High-folate diets and breast cancer survival in a prospective cohort study.

Recent evidence suggests that adequate dietary folate may attenuate the risk of breast cancer associated with intake of alcohol. However, patients with breast cancer have been commonly treated with antifolate chemotherapies. The present analysis was performed to test the hypothesis that high folate intake may diminish the effectiveness of chemotherapy and, therefore, adversely influence survival. Women at risk of postmenopausal breast cancer (n = 37,105) participated in the Iowa Women's Health Study. Total folate intake (diet + supplements) was estimated from a food frequency questionnaire administered at baseline in 1986 and categorized into tertiles. From all incident breast cancer cases ascertained in the cohort, we selected those with a diagnosis between 1986 and 1994, chemotherapy as first course of treatment, and adequate diet assessment. Mortality was determined through the State Health Registry of Iowa and the National Death Index. Cox regression was used to estimate survival while adjusting for important covariates. Through 14 yr of follow-up, 80 deaths occurred among the 177 breast cancer cases treated with chemotherapy. Among these patients, high folate intake was not associated with worse survival. After adjustment for age, extent of disease, total calories, alcohol, and estrogen receptor status, women with total folate intake in the highest tertile had a mortality risk ratio of 0.88 (95% confidence interval = 0.44-1.76) compared with cases in the lowest tertile of folate. These findings, although preliminary, afford some reassurance that folate supplementation is unlikely to have a significant adverse effect on survival after chemotherapy for breast cancer.