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Nutritional Epidemiology

Various Doses of Soy Isoflavones Do Not Modify Mammographic Density in Postmenopausal Women^1,2

³ Cancer Research Center of Hawaii, Honolulu, HI 96813; ⁴ University of California, Davis, CA 95616; ⁵ Oregon Health and Science University, Portland, OR 97239; ⁶ University of Georgia, Athens, GA 30602; ⁷ Northern California Fertility Medical Center, Roseville, CA 95661; and ⁸ Baylor College of Medicine, Houston, TX 77030

Soy isoflavones have functional similarity to human estrogens and may protect against breast cancer as a result of their antiestrogenic activity or increase risk as a result of their estrogen-like properties. We examined the relation between isoflavone supplementation and mammographic density, a strong marker for breast cancer risk, among postmenopausal women. The Osteoporosis Prevention Using Soy (OPUS) study, a multi-site, randomized, double-blinded, and placebo-controlled trial assigned 406 postmenopausal women to 80 or 120 mg/d of isoflavones each or a placebo for 2 y. Percent densities were assessed in digitized mammograms using a computer-assisted method. The mammogram reader did not know the treatment status and the time of mammograms. We applied mixed models to compare breast density by treatment while considering the repeated measures. The mammographic density analysis included 358 women, 88.2% of the OPUS participants; 303 had a complete set of 3 mammograms, 49 had 2, and 6 had only 1 mammogram. At baseline, the groups were similar in age, BMI, and percent density, but mean breast density differed by study site (P = 0.02). A model with all mammograms did not show a treatment effect on any mammographic measure, but the change over time was significant; breast density decreased by 1.6%/y across groups (P < 0.001). Stratification by age and BMI did not reveal any effects in subgroups. In this randomized 2-y trial, isoflavone supplements did not modify breast density in postmenopausal women. These findings offer reassurance that isoflavones do not act like hormone replacement medication on breast density.

^* To whom correspondence should be addressed. E-mail: gertraud{at}crch.hawaii.edu.

Manuscript received 29 November 2008. Initial review completed 3 January 2009. Revision accepted 4 February 2009.

Published online 25 March 2009.

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