Soy and its components, particularly isoflavones such as genistein and daidzein, have been extensively studied for their potential role in cancer prevention, especially in hormone-related cancers like breast and prostate cancer. Epidemiological studies have observed that populations with high dietary soy intake, such as those in some Asian countries, tend to have lower incidences of certain cancers compared to Western populations. Laboratory and animal studies suggest that soy isoflavones may exhibit anti-cancer properties through mechanisms such as modulation of estrogen receptors, inhibition of cell proliferation, induction of apoptosis, and antioxidant effects.

Clinical evidence in humans is mixed but generally suggests that moderate soy consumption is safe and may have a protective effect, particularly when consumed from whole food sources rather than supplements. For example, some cohort studies have reported a reduced risk of breast cancer recurrence and improved survival in women with higher dietary soy intake, particularly among Asian populations. However, results are less consistent in Western populations and in studies using soy supplements. Furthermore, concerns about soy acting as a phytoestrogen and potentially stimulating hormone-sensitive cancers have not been substantiated in most human research.

Overall, while there is scientific rationale and some epidemiological support for the use of soy in cancer prevention, the evidence is not uniformly strong or conclusive, leading to a moderate evidence rating.