Soy and Breast Cancer: The Controversy Continues

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Given that there are few modifiable risk factors for breast cancer, identifying cost-effective, acceptable dietary changes that can reduce breast cancer risk is of tremendous importance. In this issue of the Journal, Trock and colleagues ( 1 ) took on the remarkable challenge of quantifying the association between soy intake and breast cancer risk from the published literature. We commend the authors for tackling this complex and controversial, yet highly important, issue.

Although there is much interest in gaining a better understanding of the relationship between soy consumption and the risk of breast cancer, Trock et al. highlight the potential limitations that occur in summarizing published results of diet–disease associations. Studies examining these associations can assess nutrients, food groups, specific foods, or even biomarkers of exposure for a specific dietary constituent. Which exposure is reported by a study will depend on the hypothesis tested in each study, the characteristics of the dietary assessment method (i.e., whether it was designed to estimate overall nutrient intake or was targeted to measure the consumption of a specific food or nutrient), the availability of food composition data for a specific nutrient [i.e., isoflavone food composition data only recently became available ( 2 ) ], and/or the availability of an appropriate biomarker. Another important consideration that is applicable to all dietary studies pertains to measurement error of dietary intake and its impact on risk estimates ( 3 , 4 ) . Although the authors standardized their exposure of “soy measures”, additional variability is introduced by issues such as fermented versus nonfermented soy foods, total soy versus soy protein, or soy versus urinary isoflavone estimates. As a result, summarizing the published literature for a complex exposure such as soy consumption poses remarkable challenges.