Ever since the US National Cancer Institute first expressed interest 30 years ago in exploring the role of soy in cancer prevention, this topic has been rigorously investigated. Much of the focus has been specifically on cancer of the breast. That is understandable given that compared to the West, breast cancer mortality rates in soyfood-consuming countries are extremely low. But the rates of prostate cancer in soyfood-consuming countries are just as low. And new research suggests soyfoods could very well be one of the reasons.

To better understand the relationship between soy and prostate cancer a team of researchers from the University of Illinois analyzed 30 observational or epidemiologic studies. Observational studies examine how exposure to a particular factor, such as soy, among a given population, affects risk of developing a particular outcome, such as prostate cancer. When all studies were included in the analysis, those men consuming the most soy were 29% less likely to develop prostate cancer in comparison to men infrequently consuming soy. The results were similar when looking at either Asian or North American studies. However, whereas the consumption of unfermented soyfoods, such as tofu and soymilk, was very protective, no such protective effects were found for fermented soyfoods such as miso. Exactly why fermented foods weren’t protective isn’t clear.

Why is soy protective against prostate cancer? The answer appears to be because soyfoods are such rich sources of isoflavones. Isoflavones are naturally-occurring compounds found in very high amounts in soybeans. The results showed that among the Asian studies, the intake of genistein and daidzein, the two primary isoflavones in soybeans, was inversely related to risk. That is, the more isoflavones consumed, the less likely men were to develop prostate cancer.

In Asian studies, men in the highest intake group, consume about two servings of soyfoods per day. That amount is easy to incorporate into the diet given that a serving is one cup of soymilk, one-half cup of tofu or an ounce of soynuts. It isn’t clear why the isoflavones in soy are protective against prostate cancer, but it isn’t because they lower testosterone levels. Studies show quite clearly that consuming even very high amounts of isoflavone-rich soy doesn’t lower blood testosterone.

 

Applegate CC, Rowles JL, Ranard KM et al. (2018) Soy consumption and the risk of prostate cancer: An updated systematic review and meta-analysis. Nutrients 10.