A new study has reported that consumption of soy foods is not associated with worse breast cancer outcomes. The study was designed to investigate the influence of soy food consumption on breast cancer survival. Earlier clinical studies suggested that soy may promote breast tumor growth, but two recent studies have reported that soy foods do not adversely affect breast cancer prognosis. The authors used data from the Women's Healthy Eating and Living (WHEL) study to conduct the analysis.

The study included 3,088 WHEL participants who were diagnosed with early-stage breast cancer between 1991 and 2000 and were followed for a median of 7.3 years. Isoflavone intakes were measured after breast cancer diagnosis using a food frequency questionnaire. Soy isoflavones consist primarily of genistein (which accounts for approximately 50% of total soy isoflavones) and daidzein (40%). Soybean oil is not a good source of isoflavones. The women reported new breast cancer recurrences themselves semiannually. These reports were then verified using medical records. Death certificates were also obtained where appropriate.

Risk of death was found to decrease as isoflavone intake increased. Women with the highest levels of isoflavone intake were observed to have a 54% reduction in risk of death. However, neither of these results were statistically signficiant. The authors comment that this is the third epidemiologic study to report no adverse effects of soy foods on breast cancer prognosis. These studies, taken together, which vary in ethnic composition (two from the United States and one from China) and by level and type of soy consumption, provide the necessary epidemiologic evidence that clinicians no longer need to advise against soy consumption for women with a diagnosis of breast cancer according to the authors.

Comments regarding the study

While we agree that soy foods such as tofu and edamame appear to be safe for breast cancer survivors, most U.S. soy consumption consists of soy protein isolate and soybean oil, both of which are on our foods to avoid list. Nor do we recommend taking isoflavone supplements. Please see our articles on tofu, soybeans, soy protein isolate, soybean oil, soybean paste, and genistein and daidzein.