Soy isoflavones, estrogen therapy, and breast cancer risk: analysis and commentary

Isoflavones - Mechanism of Action and Impact on Breast Cancer Risk

Isoflavones are plant-derived substances with weak es-trogenic effects. Asian populations are high consumers of soy products which are rich in isoflavones. The lower breast cancer incidence in Asian women compared with Western women has been associated with the possibility of a preventive isoflavone effect on cancer risk. The aim of this review is to give an overview of current research data on the influence of isoflavones on the risk of primary breast cancer development as well as the risk of recurrence in breast cancer patients. Despite inconsistencies in the available data, an inverse correlation between isoflavone intake and risk of breast cancer is likely. However, a negative impact on breast cancer disease, especially on hormone receptor-positive tumors, cannot be excluded at present.

Prevalence, sources, and predictors of soy consumption in breast cancer

BACKGROUND

A number of components in soy appear to have anticancer properties, including the isoflavones, genistein and daidzein. The use of soy by women with breast cancer is now being questioned because of the estrogen-like effects of isoflavones and possible interactions with tamoxifen. Clinicians providing nutrition counseling to these women are concerned because the availability of soy foods has increased dramatically in the past few years. The goal of this study was to quantify the intake of isoflavones in women with breast cancer.

METHODS

A cross-sectional study of 100 women with breast cancer treated at Cancer Treatment Centers of America(R) between 09/03 and 02/04. Each patient completed a soy food frequency questionnaire (FFQ) that was scored by Fred Hutchinson Cancer Research Center. Demographic and clinical predictors of soy intake were evaluated using one-way non-parametric Mann Whitney test and non-parametric spearman's rank correlation.

RESULTS

Mean age was 50.5 years (std. dev. = 9.4; range 31-70) and mean BMI was 27.3 kg/m2 (std. dev. = 6.75; range 17-59). Genistein and Daidzein consumption was limited to 65 patients with a mean intake of 11.6 mg/day (std. dev. = 21.9; range 0-97.4) and 7.6 mg/day (std. dev. = 14.1; range 0-68.9) respectively. Soy milk (37%) and pills containing soy, isoflavones, or "natural" estrogen (24%) were the two biggest contributors to isoflavone intake.

CONCLUSION

Our study suggests that the isoflavone intake of breast cancer patients at our hospital was quite variable. Thirty-five patients reported no soy intake. The mean daily intake of 11.6 mg genistein and 7.4 mg daidzein, is the equivalent of less than 1/4 cup of tofu per day. This amount is higher than what has been previously reported in non-Asian American women.