Dietary folate intake and breast cancer risk: Results from the Shanghai Breast Cancer Study

Folate is involved in DNA synthesis, repair, and methylation. It has been h ypothesized that high intake of folate may reduce the risk of human cancers , including cancer of the breast. Using data from a population-based case-c ontrol study of breast cancer conducted in urban Shanghai during 1996-1998, we evaluated the association of dietary folate intake and breast cancer ri sk among 1321 cases and 1382 controls, 25-64 years of age, who never drank alcohol regularly or used vitamin supplements. Usual dietary habits were as sessed with an in-person, interviewer-administered food frequency questionn aire developed and tested for use in this population. Unconditional logisti c regression models were used to calculate odds ratios (ORs) and their 95% confidence intervals (95% CIS) after adjusting for potential confounding fa ctors. Dietary folate intake was inversely associated with breast cancer ri sk (P for trend, 0.05) with an adjusted OR of 0.71 (95% CI, 0.56-0.92) obse rved among women who were in the highest quintile of intake. The inverse as sociation was stronger after further adjusting for total fruit and vegetabl e and animal food intakes (OR, 0.62; 95% CI, 0.46-0.82; P for trend, 0.01). A more pronounced inverse association between folate intake and breast can cer risk (OR, 0.47; 95% CI, 0.25-0.88; P for trend, 0.01) was observed amon g women who consumed high levels of folate cofactors (methionine, vitamin B -12, and vitamin B-6.) than those whose intake levels of these nutrients we re low. Dietary intake of methionine, vitamin B-12, and vitamin B-6 were no t independently related to risk of breast cancer after adjusting for confou nding factors. Thus, our study adds additional support to the protective ro le of dietary folate in breast carcinogenesis and suggests further that the effect of folate may be modified by dietary intake of methionine, vitamin B-12, and vitamin B-6.