The Role of Soy and Its Isoflavones in Breast Cancer: Beneficial or Harmful?

Breast cancer (BC) is the most common cancer in women worldwide, resulting in a high mortality rate and high costs for public coffers. Increasing evidence has shown that lifestyle factors, including diet, may modify the risk for BC. For example, dietary consumption of soy and its association with BC risk has been highly debated. Among the components of soy, isoflavones, such as genistein and daidzein, stand out due to their chemical structure that mimics 17-β estradiol, which is known to be a risk factor for BC. This work aims to provide a comprehensive and current review of the literature, focusing on the consumption of soy and dietary isoflavones and their influence on BC risk. In vitro and in vivo tests have presented controversial and inconclusive results, showing that isoflavones can have both antiestrogenic and estrogenic effects and can stimulate or reduce the proliferation of BC cells depending on the dose and context of consumption. Clinical trials, systematic reviews, and meta-analyses have been more consistent and show that the vast majority of studies found a protective effect against BC associated with soy and isoflavone consumption. Although most scientific evidence points to protection against BC, caution should still be exercised when consuming isoflavones.