Intervention Studies
Some have suggested that the relationship between dietary fat and breast cancer can be established only by randomized trials of fat reduction. The Women’s Health Initiative sponsored by the U.S. National Institutes of Health has been developed with the goal of enrolling and randomizing several tens of thousands of women, one-half of whom will be instructed on how to reduce their total fat intake to 20% of calories from fat.

However, the difficulty of maintaining compliance with a diet very different from prevailing food consumption habits, as well as the gradual secular decline in total fat consumption already under way, which may reduce the size of the comparison in fat intake between intervention groups and controls, may compromise the ability of any trial to address the effect of reducing percentage of energy from fat.

Furthermore, as pointed out by the Women’s Health Initiative investigators, “women in the dietary intervention group will be counseled to adopt a dietary pattern that is high in fruits, vegetables, and grain products and low in total fat and saturated fat.” Thus, the trial would be unable to distinguish between a decrease in risk due to increased intake of fruits, vegetables, and grains, and a decrease due to lower fat intake. Also, this trial will not answer whether dietary fat reduction at an early age may reduce breast cancer risk decades later.

Type of Fat
In addition to overall fat intake, intake of specific types of fat could differentially affect risk of breast cancer. In most animal studies, diets high in polyunsaturated fat (linoleic acid), but typically at levels beyond human exposure, have clearly increased the occurrence of mammary tumors. As noted, a positive association has not been found in prospective epidemiologic studies.

Some animal studies have suggested that monounsaturated fat, in the form of olive oil, may be protective relative to other sources of energy. In a Spanish study specifically undertaken because of the high consumption of olive oil and low breast cancer rates in this population, no association was observed between breast cancer rates and total fat intake. Higher intake of olive oil was associated with reduced risk of breast cancer, however.

Similar inverse associations with consumption of olive oil or monounsaturated fat were seen in case control studies in Greece, Italy, and elsewhere in Spain; in the Italian study, intake of polyunsaturated oils was also related to lower risk.

High intake of omega-3 fatty acids from marine oils has inhibited the occurrence of mammary tumors in animals. Case control and cohort studies, however, have in general found little relation between intake of omega-3 fatty acids or fish (the major source of extra-long-chain omega-3 fatty acids) and risk of breast cancer.