Strong epidemiologic evidence implicates diet as a factor in the development of breast cancer. Recent theories have focused on the phyto-estrogens (isoflavonoids and lignans) that are found in many edible plants. When consumed, phyto-estrogens are metabolized by bowel microflora, and the resulting compounds have antiangiogenic, estrogenic and antiestrogenic properties, as well as the ability to inhibit several enzymes. In cell culture and animal experiments, these compounds have also been shown to inhibit tumor development. Further evidence of a possible protective effect comes from the observation that populations consuming large quantities of phyto-estrogens from soy-rich diets have lower rates of breast and other cancers. Ingram and colleagues studied the association of dietary phyto-estrogen and breast cancer in Australian women.

A total of 144 women with breast cancer were matched with control subjects selected according to age and area of residence. Women in both the study group and the control group were interviewed using a standardized questionnaire to elicit detailed information regarding demographic, reproductive and lifestyle characteristics. Participants also provided three consecutive 24-hour urine specimens and a blood sample. The urine samples were analyzed for lignans and isoflavonoid phyto-estrogens, and for urea and ammonia so that a measure of total nitrogen excretion could be obtained as an index of total food intake.

Initial analyses showed a substantial reduction in the risk of developing breast cancer in women with increasing excretion of phyto-estrogen metabolites, particularly equol (an isoflavin). This substantial effect persisted for equol and enterolactone (a lignan) after adjusting for age at menarche, parity and nutritional variables such as alcohol intake and total fat intake. For equol, the risk of developing breast cancer for women in the highest quartile of excretion (after adjusting for confounding variables) was one quarter that of women in the lowest quartile of excretion. For enterolactone, the comparable reduction in risk was one third. Other phyto-estrogens were associated with reductions in risk that did not reach significance.

The authors conclude that their findings support earlier studies that show increased excretion of phyto-estrogens is associated with a substantial reduction in the risk of developing breast cancer. This theory also has biological plausibility as these compounds inhibit the growth and development of hormone-dependent cells. Because the compounds are weakly estrogenic, it is suggested that they reduce the amount of circulating estrogen. However, the exact mechanisms of the suggested anticarcinogenic effect are not understood. In the absence of other primary prevention for breast cancer, the cultural movement toward increased consumption of foods containing phyto-estrogens appears to be potentially beneficial.