Phytoestrogen intake from foods, during adolescence and adulthood, and risk of breast cancer by estrogen and progesterone receptor tumor subgroup among Ontario women

Phytoestrogen intake may reduce breast cancer risk and limited evidence suggests this association may hold for hormone receptor-positive tumors only. The study aims were to assess whether the association between phytoestrogen intake during adolescence and adulthood and breast cancer risk varies by estrogen and progesterone receptor (ERPR) tumor subgroup. Cases were identified from the Ontario Cancer Registry (2002-2003), and ERPR status was ascertained from pathology reports for 81% of cases (n = 2,438). Controls were identified through random digit dialing of Ontario households (n = 3,370). Published phytoestrogen food values were applied to food frequency questionnaire responses to assess isoflavone, lignan and total phytoestrogen intake, during adolescence and adulthood. Polytomous multivariate logistic regression was used to estimate adjusted odds ratios (ORs) for association between phytoestrogen intake and breast cancer risk by hormone receptor ERPR tumor subgroups. Among premenopausal women, few associations were observed for adolescent or adult phytoestrogen intake across all tumor subgroups. Among postmenopausal women, adolescent phytoestrogen intake (isoflavone, lignan and total) was associated with reduced risk across all hormone receptor subgroups; however, statistical significance was most consistent within the ER+PR+ subgroup. For example, ER+PR+ postmenopausal breast cancer risk was associated with adolescent phytoestrogen intake (highest vs. lowest: OR = 0.79; 95% confidence interval: 0.65-0.96). Among all women and postmenopausal women, ORs for high adult lignan intake were all below 1.0 within each tumor subgroup, suggesting reduced breast cancer risk, although none reached statistical significance. In conclusion, adolescent phytoestrogen intake was associated with reduced postmenopausal breast cancer, particularly for ER+PR+ tumor subgroup.