Dietary phytoestrogens and endogenous hormones as determinants of mammographic density in postmenopausal women
Fuhrman, Barbara J
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Only 25--50% of people produce the daidzein-metabolite equol after eating soy. Equol is more bioavailable to breast tissues and may have more potent anti-cancer effects than other soy isoflavones. Thus any association between soy intake and risk of breast cancer may be modified by inter-individual differences in equol production. This is 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 in Western New York. Participants reported at least one year since the last menses, were over 45 years of age (or >51 years, if they reported menopause due to hysterectomy and at least one remaining ovary), had no history of cancer or breast augmentation/reduction surgery, and no current hormone use. Percent density (PD) was measured using computer-assisted analysis of digitized images of retrieved cranio-caudal films. Equol status was assessed using a soy-challenge protocol; usual soy intake was assessed using a questionnaire. General linear models were used to assess independent and joint effects of equol status and regular intake of soy foods on PD. Of 325 enrolled, 248 participants (76%) completed all study assessments and are included in the present analysis. Mean PD was 34% (±18). Seventy-one participants (29%) were classified as equol producers. Forty-three participants (17%) reported regularly eating at least one soy food weekly. There were no significant independent associations of soy status or soy intake with PD, but there was a significant interaction between these factors (p=0.02) in determining PD. Among equol producers, those with weekly intake of soy foods had lower PD (39.4% in weekly consumers of soy vs. 29.6% in non-consumers after adjusting for age and BMI, p for difference=0.06); in contrast, among non-producers of equol, weekly soy intake was associated with higher PD (36.4% in weekly soy consumers vs. 31.0% in non-consumers, after adjustment, p for difference=0.07). Our results suggest that equol producers and non-producers may experience different effects of exposure to dietary soy on breast tissue and perhaps, also on breast cancer risk. Further research will be needed to understand the nature of these associations, and whether they represent causal or non-causal relationships. Secondary analyses explored associations of dietary macronutrients with mammographic density, and a novel finding that equol status is associated with history of hormone use.