Soybeans are concentrated sources of isoflavones. Studies in laboratories have shown that genistein has a strong inhibitory ability in a number of breast and prostate cancer cell lines. The exact system that genistein works by is not fully understood. There is the belief that genistein blocks a key cellular pathway in cancer cells. Much of the interest in soy stems from the fact that Asian men who have diets high in soy have a lower rate of death for prostate cancer than American men do.
Asian men tend to have higher levels of phytoestrogens in their bodies.
This may partially account for the large difference in mortality rates. There are limited human data available for evaluating the link between soy foods and prostate cancer. Studies in mice and rats fed high soy diets show good outcomes in retarding tumor growth. Varies according to product. Not all soy foods contain isoflavones.