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Soy Protein Containing Isoflavones and Mammographic Density in a Randomized Controlled Trial in Postmenopausal Women

¹ Julius Center for Health Sciences and Primary Care, University Medical Center, Utrecht, the Netherlands; ² Cancer Research Center of Hawaii, Honolulu, Hawaii; ³ One Stop Medical Pty, Ltd., Mackay, Queensland, Australia; ⁴ National Institute for Public Health and the Environment, Centre for Prevention and Health Services Research, Bilthoven, the Netherlands; and ⁵ Department Epidemiology and Public Health, Faculty of Medicine, Imperial College, St. Mary's Campus, London, United Kingdom

Requests for reprints: Yvonne T. van der Schouw, Julius Center for Health Sciences and Primary Care, University Medical Center, Room Str 6.131, P. O. Box 85500, Utrecht 3508 GA, Utrecht, the Netherlands. Phone: 31-30-088-755-9360; Fax: 31-30-088-755-5485. E-mail: y.t.vanderschouw{at}umcutrecht.nl

Background: The relatively high dietary intake of soy in Asian countries has been hypothesized to, at least partly, explain the lower breast cancer incidence patterns in these countries compared with the Western world. The aim of the present study was to determine the effect of daily soy supplementation on mammographic density, one of the strongest known risk factors for breast cancer.

Methods: A double-blind, randomized, controlled trial was conducted to compare the effects of soy protein intake containing 99 mg isoflavones daily with intake of milk protein (placebo) for the duration of 1 year. Two hundred and two Dutch postmenopausal women ages 60 to 75 years were randomized. Mammographic density was assessed using a quantitative computer-assisted method on digitized mammograms. Equol producer status was assessed in plasma provided at the final visit (soy group) or after a 3-day challenge with soy after the trial was finished (placebo group).

Results: A total of 175 women completed the baseline visits and at least one follow-up visit and were included in the intention-to-treat analyses. For 126 women, both pre- and post-trial mammograms were available. Mammographic density decreased in both study arms, but the decrease did not differ significantly between intervention and placebo groups. Equol producer status did not modify the results.

Conclusion: The results of this trial do not support the hypothesis that a diet high in soy protein among postmenopausal women decreases mammographic density. (Cancer Epidemiol Biomarkers Prev 2008;17(10):2632–8)

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