Dietary fat intake in relation to lethal breast cancer in two large prospective cohort studies

Whether fat intake influences risk of developing more aggressive, lethal breast tumors is unknown. We evaluated intakes of total fat, specific types of fat, and cholesterol prior to diagnosis in relation to lethal breast cancer risk in 88,759 women in the Nurses' Health Study (NHS; 1980-2010) and 93,912 women in the Nurses' Health Study II (NHSII; 1991-2010). Diet was assessed every 4 years using a semi-quantitative food frequency questionnaire. Breast cancers were confirmed with pathology reports; deaths were confirmed by next of kin or the National Death Index. We defined lethal cases as women with invasive breast cancer who died of breast cancer. We pooled the cohorts and used multivariable Cox proportional hazards models. We identified 1,529 lethal breast cancer cases (1,279 in NHS and 250 in NHSII). Higher total fat intake was associated with a slightly lower lethal breast cancer risk (top vs. bottom quintile hazard ratio [HR] 0.85; 95 % CI 0.72, 1.01; p trend = 0.05). Specific types of fat were generally not associated with lethal breast cancer risk. For example, compared with those in the lowest quintile of saturated fat intake, those in the highest quintile had a HR of 0.98 (95 % CI 0.75, 1.26; p trend = 0.96). Among women diagnosed with breast cancer, pre-diagnosis fat intake was not associated with survival. Higher pre-diagnosis fat intake was not associated with greater risk of lethal breast cancer in these large prospective cohort studies, consistent with the weight of the evidence against a causal role for fat intake and breast cancer incidence.