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Equol Status Modifies the Association of Soy Intake and Mammographic Density in a Sample of Postmenopausal Women

¹ Department of Epidemiology, Regina Elena Italian National Cancer Institute, Rome, Italy; ² NYS Multiple Sclerosis Consortium, The Jacobs Neurological Institute; ³ Division of Pulmonary, Critical Care, and Sleep Medicine, School of Medicine and Biomedical Sciences; ⁴ Departments of Biostatistics and of Social and Preventive Medicine, School of Public Health and Health Professions; ⁵ Department of Physiology and Biophysics, School of Medicine and Biomedical Sciences; and ⁶ Department of Exercise and Nutrition Sciences, School of Public Health and Health Professions, University at Buffalo, Buffalo, New York; ⁷ Lombardi Comprehensive Cancer Center, Georgetown University, Washington, District of Columbia; ⁸ Hormone Research Laboratory, Nuclear Medicine Operative Unit, Italian National Cancer Institute; and ⁹ Clinical Analysis Laboratory, Department of Experimental Oncology, National Cancer Institute, Milan, Italy

Requests for reprints: Barbara Fuhrman, Department of Epidemiology, Italian National Cancer Institute Regina Elena, Via Elio Chianesi 53, Rome 00144, Italy. Phone: 39-6-716-829-9160; Fax: 39-6-716-878-7101. E-mail: barbara.fuhrman{at}gmail.org

Only 30% to 50% of people produce the daidzein-metabolite equol after eating soy. We conducted a cross-sectional study of the associations between equol status, intake of soy foods, and mammographic density in a sample of postmenopausal women recruited at a radiology clinic near Buffalo, New York. Participants were 48 to 82 years old, had no history of cancer or breast reduction/augmentation, and no recent use of antibiotics or hormones. Percent density was measured by computer-assisted analysis of digitized images of craniocaudal films. Equol status was assessed using a soy-challenge protocol and usual soy intake by questionnaire. General linear models were used to assess independent and joint effects of equol status and intake of soy on multivariate adjusted percent density (covariates included age, body mass index, parity, age at first birth, and ever use of combined hormone therapy). Of 325 enrolled, 232 (71%) participants completed study assessments and are included in the present analysis. Mean percent density was 34% (±18%). Seventy-five (30%) participants were producers of equol. Forty-three (19%) participants reported regularly eating >1 soy food or supplement/wk. There were no significant independent associations of equol status or soy intake with percent density, but the interaction between these factors was significant (P < 0.01). Among equol producers, those with weekly soy intake had lower percent density (30.7% in weekly consumers of soy versus 38.9% in others; P = 0.08); among nonproducers, weekly soy intake was associated with higher percent density (37.5% in weekly soy consumers versus 30.7% in others; P = 0.03). Results suggest that equol producers and nonproducers may experience different effects of dietary soy on breast tissue. (Cancer Epidemiol Biomarkers Prev 2008;17(1):33–42)