Phytoestrogens, plant food components with estrogen-like biological properties, are hypothesized to contribute to the 5-fold lower breast cancer incidence in Asian compared with Western countries (1). Isoflavones comprise the phytoestrogens most abundant in soy, the traditional staple food in Asia, and a recent meta-analysis concluded that there was a slight reduction in premenopausal breast cancer risk with higher soy consumption (1). Because consumption of soy and isoflavones is typically low in Western countries, lignans and their derived metabolites, the enterolignans, might be more relevant for breast cancer prevention in these populations (2). Further large prospective studies of phytoestrogens in breast cancer are needed in Western populations to test this hypothesis. We thus examined the association between the usual dietary intake of phytoestrogens and the risk of premenopausal invasive breast cancer in a large French cohort.

E3N (Etude Epidémiologique auprès de femmes de la Mutuelle Générale de l'Education Nationale) is a large ongoing prospective cohort consisting of 98,995 French women born between 1925 and 1950, subscribing to the health insurance plan for public education system employees, and who voluntarily enrolled in 1990-1991 (3). After a baseline questionnaire, follow-up questionnaires have been sent biennially to ascertain occurrence of diseases and to update menopausal status and exposure factors. Usual diet over the previous year was assessed using a validated 208-item diet history questionnaire administered between 1993 and 1995 and available for 74,524 participants (4). We estimated daily phytoestrogen intake using a food composition table updated for four isoflavones (genistein, daidzein, formononetin, and biochanin A), one coumestan (coumestrol), four plant lignans (pinoresinol, lariciresinol, secoisolariciresinol, and matairesinol), and two enterolignans (enterodiol and enterolactone). Dietary intake of total phytoestrogens was computed as the sum of isoflavones, coumestrol, and plant lignans.

All premenopausal women with dietary data, without a history of cancer (except for skin basal cell carcinoma or breast lobular carcinoma in situ), and who were not consuming soy dietary supplements were included in the present analysis (n = 26,868). Participants contributed person-years of follow-up starting from the date they had completed the dietary questionnaire to the date of diagnosis of premenopausal invasive breast cancer as first primary cancer (for the cases), date of diagnosis of another cancer, date of menopause, date of death, or July 2002, whichever came first. We calculated multivariate relative risks and their two-sided 95% confidence intervals in Cox proportional hazards regression models for quartiles of phytoestrogen intake, adjusting for potential confounding variables as listed in the footnotes to Table 1. We also conducted analyses stratified on the joint estrogen receptor (ER) and progesterone receptor (PR) status of the tumors.

During 117,652 person-years of follow-up (median duration, 4.2 years), 402 cases of invasive breast cancer were diagnosed among 26,868 premenopausal women (mean age, 47 years at baseline). Median dietary intake of total phytoestrogens was 1,101 μg/d, mostly consisting of plant lignans (97%).

Premenopausal breast cancer risk was not related to isoflavone, coumestrol, plant lignan, or enterolignan intakes (Table 1). Nor was any association observed with individual intakes of genistein, daidzein, formononetin, biochanin A, coumestrol, pinoresinol, lariciresinol, secoisolariciresinol, matairesinol, enterodiol, or enterolactone (data not shown).

Most (80%) of the 322 breast cancer cases with known receptor status were positive for both ER and PR [191 (59%) ER+PR+, compared with 51 (16%) ERPR, 44 (14%) ER+PR, and 36 (11%) ERPR+]. When we stratified the analysis on the joint ER/PR status, no association was found (data not shown).

In this prospective study, we found no evidence of an association between dietary intake of phytoestrogens and risk of premenopausal invasive breast cancer, either overall or by ER/PR status.

Dietary isoflavone (<120 μg/d) and coumestrol (<1 μg/d) intakes were close to those reported for other Western populations consuming little or no soy (2, 5, 6). In comparison, mean isoflavone intakes were ∼15 mg/d in European soy consumers (7) and varied between 5 and 45 mg/d in Asian populations (8, 9). Our dietary questionnaire did not cover soy foods, but the proportion of soy consumers is marginal in France, with only 1% to 3% women (10, 11). Chronic intake of >1 g/d soy protein, corresponding to >3 mg/d isoflavones (9), was recently suggested for reducing premenopausal breast cancer risk (1). In line with this hypothesis, isoflavone intake levels in this study may have been too low to reveal an association. The use of soy supplements in Western premenopausal women would enable attaining Asian isoflavone intakes; however, this is not recommended for reducing breast cancer risk insofar as its safety has not yet been shown (1).

In our study, plant lignan intakes were 30 times higher than isoflavone intakes. Our study evaluated the contributions of two major lignans newly identified (12) and reported higher lignan intakes compared with other authors. Whereas previous studies may have been limited by too low lignan intakes, inverse associations were shown for premenopausal breast cancer of certain CYP17 genotypes and ER status (13-15) and of ER+ and PR+ status (16); conversely, two dietary reports found no association, just as we did (17, 18). Limited population size and follow-up duration might explain the absence of an association in our study despite a larger range of lignan intakes. However, we had adequate statistical power (80%, with 5% two-sided significance) to detect a relative risk of ≥1.43 for the higher quartile of lignan intakes, with the sample size available. The lack of trend across quartiles of lignan intakes and our finding of inverse associations at similar levels in postmenopausal women3

3

M.S. Touillaud, A.C.M. Thiébaut, A. Fournier, M. Niravong, M.C. Boutron-Ruault, and F. Clavel-Chapelon. Dietary intakes of lignans and risk of postmenopausal breast cancer by estrogen and progesterone receptor status, submitted for publication.

suggest no real association with lignans in this study.

Enterolignans are metabolized from ingested dietary lignans in the gut and are the bioactive compounds absorbed. The absence of an association in our study does not confirm results of a dietary case-control study with similar intake levels that showed a reduced risk of premenopausal breast cancer with higher enterolignan intakes (16). Three prospective (19-21) and one case-control (22) biomarker studies of enterolignans in premenopausal breast cancer showed inconsistent results. As our study had sufficient statistical power to detect a substantial risk reduction with enterolignan intakes (80% power to detect a relative risk of ≤0.65), it suggests that enterolignans are unlikely to be associated with risk.

In summary, the absence of an association in the present study probably indicates that there are no effects of low isoflavone and high lignan levels in premenopausal breast cancer. However, a balanced diet rich in plant foods remains recommended for Western premenopausal women, as a healthy diet is likely to be beneficial over the long term (23).

Grant support: Fondation de France (M. Touillaud and Thiébaut) and Ligue Nationale Contre le Cancer (A. Thiébaut); Ligue Nationale contre le Cancer, European Community, 3M Company, Mutuelle Générale de l'Education Nationale, Institut National de la Santé et de la Recherche Médicale, Institut Gustave-Roussy, Fondation de France, and several departmental councils in France (E3N study).

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Current address for A.C.M. Thiébaut: National Cancer Institute, Division of Cancer Epidemiology and Genetics, Nutritional Epidemiology Branch, NIH, Department of Health and Human Services, Bethesda, Maryland.

We thank Dr. Francesco Branca of the WHO, Regional Office for Europe, Copenhagen, Denmark for allowing us to access the VENUS database; Rafika Chaït, Lyan Hoang, Marie Fangon, Estelle Gauthier-Djerah, Agnès Fournier, and Grégory Guernec for their contributions in data acquisition or management; Jerri Bram for proof-reading the English; and all the participants for providing the data and for their commitment to the E3N study and the practitioners for their active collaboration.

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