Dietary phytoestrogen intake--lignans and isoflavones--and breast cancer risk (Canada).

OBJECTIVE: To evaluate whether phytoestrogen intake is associated with reduced breast cancer risk, using a novel phytoestrogen database.
METHODS: Population-based breast cancer cases aged 25-74 years (diagnosed 2002-2003) were identified using Ontario Cancer Registry (n = 3,063) and controls (n = 3,430) were an age-stratified random sample of women identified through random digit dialing. An epidemiologic and Block food frequency questionnaire--expanded to include phytoestrogen-containing foods--was mailed to all subjects. The recently published Ontario phytoestrogen database was applied to FFQ responses to estimate intake. Multivariate logistic regression provided odds ratio (OR) estimates, while controlling for confounders.
RESULTS: Among all women, lignan intake was associated with a reduced breast cancer risk (Q5 vs. Q1 MVOR: 0.81, 95% CI: 0.65, 0.99); however, following stratification by BMI, this reduction in risk was statistically significant only among overweight (BMI > 25) women. Total phytoestrogen intake was also associated with a risk reduction among overweight women only. Among pre-menopausal women, total phytoestrogen intake was associated with a significant reduction in breast cancer risk among overweight women only (Q5 vs. Q1 MVOR: 0.51, 95% CI: 0.30, 0.87). Among post-menopausal women, no statistically significant association was observed between breast cancer risk and isoflavones or lignans.
CONCLUSION: Lignan intake may be associated with reduced breast cancer risk among pre-menopausal women, and our data suggest BMI modifies this association.