Context: High intakes of fat and specific fatty acids, including total, animal, saturated, polyunsaturated, and trans-unsaturated fats, have been postulated to increase breast cancer risk.

Objective: To determine whether intakes of fat and fatty acids are associated with breast cancer.

Design and setting: Cohort study (Nurses' Health Study) conducted in the United States beginning in 1976.

Participants: A total of 88795 women free of cancer in 1980 and followed up for 14 years.

Main outcome measure: Relative risk (RR) of invasive breast cancer for an incremental increase of fat intake, ascertained by food frequency questionnaire in 1980, 1984, 1986, and 1990.

Results: A total of 2956 women were diagnosed as having breast cancer. Compared with women obtaining 30.1% to 35% of energy from fat, women consuming 20% or less had a multivariate RR of breast cancer of 1.15 (95% confidence interval [CI], 0.73-1.80). In multivariate models, the RR (95% CI) for a 5%-of-energy increase was 0.97 (0.94-1.00) for total fat, 0.98 (0.96-1.01) for animal fat, 0.97 (0.93-1.02) for vegetable fat, 0.94 (0.88-1.01) for saturated fat, 0.91 (0.79-1.04) for polyunsaturated fat, and 0.94 (0.88-1.00) for monounsaturated fat. For a 1% increase in energy from trans-unsaturated fat, the values were 0.92 (0.86-0.98), and for a 0.1% increase in energy from omega-3 fat from fish, the values were 1.09 (1.03-1.16). In a model including fat, protein, and energy, the RR for a 5% increase in total fat, which can be interpreted as the risk of substituting this amount of fat for an equal amount of energy from carbohydrate, was 0.96 (95% CI, 0.93-0.99). In similar models, no significant association of risk was evident with any major types of fat.

Conclusion: We found no evidence that lower intake of total fat or specific major types of fat was associated with a decreased risk of breast cancer.