Premenopausal dietary fat in relation to pre- and post-menopausal breast cancer.

We examined the association between fat intake and breast cancer incidence in the Nurses' Health Study II. We followed 88,804 women aged 26-45 years from 1991 to 2011 and documented incident breast cancers. Dietary fat, assessed by questionnaires in 1991, was examined in relation to total, premenopausal, and postmenopausal breast cancers. Multivariable-adjusted Cox proportional hazards models were used to estimate relative risk (RR) and 95 % confidence intervals (95 % CI). During 20 years of follow-up, 2,830 incident invasive breast cancer cases were diagnosed. Total fat intake was not associated with risk of breast cancer overall. After adjustment for demographic, anthropometric, lifestyle, and dietary factors, a positive association was observed between animal fat intake and breast cancer overall (RR for highest vs lowest quintile, 1.18; 95 % CI 1.04-1.33; P trend = 0.01). A positive association with animal fat intake was also seen among premenopausal women, but not among postmenopausal women. Higher intakes of saturated fat and monounsaturated fat were each associated with modestly higher breast cancer risk among all women, and higher cholesterol intake was associated with higher premenopausal breast cancer risk. However, the associations of saturated fat, monounsaturated fat and animal fat, were attenuated and non-significant after adjustment for red meat intake. Intakes of other types of fat including vegetable fat, dairy fat, polyunsaturated fat, and trans fat were not associated with breast cancer risk. Our finding suggests a positive association between early adult intake of animal fat and breast cancer risk.