Originally published as JCO Early Release 10.1200/JCO.2006.06.0244 on January 2 2007

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Plasma Phytoestrogens and Subsequent Breast Cancer Risk

Martijn Verheus, Carla H. van Gils, Lital Keinan-Boker, Philip B. Grace, Sheila A. Bingham, Petra H.M. Peeters

From the Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, the Netherlands; Israel Center for Disease Control, Ministry of Health, and School of Public Health, University of Haifa, Haifa, Israel; Horseracing Forensic Laboratory Ltd, Fordham; and the Medical Research Council Dunn Human Nutrition Unit, Cambridge, United Kingdom

Address reprint requests to Petra H.M. Peeters, MD, PhD, Julius Center for Health Sciences and Primary Care, Room Str 6.131, PO Box 85500, University Medical Center, Utrecht 3508 GA, Utrecht, the Netherlands; e-mail: P.H.M.Peeters{at}umcutrecht.nl

	ABSTRACT

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PURPOSE: Phytoestrogens are plant compounds that are structurally and functionally similar to mammalian estrogens. By competing for estrogen receptors, phytoestrogens possibly inhibit binding of the more potent endogenous estrogens and decrease their potential effects on breast cancer risk. We investigated the association between plasma phytoestrogen levels and breast cancer risk in a prospective manner.

PATIENTS AND METHODS: We performed a nested case-control study within the Prospect cohort, one of the two Dutch cohorts participating in the European Prospective Investigation into Cancer and Nutrition. A total of 383 women (87 pre- or perimenopausal women [mean age, 52 years] and 296 postmenopausal women [mean age, 59 years]) who developed breast cancer were selected as case subjects and were matched to 383 controls, on date of blood sampling. Plasma levels of isoflavones (daidzein, genistein, glycitein, O-desmethylangolensin, and equol) and lignans (enterodiol and enterolactone) were measured. The isotope dilution liquid chromatography/tandem mass-spectrometry method incorporating triply ¹³C-labeled standards was used for all analyses. Breast cancer odds ratios were calculated for tertiles of phytoestrogen plasma levels using conditional logistic regression analysis.

RESULTS: For genistein, the risk estimate for the highest versus the lowest tertile was 0.68 (95% CI, 0.47 to 0.98). Similar protective effects, although not statistically significant, were seen for the other isoflavones. Lignan levels did not appear to be related to breast cancer risk. Results were the same in pre- or perimenopausal women, and in postmenopausal women.

CONCLUSION: High genistein circulation levels are associated with reduced breast cancer risk in the Dutch population. No effects of lignans on breast cancer risk were observed.

	INTRODUCTION

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Phytoestrogens are plant compounds that are structurally and functionally similar to mammalian estrogens. The two main classes found in the human diet are isoflavones (daidzein, genistein, and glycitein) and lignans (enterodiol and enterolactone). The isoflavone daidzein can be metabolized by intestinal bacteria into O-desmethylangolensin (O-DMA) and, in approximately 30% to 50% of individuals, into equol. The primary food sources of isoflavones are soy and soy products, and sources for lignans are cereals, flaxseed, and berries.^1-4

In Asian countries, the incidence of breast cancer is much lower compared with the incidence in Western countries.⁵ Studies of Asian women who migrated to Western countries show that breast cancer incidence rates change in only a few generations.^6,7 Hence, the role of genetics is probably relatively small compared with lifestyle habits, such as dietary intake. The intake of phytoestrogens is much higher among Asian women living in Asia, compared with the intake among women living in the Western world. It has therefore been hypothesized that high phytoestrogen intake could protect against breast cancer.⁸

Estrogens, estradiol in particular, are known to have strong mitogenic properties,⁹ and epidemiologic studies on circulating estrogen levels and breast cancer showed increased risk with higher levels of both estrone and estradiol, although only in postmenopausal women.^10-12 Phytoestrogens are capable of binding to the estrogen receptor (ER) and have weak estrogenic potential,^13-16 possibly increasing breast cancer risk. They can, however, also compete with endogenous estrogens for ERs, and in this way inhibit binding of the estrogenic more potent endogenous estrogens.^13,15,17 It has been postulated that in an environment with low circulating levels of endogenous estrogens, phytoestrogens may act as weak estrogens, but have an antiestrogenic effect in an environment with high circulating levels of endogenous estrogens. Hence, they could protect against premenopausal breast cancer and increase breast cancer risk after menopause.¹⁸ Other properties of phytoestrogens, like inhibition of aromatase and tyrosine kinase activity, have also been described.^1-4,18

Prospective studies on soy intake and breast cancer risk found no significant protective effect for higher intake.^19,20 Studies on phytoestrogen levels in blood or urine samples are scarce and have only been performed in Western study populations. Higher levels of isoflavones in blood and urine were found to be associated with (nonsignificantly) increased breast cancer risk in a study of premenopausal German women, and also in a study in the United Kingdom with predominantly postmenopausal women.^21,22 This direct association was not found by den Tonkelaar et al²³ studying urinary isoflavone levels in a Dutch population of postmenopausal women. Prospective epidemiologic studies with circulating levels or urinary excretion levels of lignans show conflicting results. In premenopausal women, both increased and decreased breast cancer risk were found.^22,24,25 Most studies with postmenopausal women, or studies showing results of pre- and postmenopausal women combined, did not report a relationship between lignans and breast cancer risk,^21,23-26 although decreased risk, especially for ER-negative breast tumors, was found in a Danish study.²⁷

In this large prospective study, we investigated the effect of phytoestrogens measured in plasma on subsequent breast cancer occurrence in pre- or perimenopausal women and in postmenopausal women in the Prospect cohort.

	PATIENTS AND METHODS

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Prospect Cohort and Subject Selection
We designed a nested case-control study within Prospect, one of two Dutch cohorts participating in the European Prospective Investigation into Cancer and Nutrition (EPIC). Rationale and design of both EPIC and Prospect-EPIC are described in detail elsewhere.^28-30 All women with an incident breast cancer diagnosis before January 2003 were selected for the present study. In a mean follow-up period of 6.5 years, 395 breast cancer cases were diagnosed. One control subject was matched to every case subject by recruitment date (plus or minus 6 months), making storing time of biologic samples comparable. The majority of women participating in the Prospect-EPIC cohort were postmenopausal. To increase power in the premenopausal subgroup, pre- and perimenopausal women were grouped together. A plasma sample was available for 87 pre-/perimenopausal and 296 postmenopausal case/control sets.

All participants signed informed consent and the study was approved by the institutional review board of the University Medical Center Utrecht (Utrecht, the Netherlands).

Menopausal Status
Women were categorized as premenopausal when they had had at least 6 menstrual periods in the 12 months before inclusion and were not currently using oral contraceptives (OC) or postmenopausal hormone therapy (HT). Women without any menstrual periods were categorized as postmenopausal. Women who experienced menstrual periods in the 12 months before inclusion were classified as perimenopausal if they had experienced fewer than six menstrual periods, if they were using OC or HT at the time of inclusion, or if they had equivocal data.

Laboratory Analyses
Plasma samples were analyzed for three isoflavones (daidzein, genistein, and glycitein), two metabolites of daidzein (O-DMA and equol), and two mammalian lignans (enterodiol and enterolactone). Laboratory personnel were blinded for case-control status when performing the analyses. Information on the methodology of the analyses and quality assurance are described in detail elsewhere.³¹ Empirically calculated limits of detection were less than 10 pg/mL for all phytoestrogens except equol. The detection limit for equol was calculated to be 100 pg/mL. All analyses were performed at the MRC Dunn Human Nutrition Unit (Cambridge, United Kingdom).

Statistical Analyses
Participants with plasma phytoestrogen values under the detection limit were given the value of the detection limit. This was done for all phytoestrogens except equol, as only an estimated 30% to 50% of the population is able to produce equol in the intestinal tract.³ Plasma phytoestrogen levels were log transformed to normalize distributions. These transformed values were then used to compute geometric means.

Odds ratios (ORs) and 95% CIs were estimated for cancer risk by tertile level of the plasma phytoestrogens with conditional logistic regression models using the SAS PHREG procedure (SAS Institute, Cary, NC). Tertile cutoff points were based on the frequency distribution of controls. Due to small numbers of women with detectable equol levels (24%), these women were compared with women without detectable levels. Potential confounding was evaluated using conditional logistic regression models with breast cancer as the outcome, and by adding one potential confounder as a covariate to the model in addition to one of the phytoestrogens as the variable of interest. Variables were judged to be a confounder when adding it to the model changed the ORs for the phytoestrogen of interest by at least 10%. The following characteristics were evaluated for confounding: age at baseline, body mass index, family history of breast cancer, age at menarche, parity, age at first childbirth, age at menopause (postmenopausal women only), current and ever use of OC and HT, current and ever smoking of cigarettes, and physical activity. Analyses were done separately for pre- or perimenopausal women and postmenopausal women. To test for linear trends in ORs over the tertiles, median values within tertiles were calculated and evaluated as a continuous variable, using conditional logistic regression analysis.

All P values are two-sided and values below .05 were considered significant. All analyses were conducted using the Statistical Analysis System (SAS) software package, version 8.02 (SAS Institute, Cary, NC).

	RESULTS

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Basic characteristics at baseline are given in Table 1. Among isoflavones, genistein had the highest geometric mean concentration (Table 2). Enterolactone was the main lignan.

Isoflavone levels were mutually adjusted by analyzing genistein and one of the other isoflavones in the same model. The beneficial effect of genistein became slightly stronger, whereas the effects of the other isoflavones (except equol) changed toward the null relationships (data not shown).

	DISCUSSION

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In this study, high levels of plasma genistein were associated with significantly lower risk of breast cancer development. Similar trends, although not statistically significant, were shown for daidzein, glycitein, O-DMA, and equol. Lignans were not related to breast cancer occurrence. Results were the same in both the pre- or perimenopausal women and postmenopausal women subgroups.

The main advantage of the present study is its prospective design. Information about lifestyle factors and dietary habits, as well as blood samples, was collected before diagnosis. Therefore, our results cannot have been biased by behavioral or metabolic changes after breast cancer diagnosis or by cancer treatment. Because of the prospective design, we were also able to select controls from the same source population from which the cases arose, avoiding selection bias. A second major strength of the study was the use of levels of phytoestrogens from plasma instead of from dietary intake. Blood levels may more accurately reflect relevant biologic dose levels, since phytoestrogen metabolism by bacteria in the gastrointestinal tract is highly variable. This is not reflected by dietary intake. Another important advantage is the size of our study, which exceeds the sample size of three other prospective studies on circulating levels or urinary excretion levels of isoflavones.^21-23 Furthermore, our study is one of the largest prospective studies to investigate the association between lignans and breast cancer risk.^21-27

We have measured seven different phytoestrogens. It should be noted, however, that their relationships with breast cancer development are unlikely to be independent. Levels of these phytoestrogens are highly correlated, especially among isoflavones. Relationships of isoflavones with breast cancer risk must therefore be interpreted with caution, because they may have been caused by the relationship between breast cancer risk and genistein, as it is this compound that has the highest affinity for the estrogen receptor^15,32-34 and is the most abundant isoflavone in the bloodstream. Moreover, when daidzein, glycitein, or O-DMA were added to a regression model with genistein, the association between genistein and breast cancer risk became slightly stronger, while the protective effect of the other isoflavones disappeared.

Formally, the capability of producing equol in the intestinal tract from its precursor daidzein can only be determined after challenging persons for several days with a high-dose of dietary daidzein.³ In our population, without preceding such dietary challenge, detectable or nondetectable levels might reflect the capability of producing equol. However, as detectable levels also depend on daidzein levels, some women capable of metabolizing equol may have had undetectable levels. Hence, the association between metabolizing capability and breast cancer risk might be stronger when we would have been able to compare metabolizers with nonmetabolizers.

We did not have information on race or ethnicity. However, only 5.5% of the women in our study noted on their questionnaires that they were not born in the Netherlands. Furthermore, women had to speak proper Dutch to fill out the questionnaires. In the Netherlands, we can then assume that most women of the study population are white.

Our results are the opposite of the results of a recently published study with premenopausal women in Germany that showed increased breast cancer risk in women with higher levels of genistein in plasma,²² which was, however, nonsignificant. Grace et al²¹ also found increased breast cancer risk with elevated levels of five different types of isoflavones, measured in both serum and urine samples in women living in the United Kingdom. Results were based on analyses with pre- and postmenopausal women combined, but the majority of women were postmenopausal. This direct association was not found by den Tonkelaar et al²³ in a study with urinary genistein levels in a Dutch population of postmenopausal women.²³

In our study, we found no relationship between enterodiol or enterolactone levels and breast cancer occurrence, although in pre- and perimenopausal women, there was a tendency of increased risk, but confidence intervals were wide and the results were not statistically significant. Only two other prospective studies with circulating levels of enterolactone present results for pre- and postmenopausal women separately. Results of our study are in line with results of a large case-control study, nested within the New York University Women’s Health Study cohort (New York, NY), showing increased breast cancer risk with high circulating levels of enterolactone in premenopausal women, but not in postmenopausal women. As in our study, results were not statistically significant.²⁴ A second study reported weak nonsignificant effects in the opposite direction: high levels of enterolactone were associated with decreased breast cancer risk in premenopausal women, but with increased risk in postmenopausal women in Finland.²⁵ Piller et al²² also found a protective effect in women with high levels of circulating enterolactone in a German study that consisted of premenopausal women only. A Dutch study with exclusively postmenopausal women found the opposite effects with nonsignificantly increased breast cancer risk in women with high urinary excretion levels of enterolactone.²³ Other prospective studies with enterolactone report combined results of pre- and postmenopausal women. Two such studies from England and Sweden did not find breast cancer occurrence to be associated with enterodiol and enterolactone in blood and urine.^21,26 In Denmark, however, plasma enterolactone levels significantly decreased breast cancer risk, especially for ER-negative tumors.²⁷ Median plasma levels in the latter study were higher compared with our study (28.1 v 10.0 nmol/L), suggesting that the lack of association between enterolactone levels in our study may have been as a result of absolute levels of circulating enterolactone being too low. Piller et al,²² however, were able to show significantly decreased breast cancer risk with comparable plasma levels to our study (9.7 nmol/L).

We have no clear explanation for the opposing effects of isoflavones between our study and the studies by Grace et al²¹ and Piller et al.²² An explanation may be that serum levels of isoflavones, besides their own effects, are also markers for other dietary compounds. Only a small part of the dietary intake of isoflavones in Western women is via soy or soy products. If other sources of isoflavones differ between countries, and isoflavones therefore reflect different dietary components in each country, this may also cause different effects. In addition, variants of genes involved in the metabolism of sex steroid hormones may be important for the effects of phytoestrogens on breast cancer risk.³⁵ Stronger decreased breast cancer risk in women with high dietary intake and in women with high plasma concentrations of lignans was shown for a certain variant of the cyp17 gene.^36,37

In our opinion, the hypothesis that phytoestrogens protect against breast cancer in an environment of high circulating levels of endogenous estrogens (ie, in premenopausal women), but increase risk in environments of relatively low levels of endogenous estrogens (ie, after menopause), is not generally supported by the results of prospective studies published so far, including our own. Furthermore, because endogenous estrogen levels severely decrease after menopause, the excess of circulating levels of phytoestrogens over circulating estradiol levels is much larger in postmenopausal women compared with premenopausal women. Hence, competition between phytoestrogens and estradiol may be more effective after menopause.

Circulating levels of isoflavones in women living in Western countries are much lower compared with circulating levels in Asian women. However, comparing endogenous estrogen levels with phytoestrogen levels from Western women shows that phytoestrogen levels are 50 to 1,000 times higher than endogenous estrogen levels.^{10-12,21,22,24-27} Hence, an effect can be expected even at low circulating levels of isoflavones, as are common in European and American populations.

Other proposed mechanisms of action of phytoestrogens, like scavenging of free radicals and inducement of apoptosis and of tyrosine kinase activity, may also result in decreased breast cancer risk. However, some of these effects were only shown in in vitro experiments with much higher phytoestrogens levels, compared with in vivo levels, and it is questionable whether these mechanisms explain our results.^1-4,18 Inhibition of aromatic enzymes, however, was described with levels that can be reached with a phytoestrogen-enriched diet.² This effect is particularly interesting for relationships after menopause, when there is no estrogen production from the ovaries and aromatic conversion of androgens in fat tissue is the main pathway of estrogen production.

In conclusion, high circulating levels of genistein were associated with decreased breast cancer risk in Dutch women with relatively low overall circulating levels of isoflavones that are characteristic for Western populations. These results suggest that this compound may be protective against breast cancer development. Other isoflavones may have similar properties, but evidence is less strong. Circulating levels of lignans did not seem to be associated with breast cancer occurrence.

	AUTHORS’ DISCLOSURES OF POTENTIAL CONFLICTS OF INTEREST

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The authors indicated no potential conflicts of interest.

	AUTHOR CONTRIBUTIONS

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Conception and design: Carla H. van Gils, Lital Keinan-Boker, Petra H.M. Peeters

Financial support: Petra H.M. Peeters

Provision of study materials or patients: Carla H. van Gils, Petra H.M. Peeters

Collection and assembly of data: Lital Keinan-Boker, Philip B. Grace, Sheila A. Bingham, Petra H.M. Peeters

Data analysis and interpretation: Martijn Verheus, Carla H. van Gils, Petra H.M. Peeters

Manuscript writing: Martijn Verheus, Carla H. van Gils, Petra H.M. Peeters

Final approval of manuscript: Martijn Verheus, Carla H. van Gils, Lital Keinan-Boker, Philip B. Grace, Sheila A. Bingham, Petra H.M. Peeters

	ACKNOWLEDGMENTS

 
The study was supported by Grant No. 21000027 of the Dutch ‘Zorg Onderzoek Nederland’ Fund (ZON), Grant No. 2000/30 of the World Cancer Research Fund (WCRF) and the European Committee ‘Europe Against Cancer’.

	NOTES

 
published online ahead of print at www.jco.org on January 2, 2007.

Authors’ disclosures of potential conflicts of interest and author contributions are found at the end of this article.

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Submitted March 3, 2006; accepted November 27, 2006.

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