Dietary fiber, phytoestrogens, and breast cancer.

The hypothesis that diet exerts an influence on breast cancer risk has emphasized a role for fat, and current dietary intervention trials to reduce that risk are designed specifically to decrease fat consumption to 15-20% of total calories. There is, however, mounting evidence that dietary fiber has a protective effect and may favorably modify the enhanced breast cancer risk associated with the typical American high-fat low-fiber diet. These data come largely from epidemiological studies, but a few experiments with animal models have also been performed. The mechanisms concerned probably involve estrogen metabolism and bioactivity, both by effects on the enterohepatic circulation of estrogens and the actions of fiber-associated phytoestrogens. More studies are essential to determine the specific types of dietary fiber that are likely to affect the risk of breast cancer, and an appropriate modification in fiber intake should then be added to dietary fat reduction in any future clinical intervention trials designed to demonstrate a favorable influence on breast cancer incidence.