Effect of soy isoflavones on breast cancer risk among pre- and post-menopausal women: a systematic review ofrandomized controlled trials

Abstract

Background: Breast cancer is the most frequent female cancer in both developed and developing world which comprising 16% of all female cancer according to WHO GLOBOCAN 2008. The statistic from Hong Kong Cancer Registry reported that breast cancer is the third commonest cause of female death in Hong Kong. Breast cancer incidence varies remarkably among developed countries. The high dietary consumption of soy isoflavones has been hypothesized to explain the lower breast cancer incidence among women in Asian countries in observational studies, but whether soy isoflavones exert estrogenic or anti-estrogenic in breast tissue remains uncertain.

Objective: This systematic review was to assess the effects of isoflavone-rich soy consumption on breast cancer risk in pre- and post-menopausal women

Methods: Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines for conducting and reporting randomized controlled trials were followed.

Data sources: A systematic review of randomized controlled trials was conducted through searching databases: MEDLINE, PubMed and Cochrane Library (2002 until March 2012). Keywords for electronic searches included: [(soy OR isoflavones) AND (breast cancer OR breast neoplasms)] limited study types to human & randomized controlled studies.

Study selection: RCTs of the effects of isoflavones or supplement versus placebo or control diet among pre- and post-menopausal participants who were currently free from breast cancer.

Outcome measurements: serum sex hormones and IGF profile, mammographic density and menstruation cycle length

Results: 15 RCTs (1527 women) compared isoflavones with placebo or control diet for study duration ranged from 2 months to 2 years. No significant effect was found on serum sex hormones, IGF profile, mammographic density or menstrual cycle length. The effect of menstrual cycle on mammographic densities was noticed.

Conclusion: The results of the systematic review did not support the hypothesis that short-term isoflavones exposure has an effect on modulating breast cancer risk. The effect of menstrual cycle on mammographic densities probably reflects the effect of hormonal changes. Null results did not necessarily contradict the inverse association between soy intake and breast cancer risk from the results of epidemiologic studies. The absence of conclusive data on the effects might be attributable to the insufficient exposure duration in the RCTs. Longer duration of soy exposure and early life exposure might be a scope for future research.