Men who consume soy foods may be at lower risk for prostate cancer, according to an updated systematic review and meta-analysis of 30 observational studies, published in the journal Nutrients in early 2018. It included diet and health data from 267,000 men in Asia, Europe, and North America.
Overall, total soy intake was associated with a reduced risk of prostate cancer. In further analyses, this association held up for unfermented soy foods (soybeans, tofu, and soy milk), while fermented soy (miso and natto) showed neither benefit nor risk (that’s good news since there has been some concern that fermented foods may increase prostate cancer risk). No link was seen between soy and the risk of advanced prostate cancer (perhaps due to the few studies addressing this), though a few studies not included in this review have suggested a protective effect. The studies did not look at soy supplements, which are of questionable value.
The researchers noted, however, that variability in participants, methodology, and findings across studies “makes it difficult to draw conclusions about whether ethnic differences, preparation methods, or eating patterns exist in soy food or isoflavone consumption and prostate cancer risk.” Furthermore, the results “should be interpreted with caution” because other dietary and lifestyle factors not controlled for in the studies could have contributed to the decreased cancer risk observed. More studies are needed to support consuming soy as a prostate cancer preventive—and there is still too little research to fully understand soy’s effects (good or bad) in men who already have the cancer.
In the U.S., prostate cancer is the second leading cause of cancer deaths in men (after lung cancer). The lower rate of prostate cancer seen in Asia has been attributed in part to soy being a staple food in most Asian diets. It’s thought that isoflavone compounds in soy—which have been shown to accumulate in prostate tissue and act as weak hormones—may suppress cancer through both hormonal and non-hormonal mechanisms. For example, the isoflavone genistein preferentially binds to estrogen receptors in prostate tissue, and this may reduce growth of tumor cells and induce cell death.
This is not the first study to show a possible protective effect of soy against prostate cancer. In fact, there have been four previous meta-analyses, including a 2009 analysis of 15 studies, published in the American Journal of Clinical Nutrition, which found that men who consumed soy regularly had a 26 percent reduced risk of prostate cancer, compared with those who ate little or none. As in the current study, the benefit was seen only with unfermented soy foods, while fermented foods had no effect.
This article first appeared in the UC Berkeley Wellness Letter.
Also see Pros and Cons of Soy Foods.