Effect of soy isoflavones on breast cancer recurrence and death for patients receiving adjuvant endocrine therapy

CMAJ. 2010 Nov 23;182(17):1857-62. doi: 10.1503/cmaj.091298. Epub 2010 Oct 18.


Background: The intake of soy isoflavones among women with breast cancer has become a public health concern, because these compounds have weak estrogenic effects. There is little clinical evidence about their safety for patients with breast cancer who are receiving adjuvant endocrine therapy.

Methods: For patients who underwent surgery for breast cancer between August 2002 and July 2003 and who were receiving adjuvant endocrine therapy, we examined associations between dietary intake of soy isoflavones and recurrence of breast cancer and death. We measured dietary intake of soy isoflavones at baseline using a validated food frequency questionnaire. We estimated hazard ratios (HRs) and 95% confidence intervals (CIs) by means of multivariable Cox proportional hazards regression models. We further stratified the analyses by hormonal receptor status and endocrine therapy.

Results: The median follow-up period for the 524 patients in this study was 5.1 years. Among premenopausal patients, the overall death rate (30.6%) was not related to intake of soy isoflavones (HR = 1.05, 95% CI 0.78-1.71 for the highest quartile [> 42.3 mg/day] v. the lowest quartile [< 15.2 mg/day], p for trend = 0.87). Relative to post-menopausal patients in the lowest quartile of soy isoflavone intake, the risk of recurrence for post-menopausal patients in the highest quartile was significantly lower (HR = 0.67, 95% CI 0.54-0.85, p for trend = 0.02). Inverse associations were observed in patients with estrogen and progesterone receptor positive disease and those receiving anastrozole therapy.

Interpretation: High dietary intake of soy isoflavones was associated with lower risk of recurrence among post-menopausal patients with breast cancer positive for estrogen and progesterone receptor and those who were receiving anastrozole as endocrine therapy.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Aged
  • Antineoplastic Agents, Hormonal / therapeutic use*
  • Breast Neoplasms / chemically induced*
  • Breast Neoplasms / drug therapy
  • Breast Neoplasms / mortality
  • Chemotherapy, Adjuvant
  • Confidence Intervals
  • Diet / adverse effects
  • Diet Surveys
  • Female
  • Humans
  • Isoflavones / adverse effects*
  • Menopause
  • Middle Aged
  • Multivariate Analysis
  • Proportional Hazards Models
  • Recurrence
  • Soy Foods / adverse effects*
  • Soy Milk


  • Antineoplastic Agents, Hormonal
  • Isoflavones