Soy as a source of isoflavones vs other sources of phytoestrogens: comparative effects on breast cancer risk and progression – a systematic review

Breast cancer remains the most prevalent malignancy among women worldwide, with notable geographic disparities in incidence that may reflect dietary and other environmental factors, including but not limited to soy intake. This review aimed to compare soy as a source of isoflavones with other sources of phytoestrogens and to assess whether habitual soy consumption within Asian dietary contexts is associated with differential breast cancer risk compared with populations with minimal soy intake. Conducted in accordance with PRISMA 2020 guidelines and registered in PROSPERO. Across all included studies, higher habitual soy isoflavone intake was consistently associated with reduced breast cancer incidence and lower breast cancer-specific mortality. Notably, the most pronounced prognostic benefit was observed at an estimated moderate intake, although this value was derived from indirect estimates of isoflavone intake from foods actually consumed by the subjects. Compared with other phytoestrogens commonly consumed in Western, Middle Eastern, and African populations, soy isoflavones appear to show a clearer and more reproducible epidemiologic association with breast cancer-specific outcomes. These findings support moderate long-term soy consumption, provided that it adheres to traditional Asian cooking methods, as a practical dietary strategy for breast cancer prevention.