Folate intake and risk of breast cancer by estrogen and progesterone receptor status in a Swedish cohort.

BACKGROUND Folate is a B vitamin involved in one-carbon metabolism and has been postulated to influence the risk of breast cancer. However, epidemiologic studies of folate intake in relation to breast cancer risk are inconclusive. We examined the association between dietary folate intake and the risk of breast cancer by estrogen receptor (ER) and progesterone receptor (PR) status of the breast tumor in the Swedish Mammography Cohort. METHODS Our study population consisted of 61,433 women who completed a food frequency questionnaire at baseline (1987-1990) and again in 1997. Cox proportional hazards models were used to estimate rate ratios (RR) with 95% confidence intervals (95% CI). RESULTS During an average of 17.4 years of follow-up, 2,952 incident cases of invasive breast cancer were ascertained. We observed no association between dietary folate intake and risk of total breast cancer or ER+/PR+ or ER-/PR- tumors. The multivariate RR of total breast cancer comparing extreme quintiles of folate intake was 1.01 (95% CI, 0.90-1.13; P(trend) = 0.84). However, folate intake was inversely associated with risk of ER+/PR- breast cancer (n = 417 cases; RR for highest versus lowest quintile, 0.79; 95% CI, 0.59-1.07; P(trend) = 0.01). Results did not vary by alcohol intake or menopausal status. CONCLUSIONS These findings do not support an overall association between folate intake and risk of breast cancer but suggest that folate intake may be inversely associated with ER+/PR- tumors.