Is There a Soy and Breast Cancer Connection?

Breast cancer is the most common type of cancer in the world, affecting over 2.2 million people in 2020 (WCRF 2022). In the United States alone, it is predicted that there will be more than 300,000 new cases of breast cancer in 2023 (Siegel et al. 2023).

Given the disease’s widespread impact, it should come as no surprise that there has been continued research into reducing the risk of breast cancer. Soy has been the topic of many such studies due to its estrogen-like components known as isoflavones. High levels of estrogen have been shown to increase the risk of breast cancer, so it would seem to follow that high levels of soy intake should do the same. But that is not the full story.

Isoflavones are a class of phytoestrogens, which are similar in structure to estradiol, a type of estrogen. Soy provides an exceptionally rich source of isoflavones, though they are present in lower amounts in other fruits, vegetables, and legumes (Reed et al. 2021). Isoflavones can mimic the actions of estrogen in the body by binding to the estrogen receptors of various organs, although importantly, the physiologic effects of this binding can differ from the effects of endogenous estrogen (Boutas et al. 2022).

Naturally, these findings have generated much controversy over the years surrounding the role of soy in breast cancer. Through a variety of mechanisms, exposure to high levels of estrogen and overactivation of estrogen receptors do seem to play a role in the development and progression of breast cancer (Clemons and Goss 2001). A few earlier studies concluded that, like estrogen, soy isoflavones can enhance cancer cell proliferation, although more recent research has suggested that soy has either a protective effect (Boutas et al. 2022, Yang et al. 2023) or no effect at all (Morimoto et al. 2014).

This may seem contradictory at first glance. If isoflavones are known to bind to and activate estrogen receptors, then how can they protect against breast cancer?

As it turns out, there are two types of estrogen receptors in the body—alpha-receptors and beta-receptors. Depending on the tissue, the distribution and action of these receptors will differ. Whereas estrogen binds to both receptors equally, isoflavones preferentially bind to the beta-receptors (Reed et al. 2021, Fritz et al. 2013). Alpha-receptors have been found in higher concentrations in breast cancer tissue, suggesting that their activation may play role in promoting the growth of cancer cells. Beta-receptors, on the other hand, appear to have antiproliferative effects (Clemons and Goss 2001, Fritz et al. 2013). Moreover, through the process of negative feedback, increased levels of isoflavones can decrease the body’s own production of estrogen, thus reducing the risk of breast cancer (Boutas et al. 2022).

Apart from their effects on estrogen receptors, soy isoflavones have been shown to inhibit vascular endothelial growth factor, prevent the production of reactive oxygen species, and promote tumor suppressive proteins, all of which reduce the risk of cancer (Fritz et al. 2013, Yang et al. 2023). It is also worth noting that, as demonstrated by multiple studies, soy consumption does not affect estrogen or testosterone levels in men (Reed et al. 2021). This again highlights that there are significant differences in the effects of soy isoflavones and estrogen in the body.

Even with these proposed mechanisms, the research is far from conclusive. As with many dietary studies, it is impossible to ascertain the exact amount of soy or isoflavones consumed by study participants, and populations that consume more soy products may share other lifestyle factors that contribute to their lower risk of breast cancer. Additionally, few studies have looked at the effects of a high dose of isoflavone supplements, so whether such supplements afford the same benefits as soy intake remains to be seen.

All this is to say that while soy does appear to reduce the risk of breast cancer, the degree to which it provides anti-cancer benefits requires further investigation. For now, people should continue to enjoy soy as part of a balanced diet along with fruits, vegetables, and legumes.ft