A Review of the Clinical and Epidemiologic Evidence Relevant to the Impact of Postdiagnosis Isoflavone Intake on Breast Cancer Outcomes.

PURPOSE OF REVIEW

This narrative review aims to determine the impact of postdiagnosis isoflavone intake, via supplements and foods, on breast cancer outcomes. Foods derived from soybeans are uniquely rich sources of isoflavones, naturally occurring compounds that can bind to estrogen receptors although the extent to which they exert estrogen-like effects in humans is unclear. Isoflavones have been rigorously investigated for a wide range of health benefits including breast cancer prevention. However, their classification as phytoestrogens has led to concern that isoflavones and hence, soy food consumption, could worsen the prognosis of women with breast cancer and interfere with the efficacy of endocrine therapy for this disease.

RECENT FINDINGS

Research in athymic ovariectomized mice shows isoflavones stimulate the growth of existing estrogen-sensitive mammary tumors. However, extensive clinical research indicates that neither soy foods nor isolated isoflavones affect markers of breast cancer risk including mammographic density and breast cell proliferation. No effects are observed even when isoflavone exposure greatly exceeds typical intake in Asian countries. Furthermore, the results from epidemiologic studies indicate postdiagnosis isoflavone intake from soy foods reduces recurrence and possibly mortality from breast cancer. Additionally, the limited observational data do not show that isoflavones interfere with the efficacy of tamoxifen or aromatase inhibitors. Regardless of their treatment status, evidence indicates that women with breast cancer can safely consume soy foods. Limiting intake to no more than two servings of traditional Asian soy foods daily, an amount that provides approximately 50 mg isoflavones, is recommended, not because data indicate exceeding this amount is harmful, but because few population-based studies involved participants consuming more than this intake recommendation.