Soy intake and mammographic density in postmenopausal women: Modification by equol status.

A165

The association between soy intake and risk of breast cancer may be modified by inter-individual differences in the metabolism of soy isoflavones. Only 30-50% 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 in Western New York. Participants were postmenopausal, over 45 years of age (or >51 years, if they report partial hysterectomy and at least one remaining ovary), reported no history of cancer or breast augmentation / reduction surgery, and no current hormone use. Percent density (PD), an intermediate marker of breast cancer risk, 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 by 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 equol status or soy intake with PD, but there was a significant interaction between these factors in predicting PD (p=0.02). Among equol producers, those with weekly intake of soy foods had lower PD (29.6% in weekly consumers of soy vs. 39.4% in others, p=0.06) after adjustment for age and BMI; among non-producers of equol, weekly soy intake was associated with higher PD (36.4% in weekly soy consumers vs. 31.0% in others, p=0.07). Additional adjustments for history of combination hormone therapy use and for vegetarian status did not modify the statistical significance of the interaction. Our results suggest that equol producers and non-producers may experience different effects of exposure to dietary soy on breast tissue. Further research will be needed to understand the nature of these associations, and whether they represent causal or non-causal relationships.