more biologically active than its parent compound, daidzein, with a higher affinity for the oestrogen receptor and a more potent antioxidant activity. Although some observational and intervention studies suggest that the ability to produce equol is associated with reduced risk of breast and prostate cancer, CVD, improved bone health and reduced incidence of hot flushes, others have reported null or adverse effects. Studies to date have been limited and well-designed studies that are sufficiently powered to investigate the relationship between equol production and disease risk are warranted before the clinical relevance of the equol phenotype can be fully elucidated.

Abbreviations:
BMC

bone mineral content

BMD

bone mineral density

DHT

dihydrotestosterone

ER

oestrogen receptor

Equol (7-hydroxy-3-(4′-hydroxyphenyl)-chroman) is a non-steroidal oestrogen that is formed exclusively via the bacterial metabolism of the soya isoflavone daidzein in the intestine. Equol was first identified in the urine of pregnant mares in 1932(1), but was not discovered in human urine until the 1980s(2). The oestrogenic effects of equol first became apparent in the 1940s when sheep grazing on pastures containing clover in South Western Australia became infertile(3). This ‘clover disease’ was later attributed to the rich quantities of formononetin present in the clover, which is converted to daidzein in the rumen(4). The isoflavone intake associated with clover disease (20–100 g/d) was, however, far in excess to that of human subjects who consume a soya-rich diet (isoflavone intake among older adults in Japan is about 40 mg/d)(5).

Daidzein, one of the major soya isoflavones, is present in most soya foods as the glycoside daidzin. Upon consumption, daidzin is hydrolysed by intestinal β-glycosidases to form the bioavailable aglycone, daidzein. Daidzein is further metabolised by colonic bacteria to form equol or O-desmethylangolensin via a pathway that involves the formation of the intermediate dihydrodaidzein(6, 7) (Fig. 1). The importance of the gut microflora in the biotransformation of daidzein to equol is evident from studies that have demonstrated that germ-free animals(9) and infants below the age of 4 months(1012) who have a poorly developed gut microflora, do not produce equol when fed a soya diet.

Fig 1. Metabolism of the soya isoflavone daidzein to O-desmethylangolensin and equol. (From Lampe(8); reproduced with permission.)

In comparison with other isoflavones, equol is unique in structure, having a chiral carbon atom at position C-3 of the furan ring. Equol can therefore exist in two distinct enantiomeric forms, S-equol and R-equol (Fig. 2). It has recently been shown that human intestinal bacteria exclusively synthesise the S-equol enantiomer from daidzein(14). Not every individual can, however, produce equol; approximately 30% of adults among western populations(1517) and 60% of Asian adults(18, 19) have the ability to produce equol from daidzein following a soya challenge. Inter-individual variation in the ability to produce equol has been attributed to a number of factors, primarily inter-individual variation in gut microflora composition. Bacterial species that have been identified as being capable of converting daidzein to equol in vitro include Streptococcus intermedius, Ruminococcus products and Bacteriodes ovatus (20). It is evident that different bacteria are involved in different stages of the biotransformation of daidzein to equol and that these bacteria may differ between individuals(7). Lactococcus garvieae can efficiently convert daidzein to S-equol and has recently been used to produce the natural S-equol-containing supplement SE5-OH(21). Diet has also been associated with the ‘equol producer’ phenotype, with equol producers reported to consume higher amounts of carbohydrate(15, 16) and lower amounts of fat(16) as a percentage of energy than non-producers. Fibre(15), meat(22), PUFA and alcohol intake(23) have also been positively associated with the ability to produce equol among western populations. The frequency of equol producers among vegetarian adults has been reported to be much higher than non-vegetarians, with levels comparable to those of Asian adults(17). Dietary modification may therefore potentially influence equol producer status. Consequently, prebiotics and probiotics have been studied as an aid to induce equol production in adults; however, studies to date have been unsuccessful(2426). Familial correlation and segregation analysis has suggested that the ability to produce equol is under a certain degree of genetic control with other non-genetic factors also involved(27). The involvement of host genetics is credible given that the ability to produce equol appears to remain stable over time(28).

Fig 2. Comparison of the conformational structures of the diastereoisomers of equol, showing the site position of the chiral carbon centre. (From Brown et al.(13); reproduced with permission.)

Individuals with a plasma equol concentration of <40 nmol/l (10 μg/l) can be classified as ‘equol non-producers’ and those with concentrations >83 nmol/l (20 μg/l) classified as ‘equol producers’(6). Cut-offs can also be applied to urinary concentrations with an equol producer being defined as excreting >1000 nmol/l(15, 16). Defining thresholds for equol producer status is complicated by variations in the methodologies used to detect S-equol. Indeed, with sensitive methods, small quantities of equol can be detected in the urine of most individuals, as small amounts of equol can be obtained via animal foods in the diet e.g. cow's milk(29). Many studies have defined equol producers as those in whom equol is detectable according to the assay system used. Professor Ken Setchell has developed a more accurate approach to define an ‘equol producer’ that is independent of isoflavone intake and the analytical technique used. Thus, a threshold value of log10-transformed urinary S-equol:daidzein ratio of greater than −1·75 can be used to define an equol producer(17).

Biological properties of equol

Approximately 50% of equol circulates in the ‘free’ form, unbound to serum protein(30) in comparison with only 18·7% daidzein and 4·6% oestradiol(6). Equol can be classified as a selective oestrogen receptor (ER) modulator, having a selective affinity for ERβ. S-equol binds to ERβ with approximately 20% as much affinity as 17β-oestradiol and has a much stronger affinity for ERβ compared to R-equol(14). Equol also has stronger in vitro antioxidant activity compared to other isoflavones(3133). S-equol is somewhat unique in that in addition to its oestrogenic properties, it potently antagonizes dihydrotestosterone (DHT) in vivo (34). In addition, equol inhibits binding of oestradiol, testosterone and sex hormone binding globulin in a dose-dependent manner(35). Due to the biological properties of equol, it has been postulated that the ability to produce equol may be beneficial to health, in particular, in relation to hormone-dependent diseases.

A wealth of clinical studies investigating the effects of soya and isoflavones on health has been conducted to date, yielding somewhat inconsistent findings. In 2002, Professor Ken Setchell postulated that the beneficial effects of soya/isoflavones may be dependent on equol producer status; a theory now known as the ‘equol hypothesis’(6). This paper will outline research to date that has examined the effects of equol on the risk and progression of a number of chronic diseases including breast and prostate cancer and CVD. Effects on bone health and menopause will also be discussed.

Equol and breast cancer

Breast cancer is the most common cancer in women worldwide, accounting for about 23% of all female cancers(36). Incidence in western countries, including the UK and Ireland, is between two- and four-fold higher compared to Asian countries(37). In the 1980s, Setchell and Adlercreutz were the first to postulate that soya isoflavones may confer protection against breast cancer(38) and Adlercreutz et al. have demonstrated that individuals living in countries with high rates of breast cancer excrete low amounts of isoflavones(3941) in comparison with those living in areas at low risk(4246).

Few epidemiological studies have investigated the association between equol and breast cancer risk, albeit two studies have reported positive findings. In a case–control study conducted in women (144 pairs, aged 30–84 year) resident in Perth, western Australia, Ingram et al.(47) reported that equol excretion was associated with a substantial reduction in risk of breast cancer (OR 0·27, 95% CI 0·1, 0·69 highest v. lowest quartile equol excretion). Furthermore, a nested case–control study conducted within the Multiethnic Cohort Study in Hawaii and Los Angeles reported similar findings for white post-menopausal women, with the highest quartile of equol excretion associated with a reduced risk of breast cancer compared to the lowest quartile (OR 0·27 (95% CI 0·08, 0·95))(48). This finding, however, was not statistically significant (P=0·07) possibly due to the small sample size (51 white cases and 96 controls). In contrast to the above findings, a prospective study conducted within the UK arm of the European Prospective Investigation of Cancer and Nutrition study, analysed 114 spot urines and 97 serum samples from women (aged 41–76 years) who developed breast cancer 4–8 years after sampling(49). Samples were matched to 219 urine and 187 serum controls. Exposure to daidzein and equol was significantly associated with increased breast cancer risk (log2 OR=1·34 (95% CI 1·06, 1·7)), P=0·013 for urinary equol). It was noted, however, that dietary intake was low, with only 3% of the study population consuming soya foods and mean urinary equol excretion was 0·5 (sd 2·1) μg/mmol creatine. A follow-up to this study that included 237 cases and 952 controls (aged 45–75 years), however, reported no association between serum or urinary equol and breast cancer risk, although urinary equol excretion was associated with an increased risk of cancer in those women with ER-positive tumours(50). It is plausible that inconsistent findings from epidemiological studies conducted in western populations investigating the relationship between equol and breast cancer risk are due to the low isoflavone intakes among these groups. Isoflavone intake in Europe and USA is about 3 mg/d(51) and the European Prospective Investigation of Cancer and Nutrition study outlined earlier(49) reported mean dietary intakes of only 0·2 mg/d daidzein and 0·2 mg/d genistein.

Breast cancer biomarkers have also been studied, to a limited extent, in relation to equol excretion in human subjects. Duncan et al. (52) supplemented the diet of fourteen premenopausal women with 0·15, 1·0 and 2·0 mg isoflavones/kg bodyweight per day and reported that for all doses tested, equol producers had a more favourable hormonal profile compared to the equol non-producers. Equol producers generally had lower concentrations of oestrone, oestrone-sulphate, testosterone, androstenedione, dehydroepiandrosterone, dehydroepiandrosterone-sulphate and cortisol and higher levels of sex hormone binding globulin, a profile associated with lower risk of breast cancer. In addition, Nettleton et al. (53) have shown that in post-menopausal women, equol producer status is associated with increased concentrations of urinary 2-hydroxyestrogens and an increased 2-hydroxyestrogen:16α-hydroxyestrone ratio. 2-hydroxyestrogen:16α-hydroxyestrone has previously been reported to be inversely associated with breast cancer risk(5456) although not all studies support this finding(57, 58). Equol producer status may also confer protection on mammographic density, an established marker positively associated with breast cancer risk. Frankenfeld et al. (59) studied fifty-five overweight post-menopausal women given a soya challenge (soya protein containing >10 mg daidzein/d for 3 d) to determine equol status and reported that mammographic density was 39% lower in equol producers compared to non-producers (P=0·04). Although Fuhrman et al.(60) found no independent association of equol status or soya intake with percent mammographic density, among post-menopausal equol producers (n 75), those who consumed soya on a weekly basis had a significantly lower breast density. In contrast, for the equol non-producers (n 157), weekly soya intake was associated with increased mammographic density, suggesting that soya may exert differential effects on breast tissue depending on equol producer status. In a recent study among premenopausal women in USA (n 200), no differences were found in mammographic breast density between equol producers and non-producers(61).

Although the mechanisms of action of soya isoflavones in breast and prostate cancer have been well studied(62), little research has focused on equol specifically. In vitro studies have demonstrated that equol, at concentrations ⩽10 μm, stimulates the proliferation of ER positive breast cancer cells(6365), whereas at higher concentrations (≥10 μm), proliferation of the oestrogen-insensitive breast cancer cell line MDA-MB-231 is inhibited(64, 65). We have also demonstrated that racemic equol and S-equol inhibit the invasion of MDA-MB-231 breast cancer cells through matrigel and that racemic, but not S-equol, protects MCF-10A breast cells against DNA damage induced by 2-hydroxy-4-nonenal or menadione(65). This latter finding is of particular interest as it implicates the R-equol rather than the S-equol enantiomer as being responsible for this antigenotoxic effect and warrants further investigation. In support of our findings, another recent study(13) has highlighted the potential beneficial effects of the unnatural R-equol enantiomer in breast cancer. In this study, the effects of a diet supplemented with 250 mg/kg of S- or R-equol on dimethylbenz(a)anthracene-induced mammary tumours in rats were investigated. While S-equol was found to have no chemopreventive effects, R-equol significantly reduced the number of tumours detected, increased tumour latency and reduced the invasive capacity of the tumours. Both R- and S-equol enantiomers therefore deserve further investigation as potential anti-carcinogenic agents in the breast.

There have been concerns over the safety of isoflavones due to their oestrogen-like properties, particularly in relation to breast cancer. The suggestion that isoflavones may have adverse effects in women with breast cancer or at increased risk of the disease has arisen mainly from animal studies and is generally not supported by clinical and epidemiological studies. However, little is known regarding the effects of isoflavones on breast tissue and so these women should perhaps be cautious with regard to their isoflavone intake(66). The European Food Safety Authority is currently conducting a safety evaluation and will assess the hazards to human health and also the potential benefits of isoflavones, foods rich in isoflavones and isoflavone supplements.

Equol and prostate cancer

Prostate cancer is the second most common cancer in men worldwide(36). Similar to breast cancer, prostate cancer incidence is much higher (3–30-fold) in the western world in comparison with Asian countries(37). A case–control study(67) that examined the equol producer status among residents with prostate cancer and controls in Japan, Korea and USA reported that the number of equol producers was significantly lower in the prostate cancer group compared to controls for both the Japanese (29% cases, 46% controls) and Korean (30% cases, 59% controls) cohorts, suggesting that the ability to produce equol is protective against prostate cancer. As expected, the number of equol producers in USA cohort (17% cases, 14% controls) was lower compared to the Asian groups. In another case–control study nested in a community-based cohort in Japan(68), equol status was examined in relation to prostate cancer risk in 52 cases and 151 controls aged ≥40 years from 45 areas of Japan. Serum equol was inversely associated with prostate cancer risk, with the highest tertile of serum equol associated with a 60% reduction in disease risk.

As first reported in 2004, equol is a potent antagonist of DHT(34), one of the key sex hormones implicated in prostate cancer development. Unlike other 5α-reductase inhibitors, equol does not affect the synthesis of DHT. Instead equol appears to bind directly to DHT, thus preventing DHT binding to the androgen receptor. In a recent Japanese intervention trial(18), twenty-eight healthy volunteers were given 60 mg soya isoflavones (containing 19·1 mg daidzein) daily for 3 months and the effects on sex hormones implicated in prostate cancer development was investigated. Following supplementation, serum sex hormone binding globulin increased and serum-free testosterone and DHT decreased significantly compared to baseline concentrations. When subjects were stratified according to equol status, these beneficial effects were only significant in those men who were equol producers (64%), further implicating equol as a chemopreventive agent in prostate cancer. Of interest, in this study, two of the volunteers who had no detectable serum equol (<0·5 μg/l) at baseline, had detectable concentrations of equol in their serum (1·1 μg/l and 24·1 μg/l) following three months of soya supplementation, suggesting that equol producer status can be manipulated with prolonged and consistent soya isoflavone consumption. This theory is further supported by Hedlund et al.(22) who reported that Caucasian men who consumed ≥30 mg soya isoflavones/d for at least 2 years were 5·3 times more likely to produce equol than men who consumed ⩽5 mg/d.

In vitro studies have demonstrated that equol inhibits the proliferation of benign human prostatic epithelial cells, causing an accumulation of cells in G0/G1 of the cell cycle(69), with the effects of equol being 10-fold more potent than that of daidzein. Equol has also been shown to inhibit the proliferation of the prostate cancer cell lines 22Rv1(69), DU145(69), LAPC-4(65, 69), LNCaP(65, 70) and PC-3(70). Inhibition of growth occurred at physiologically relevant equol concentrations (⩽10 μm) given that equol concentrations in the prostatic fluid of men consuming a soya-rich diet can be as high as 13·5 μm(69, 71). Equol has also been shown to inhibit the invasion of PC-3 prostate cancer cells through matrigel(65) , an effect that was only significant at an equol concentration of 50 μm. Although tissue concentrations of equol are unknown, daidzein concentrations in prostate tissue have been reported to be ten-fold higher in comparison with serum levels(72) and therefore the possibility of a concentration of 50 μm equol being physiologically relevant in the tissue cannot be excluded.

Equol and CVD

CVD is one of the leading causes of premature death in the UK(73). High soya intake has been associated with a lower incidence of CVD(74, 75) and in 1999 the Food and Drug Administration approved a health claim for CHD based on the cholesterol-lowering effects of soya protein(76), a claim supported by the Joint Health Claims Initiative in the UK in 2002. Recent meta-analyses, however, demonstrate that the cholesterol-lowering effects of soya protein, though still relevant, are much smaller (about 3–5%)(7780) than initially reported (12·9%)(81). European Food Safety Authority recently concluded that a cause and effect relationship has not been established between the consumption of soya protein and the reduction of LDL-cholesterol concentrations(82). In addition, the Food and Drug Administration is currently re-evaluating its soya protein health claim.

Although a wealth of studies has investigated the effects of soya on heart health, few have stratified subjects according to equol producer status. In a randomized, placebo-controlled crossover trial in which twenty-six mildly hypercholesterolaemic subjects consumed soya foods containing 80 mg isoflavones/d or placebo for 5 weeks, no significant differences in plasma lipids, blood pressure or arterial compliance were observed(83). Retrospective analysis, however, showed that in the equol producers (35% of cohort), significant reductions in total cholesterol (8·5%), LDL cholesterol (10%), LDL:HDL (13·5%), plasma TAG (21%) and lipoprotein(a) (11%) were observed with the soya diet, suggesting that it may only be those capable of producing equol that benefit from the lipid-lowering effects of soya. In contrast, in another crossover study, Hall et al.(84) found no effect of an isoflavone-enriched cereal bar (providing 50 mg isoflavones/d) on plasma lipids or blood pressure in post-menopausal women, with no differences observed between equol producers (28%) and non-producers. This study, however, involved healthy post-menopausal women (aged 45–70 years) who likely had a low risk of CVD at baseline which may explain why no significant effects were detected. Furthermore, in addition to the fact that a lower isoflavone dose was used in this study, in contrast to the previous study(83), isolated isoflavones added to a cereal bar were used rather than a whole soya food, which highlights the potential importance of the food matrix used in these studies.

In a 12-month double-blind, randomized trial, Kreijkamp-Kaspers et al.(85) compared the effects of soya protein (providing 99 mg isoflavones/d) to milk protein on blood pressure and endothelial function in 202 post-menopausal women aged 60–75 years. For the equol producers within the soya group (28%), both systolic and diastolic blood pressure decreased and endothelial function improved following intervention, whereas blood pressure increased and endothelial function deteriorated in the non-producers (Fig. 3). These beneficial outcomes in the equol producers were not significantly different from the non-producers possibly due to the small number of equol producers within the study.

Fig 3. The effects of a soya supplement (99 mg isoflavones/d for 12 months) on blood pressure (BP) and flow-mediated dilation (FMD) in healthy postmenopausal women(85). (P values represent repeated-measures ANOVA for time×equol-status interaction.)

In one of the first studies to examine the relationship between equol production and CVD risk markers within a Chinese population(86), it was observed that equol producers, who comprised 50% of the cohort of 202 adults aged 20–69 years, had lower serum uric acid, TAG and waist:hip ratio and tended to have higher LDL cholesterol compared to non-producers, suggesting that equol phenotype may influence CVD risk.

Equol and bone health

Due to the oestrogenic properties of isoflavones, their effects on bone have been widely investigated(87); however, the European Food Safety Authority has recently concluded that there is insufficient evidence to establish a cause and effect relationship between the consumption of soya isoflavones and the maintenance of bone mineral density (BMD) in post-menopausal women(88). Few studies have stratified subjects according to equol producer status. In a two-year randomized, placebo-controlled trial, Lydeking-Olsen et al.(89) investigated the effects of soyamilk (providing 76 mg isoflavones/d), natural transdermal progesterone or the combination, on BMD and bone mineral content (BMC) in the lumbar hip and spine in post-menopausal women who either had established osteoporosis or had three or more risk-factors for the disease. The percentage change in BMD and BMC in the lumbar spine was not significantly different from baseline in the soya isoflavone or transdermal progesterone group, whereas BMD and BMC significantly decreased in the groups receiving the combined treatment (−2·8% BMD loss, −2·4% BMC loss) or the placebo (soyamilk without isoflavones) (−4·2% BMD loss, −4·3% BMC loss). Equol producers (n 10) showed a 2·4% and 2·8% increase in BMD and BMC, respectively, in comparison with equol non-producers (n 12) who had corresponding increases of 0·6% and 0·3%. Although these findings were not statistically significant, doubtless due to the small numbers involved, they highlight the potentially important role of equol in bone health.

In contrast to the above study, in exploratory analyses, Frankenfeld et al.(90) reported that among post-menopausal women who regularly consumed soya (>1 serving of tofu or soyamilk per week, n 10), spinal BMD was 20% lower among equol producers than non-producers. Numbers investigated in this study were, however, low (five equol producers and five non-producers) and clearly further studies that are appropriately designed and adequately powered are needed to investigate this association sufficiently.

Equol and menopausal symptoms

In 1992, Adlercreutz postulated that the oestrogen-like effects of soya isoflavones may account for the low prevalence of hot flushes and other menopausal symptoms in Japanese women(43). Subsequently, studies have shown that soya isoflavones reduce menopausal symptoms, in particular hot flushes, by up to 50%(91). In a study of 180 Japanese women(92)in which the severity of menopausal symptoms was assessed by questionnaire, it was found that only 5% of these women suffered from hot flushes. Of interest was the finding that the equol producers (53·5% of the group) reported the least severe menopausal symptoms. More recently, Ishiwata et al.(93) investigated the effect of a natural S-equol-containing supplement on menopausal symptoms in a randomized placebo-controlled trial in Japanese women. The supplement used in this study, as mentioned previously, was developed by a pharmaceutical company in Japan, and was prepared by the fermentation of soya germ with the bacteria L. garvieae (21). In Ishiwata's study, 134 Japanese women were randomly assigned to receive either placebo, 10 mg equol/d or 30 mg equol/d (10 mg three times daily) for 12 weeks, with habitual isoflavone intake limited to 20 mg/d. Menopausal symptom and Profile of Mood States questionnaires were completed at baseline and post intervention. Mood-related symptom scores were significantly improved following supplementation with 30 mg equol/d in perimenopausal/post-menopausal equol non-producers, the group most likely to benefit from equol supplementation. Follow-up studies have demonstrated that 10 mg S-equol significantly improved shoulder stiffness and reduced the frequency and severity of hot flushes in Japanese post-menopausal equol non-producing women(94). Thus, S-equol supplementation may be a potential alternative treatment for the management of menopausal symptoms.

Conclusions

Despite the abundance of studies that have investigated the effects of soya and isoflavones on health, a few studies have been adequately designed and appropriately powered to examine the influence of equol producer status. Although research to date suggests that the ability to produce equol may confer protection against breast and prostate cancer, CVD, bone disease and menopausal symptoms, clinical intervention trials with S-equol are needed to confirm or refute the ‘equol hypothesis’. Such trials are now possible with the development of the equol-rich soya product SE5-OH, a new functional food ingredient that relies on the bacterial conversion of daidzein to S-equol(21). The effects of this supplement on menopausal symptoms have been investigated as outlined above. SE5-OH is not genotoxic and the no-observed-adverse-effect-level was 2000 mg/kg per d (the highest dose tested) in a subchronic feeding study in rats(21). In addition, reproductive and developmental toxicity studies in rats have defined no-observed-adverse-effect-levels of 1000 mg/kg per d (6·5 mg equol/kg per d) and 2000 mg/kg per d (13 mg equol/kg per d), respectively(95). The pharmacokinetics of SE5-OH has also been investigated in healthy post-menopausal women(96). Following oral ingestion, S-equol was reported to be rapidly absorbed from the supplement, was highly bioavailable and had a terminal elimination half-life of 8 h.

Limited in vitro research has examined the mechanisms of action of equol specifically and it is important that more research is conducted in this area so that, together with clinical intervention studies, the precise role of equol in human health can be elucidated.

Acknowledgements

The author declares no conflict of interest. Some of this work was supported by the Commission of the European Communities specific RTD programme ‘Quality of Life and Management of Living Resources’ (project no. QLK-2000-00266 and Marie-Curie Programme Training Site grant no. HPMT-CT-2001-00287). It does not necessarily reflect its views and in no way anticipates the Commission's future policy in this area.

References

1.Marrian, G & Haslewood, G (1932) Equol, a new inactive phenol isolated from the ketohydroxyoestrin fraction of mare's urine. Biochem J 26, 12271232.
2.Axelson, M, Kirk, DN, Farrant, RD et al. . (1982) The identification of the weak oestrogen equol [7-hydroxy-3-(4’-hydroxyphenyl)chroman] in human urine. Biochem J 201, 353357.
3.Bennetts, HW, Underwood, EJ & Shier, FL (1946) A specific breeding problem of sheep on subterranean clover pastures in Western Australia. Aust J Agric Res 22, 131138.
4.Shutt, DA (1976) The effects of plant oestrogens on animal reproduction. Endeavour 35, 110113.
5.Messina, M, Nagata, C & Wu, AH (2006) Estimated Asian adult soy protein and isoflavone intakes. Nutr Cancer 55, 112.
6.Setchell, KDR, Brown, NM & Lydeking-Olsen, E (2002) The clinical importance of the metabolite equol – a clue to the effectiveness of soy and its isoflavones. J Nutr 132, 35773584.
7.Yuan, J-P, Wang, J-H & Liu, X (2007) Metabolism of dietary soy isoflavones to equol by human intestinal microflora – implications for health. Mol Nutr Food Res 51, 765781.
8.Lampe, JW (2009) Is equol the key to the efficacy of soy foods? Am J Clin Nutr 89, 1664S1667S.
9.Bowey, E, Adlercreutz, H & Rowland, I (2003) Metabolism of isoflavones and lignans by the gut microflora: a study in germ-free and human flora associated rats. Food Chem Toxicol 41, 631636.
10.Setchell, K, Zimmer-Nechemias, L, Cai, J et al. . (1997) Exposure of infants to phyto-oestrogens from soy-based infant formula. Lancet 350, 2327.
11.Setchell, K, Zimmer-Nechemias, L, Cai, J et al. . (1998) Isoflavone content of infant formulas and the metabolic fate of these phytoestrogens in early life. Am J Clin Nutr 68, 1453S1461S.
12.Hoey, L, Rowland, I, Lloyd, A et al. . (2004) Influence of soya-based infant formula consumption on isoflavone and gut microflora metabolite concentrations in urine and on faecal microflora composition and metabolic activity in infants and children. Br J Nutr 91, 607616.
13.Brown, NM, Belles, CA, Lindley, SL et al. . (2010) The chemopreventive action of equol enantiomers in a chemically induced animal model of breast cancer. Carcinogenesis 31, 886893.
14.Setchell, KDR, Clerici, C, Lephart, E et al. . (2005) S-equol, a potent ligand for estrogen receptor b, is the exclusive enantiomeric form of the soy isoflavone metabolite produced by human intestinal bacterial flora. Am J Clin Nutr 81, 10721079.
15.Lampe, JW, Karr, SC, Hutchins, AM et al. . (1998) Urinary equol excretion with a soy challenge: influence of habitual diet. Proc Soc Exp Biol Med 217, 335339.
16.Rowland, I, Wiseman, H, Sanders, TAB et al. . (2000) Interindividual variation in metabolism of soy isoflavones and lignans: influence of habitual diet on equol production by the gut microflora. Nutr Cancer 36, 2732.
17.Setchell, KDR & Cole, SJ (2006) Method of defining equol-producer status and its frequency among vegetarians. J Nutr 136, 21882193.
18.Tanaka, M, Fujimoto, K, Chihara, Y et al. . (2009) Isoflavone supplements stimulated the production of serum equol and decreased the serum dihydrotestosterone levels in healthy male volunteers. Prostate Cancer Prostatic Dis 12, 247252.
19.Liu, B, Liu, A, Uchiyama, S et al. . (2010) Prevalence of equol producer phenotype and its relations to lifestyle factors and dietary intakes among lifestyle factors and dietary intakes among healthy Chinese adults in Beijing. Wei Sheng Yan Jiu 39, 5964.
20.Ueno, T & Uchiyama, S (2001) Identification of the specific intestinal bacteria capable of metabolising soy isoflavone to equol. Ann Nutr Metab 45, 114.
21.Yee, S, Burdock, GA, Kurata, Y et al. . (2008) Acute and subchronic toxicity and genotoxicity of SE5-OH, an equol-rich product produced by Lactococcus garvieae. Food Chem Toxicol 46, 27132720.
22.Hedlund, TE, Maroni, PD, Ferucci, PG et al. . (2005) Long-term dietary habits affect soy isoflavone metabolism and accumulation in prostatic fluid in Caucasian men. J Nutr 135, 14001406.
23.Bolca, S, Possemiers, S, Herregat, A et al. . (2007) Microbial and dietary factors are associated with the equol producer phenotype in healthy postmenopausal women. J Nutr 137, 22422246.
24.McMullen, MH, Hamilton-Reeves, JM, Bonorden, MJ et al. . (2006) Consumption of Lactobacillus acidophilus and Bifidobacterium longum does not alter phytoestrogen metabolism and plasma hormones in men: a pilot study. J Altern Complement Med 12, 887894.
25.Larkin, TA, Price, WE & Astheimer, LB (2007) Increased probiotic yogurt or resistant starch intake does not affect isoflavone bioavailability in subjects consuming a high soy diet. Nutrition 23, 709718.
26.Larkin, TA, Astheimer, LB & Price, WE (2007) Dietary combination of soy with a probiotic or prebiotic food significantly reduces total and LDL cholesterol in mildly hypercholesterolaemic subjects. Eur J Clin Nutr 63, 238245.
27.Frankenfeld, CL, Atkinson, C, Thomas, WK et al. . (2004) Familial correlations, segregation analysis, and nongenetic correlates of soy isoflavone-metabolizing phenotypes. Exp Biol Med 229, 902913.
28.Frankenfeld, CL, Atkinson, C, Thomas, WK et al. . (2005) High concordance of daidzein-metabolizing phenotypes in individuals measured 1 to 3 years apart. Br J Nutr 94, 873876.
29.Adlercreutz, H, Bowey, E, Heinonen, SM et al. . (2004) Role of the intestine in the production of equol. J Nutr 134, 1234S.
30.Nagel, S, vom Saal, F & Welshons, W (1998) The effective free fraction of estradiol and xenoestrogens in human serum measured by whole cell uptake assays: physiology of delivery modifies estrogenic activity. Proc Soc Exp Biol Med 217, 300309.
31.Hodgson, J, Croft, K, Puddey, I et al. . (1996) Soybean isoflavonoids and their metabolic products inhibit in vitro lipoprotein oxidation in serum. J Nutr Biochem 7, 664669.
32.Mitchell, J, Gardner, P, McPhail, D et al. . (1998) Antioxidant efficacy of phytoestrogens in chemical and biological model systems. Arch Biochem Biophys 360, 142148.
33.Arora, A, Nair, M & Strasburg, G (1998) Antioxidant activities of isoflavones and their biological metabolites in a liposomal system. Arch Biochem Biophys 356, 133141.
34.Lund, T, Munson, D, Haldy, M et al. . (2004) Equol is a novel anti-androgen that inhibits prostate growth and hormone feedback. Biol Reprod 70, 11881195.
35.Martin, M, Haourigui, M, Pelissero, C et al. . (1996) Interactions between phytoestrogens and human sex steroid binding protein. Life Sci 58, 429436.
36.World Cancer Research Fund/American Institute for Cancer Research (2007) Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective. Washington DC: AICR.
37.Ferlay, J, Shin, HR, Bray, F et al. (2008) GLOBOCAN 2008, Cancer Incidence and Mortality Worldwide: IARC CancerBase No. 10 [Internet]. Lyon, France: International Agency for Research on Cancer; available at http://globocan.iarc.fr (accessed June 2010).
38.Setchell, KDR & Adlercreutz, H (1988) Mammalian lignans and phyto-oestrogens. Recent studies on their formation, metabolism and biological role in health and disease. In Role of the Gut Flora in Toxicity and Cancer, pp. 315345 [Rowland, I]. London: Academic Press.
39.Adlercreutz, H, Fotsis, T, Heikkinen, R et al. . (1982) Excretion of the lignans enterolactone and enterodiol and of equol in omnivorous and vegetarian women and in women with breast cancer. Lancet 2, 12951299.
40.Adlercreutz, H, Hockerstedt, K, Bannwart, C et al. (1988) Association between dietary fiber, urinary excretion of lignans and isoflavonic phytoestrogens, and plasma non-protein bound sex hormones in relation to breast cancer. In Progress in Cancer Research Therarpy, Vol 35: Hormones and Cancer 3, pp. 409412 [Bresciani, F, King, R, Lippman, M and Raynaud, J-P, editors]. New York: Raven Press.
41.Adlercreutz, H, Fotsis, T, Bannwart, C et al. . (1986) Determination of urinary lignans and phytoestrogen metabolites, potential antiestrogens and anticarcinogens, in urine of women on various habitual diets. J Steroid Biochem 25, 791797.
42.Adlercreutz, H, Hockerstedt, K, Bannwart, C et al. . (1987) Effect of dietary components, including lignans and phytoestrogens, on enterohepatic circulation and liver metabolism of estrogens and on sex hormone binding globulin (SHBG). J Steroid Biochem 27, 11351144.
43.Adlercreutz, H, Honjo, H & Higashi, A (1991) Urinary excretion of lignans and isoflavonoid phytoestrogens in Japanese men and women consuming traditional Japanese diet. Am J Clin Nutr 54, 10931100.
44.Adlercreutz, H, Hamalainen, E, Gorbach, S et al. . (1992) Dietary phyto-oestrogens and the menopause in Japan. Lancet 339, 1233.
45.Adlercreutz, H, Markkanen, H & Watanabe, S (1993) Plasma concentrations of phyto-oestrogens in Japanese men. Lancet 342, 12091210.
46.Adlercreutz, H, Goldin, B, Gorbach, S et al. . (1995) Soybean phytoestrogen intake and cancer risk. J Nutr 125, 757S770S.
47.Ingram, D, Sanders, K, Kolybaba, M et al. . (1997) Case-control study of phyto-oestrogens and breast cancer. Lancet 350, 990994.
48.Goodman, MT, Shvetsov, YB, Wilkens, LR et al. . (2009) Urinary phytoestrogen excretion and postmenopausal breast cancer risk: The Multiethnic Cohort Study. Cancer Prev Res 2, 887894.
49.Grace, PB, Taylor, JL, Low, Y-L et al. . (2004) 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. Cancer Epidemiol Biomarkers Prev 13, 698708.
50.Ward, H, Chapelais, G, Kuhnle, GGC et al. . (2008) Breast cancer risk in relation to urinary and serum biomarkers of phytoestrogen exposure in the European Prospective into Cancer-Norfolk cohort study. Breast Cancer Res 10:R32.
51.Messina, M (2010) A brief historical overview of the past two decades of soy and isoflavone research. J Nutr 140, 1350S1354S.
52.Duncan, AM, Merz-Demlow, BE, Xu, X et al. . (2000) Premenopausal equol excretors show plasma hormone profiles associated with lowered risk of breast cancer. Cancer Epidemiol Biomarkers Prev 9, 581586.
53.Nettleton, JA, Greany, KA, Thomas, W et al. . (2005) The effect of soy consumption on the urinary 2:16-hydroxyestrone ratio in postmenopausal women depends on equol production status but is not influenced by probiotic consumption. J Nutr 135, 603608.
54.Kabat, GC, Chang, CJ, Sparano, JA et al. . (1997) Urinary estrogen metabolites and breast cancer: a case control study. Cancer Epidemiol Biomarkers Prev 6, 505509.
55.Muti, P, Bradlow, HL, Micheli, A et al. . (2000) Estrogen metabolism and risk of breast cancer: a prospective study of the 2:16alpha-hydroxyestrone ratio in premenopausal and postmenopausal women. Epidemiology 11, 635640.
56.Fishman, J, Schneider, J, Hershcope, RJ et al. . (1984) Increased estrogen-16 alpha-hydroxylase activity in women with breast and endometrial cancer. J Steroid Biochem 20, 10771081.
57.Ursin, G, London, S, Stanczyk, FZ et al. . (1999) Urinary 2-hydroxyestrone/16alpha-hydroxyestrone ratio and risk of breast cancer in postmenopausal women. J Natl Cancer Inst 91, 10671072.
58.Adlercreutz, H, Fotsis, T, Hockerstedt, K et al. . (1989) Diet and urinary estrogen profile in premenopausal omnivorous and vegetarian women and in premenopausal women with breast cancer. J Steroid Biochem 34, 527530.
59.Frankenfeld, CL, McTiernan, A, Aiello, EJ et al. . (2004) Mammographic density in relation to daidzein-metabolizing phenotypes in overweight, postmenopausal women. Cancer Epidemiol Biomarkers Prev 13, 11561162.
60.Fuhrman, BJ, Teter, BE, Barba, M et al. . (2008) Equol status modifies the association of soy intake and mammographic density in a sample of postmenopausal women. Cancer Epidemiol Biomarkers Prev 17, 3342.
61.Atkinson, C, Newton, KM, Aiello Bowles, EJ et al. . (2009) Daidzein-metabolizing phenotypes in relation to mammographic breast density among premenopausal women in the United States. Breast Cancer Res Treat 116, 587594.
62.Magee, PJ & Rowland, IR (2004) Phyto-oestrogens, their mechanism of action: current evidence for a role in breast and prostate cancer. Br J Nutr 91, 513531.
63.Zava, DT, Blen, M & Duwe, G (1997) Estrogenic activity of natural and synthetic estrogens in human breast cancer cells in culture. Environ Health Perspect 105, 637645.
64.Schmitt, E, Dekant, W & Stopper, H (2001) Assaying the estrogenicity of phytoestrogens in cells of different estrogen sensitive tissues. Toxicol In Vitro 15, 433439.
65.Magee, PJ, Raschke, M, Steiner, C et al. . (2006) Equol: a comparison of the effects of the racemic compound with that of the purified S-enantiomer on the growth, invasion, and DNA integrity of breast and prostate cells in vitro. Nutr Cancer 54, 232242.
66.Messina, M & Wu, AH (2009) Perspectives on the soy-breast cancer relation. Am J Clin Nutr 89, 1673S1679S.
67.Akaza, H, Miyanaga, N, Takashima, N et al. . (2004) Comparisons of percent equol producers between prostate cancer patients and controls: case-controlled studies of isoflavones in Japanese, Korean and American residents. Jpn J Clin Oncol 34, 8689.
68.Ozasa, K, Nakao, M, Watanabe, Y et al. . (2004) Serum phytoestrogens and prostate cancer risk in a nested case-control study among Japanese men. Cancer Sci 95, 6571.
69.Hedlund, TE, Johannes, WU & Miller, GJ (2003) Soy isoflavonoid equol modulates the growth of benign and malignant prostatic epithelial cells in vitro. Prostate 54, 6878.
70.Mitchell, JH, Duthie, SJ & Collins, AR (2000) Effects of phytoestrogens on growth and DNA integrity in human prostate tumor cell lines: PC-3 and LNCaP. Nutr Cancer 38, 223228.
71.Morton, MS, Chan, PS, Cheng, C et al. . (1997) Lignans and isoflavonoids in plasma and prostatic fluid in men: samples from Portugal, Hong Kong and the United Kingdom. Prostate 32, 122128.
72.Gardner, CD, Oelrich, B, Liu, JP et al. . (2009) Prostatic soy isoflavone concentrations exceed serum levels after dietary supplementation. Prostate 69, 719726.
73.British Heart Foundation Statistics Website (2010) Numbers dying from CVD and CHD, available at http://www.heartstats.org/datapage.asp?id=713 (accessed August 2010).
74.Lichtenstein, AH (1998) Soy protein, isoflavones and cardiovascular disease risk. J Nutr 128, 15891592.
75.Nagata, C (2000) Ecological study of the association between soy product intake and mortality from cancer and heart disease in Japan. Int J Epidemiol 29, 832836.
76.Food and Drug Administration (1999) Food Labeling: Health Claims, Soy protein and Coronary Heart Disease. Fed Register 64, 5769957733.
77.Sacks, FM, Lichtenstein, AH, Van Horn, L et al. . (2006) Soy protein, isoflavones and cardiovascular health. An American Heart Association science advisory for professionals from the nutrition committee. Circulation 113, 10341044.
78.Balk, E, Chung, M, Chew, P et al. (2005) Effects of soy on health outcomes. Evidence report/technology assessment No. 126 (prepared by Tufts-New England Medical Center Evidence-based Practice Center under Contract No. 290-02-0022). AHRQ Publication No. 05-E024–2. Rockville, MD: Agency for Healthcare Research and Quality.
79.Zhan, S & Ho, SC (2005) Meta-analysis of the effects of soy protein containing isoflavones on the lipid profile. Am J Clin Nutr 81, 397408.
80.Taku, K, Umegaki, K, Sato, Y et al. . (2007) Soy isoflavones lower serum total and LDL cholesterol in humans: a meta-analysis of 11 randomized controlled trials. Am J Clin Nutr 85, 11481156.
81.Anderson, JW, Johnstone, BM & Cook-Newell, ME (1995) Meta-analysis of the effects of soy protein intake on serum lipids. N Engl J Med 333, 276282.
82.EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) (2009). Scientific opinion on the substantiation of a health claim related to soy protein and reduction of blood cholesterol concentrations pursuant to Article 14 of the Regulation (EC) No 1924/2006. EFSA J 8(7):1688.
83.Meyer, BJ, Larkin, TA, Owen, AJ et al. . (2004) Limited lipid-lowering effects of regular consumption of whole soybean foods. Ann Nutr Metab 48, 6778.
84.Hall, WL, Vafeiadou, K, Hallund, J et al. . (2006) Soy-isoflavone-enriched foods and markers of lipid and glucose metabolism in postmenopausal women: interactions with genotype and equol production. Am J Clin Nutr 83, 592600.
85.Kreijkamp-Kaspers, S, Kok, L, Bots, ML et al. . (2005) Randomized controlled trial of the effects of soy protein containing isoflavones on vascular function in postmenopausal women. Am J Clin Nutr 81, 189195.
86.Guo, K, Zhang, B, Chen, C et al. . (2010) Daidzein-metabolising phenotypes in relation to serum lipids and uric acid in adults in Guangzhou, China. Br J Nutr 8, 17.
87.Ma, D-F, Qin, L-Q, Wang, P-Y et al. . (2008) Soy isoflavone intake inhibits bone resorption and stimulates bone formation in menopausal women: meta-analysis of randomized controlled trials. Eur J Clin Nutr 62, 155161.
88.EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) (2009). Scientific opinion on the substantiation of health claims related to soy isoflavones and maintenance of bone mineral density (ID 1655) pursuant to Article 13(1) of Regulation (EC) No 1924/2006. EFSA J 7(9):1270.
89.Lydeking-Olsen, E, Beck-Jensen, J-E, Setchell, KDR et al. . (2004) Soymilk or progesterone for prevention of bone loss. A 2 year randomized, placebo-controlled trial. Eur J Nutr 43, 246257.
90.Frankenfeld, CL, McTiernan, A, Thomas, WK et al. . (2006) Postmenopausal bone mineral density in relation to soy isoflavone-metabolizing phenotypes. Maturitas 53, 315324.
91.Kurzer, MS (2008). Soy consumption for reduction of menopausal symptoms. Inflammopharmacology 16, 227229.
92.Uchyama, S, Ueno, T & Shirota, T (2001) The relationship between soy isoflavones and the menopausal symptoms in Japanese perimenopausal women. Ann Nutr Metab 45, 113.
93.Ishiwata, N, Melby, MK, Mizuno, S et al. . (2009) New equol supplement for relieving menopausal symptoms: randomized, placebo-controlled trial of Japanese women. Menopause 16, 141148.
94.Aso, T (2010) Equol improves menopausal symptoms in Japanese women. J Nutr 140, 1386S1389S.
95.Matulka, RA, Matsuura, I, Uesugi, T et al. . (2009) Developmental and reproductive effects of SE5-OH: an equol-rich soy-based ingredient. J Toxicol ID: 307618, [Epub 15 December 2008].
96.Setchell, KDR, Zhao, X, Shoaf, SE et al. . (2009) The pharmacokinetics of S-(-)equol administered as SE5-OH tablets to healthy postmenopausal women. J Nutr 139, 20372043.