Certain types of fats have been shown to increase estrogen levels, which, in turn, may raise breast cancer risk. Opt for monounsaturated oils, such as olive and canola, as well as omega-3 fatty acids, found in salmon, sardines, and herring. Avoid trans-fatty stick margarine, packaged baked goods, and snack foods, as well as corn, sunflower, and safflower oils, which tend to be high in polyunsaturated fats.