Phytoestrogens and breast cancer risk. Review of the epidemiological evidence.

Phytoestrogens are natural plant substances. The three main classes are isoflavones, coumestans, and lignans. Phytoestrogens have anticarcinogenic potential, but they have also significant estrogenic properties. For an evaluation of the effect of phytoestrogens on breast cancer risk we reviewed the analytical epidemiological data. A total of 18 studies were included. Up to now, there are 13 studies that have assessed the direct relation between the individual dietary intake of soy products and the risk of breast cancer. Overall, results do not show protective effects, with the exception maybe for women who consume phytoestrogens at adolescence or at very high doses. Only four of these 13 studies are prospective, and none of them found statistically significant breast cancer reductions. Four studies assessed urinary isoflavones excretion in relation to breast cancer. Three of these are case control studies, where excretion was measured after breast cancer occurrence and thus seriously limiting causal interpretation of the results. The only prospective study with urinary measurements before breast cancer occurrence was done in a Dutch postmenopausal population and showed a non-significant breast cancer risk reduction for high excretion. Three studies measured enterolactone (lignan): two case control studies reported a preventive effect on breast cancer risk, but the only prospective study did not . In conclusion, few prospective studies (n = 5) were done to assess the effects of phytoestrogens on breast cancer risk. None of them found protective effects. However, these prospective studies did not focus on 'age at consumption', which seems to be important based on results from dietary case control studies done so far.