World-wide prostate cancer is the 4th most common cancer; however, the incidence of the disease varies markedly geographically. 
The United States has one of the highest rates of prostate cancer. According to the American Cancer Society, over 240,000 men are diagnosed with prostate cancer every year; this translates into over 2 million prostate cancer survivors currently in the United States. Asian countries on the other hand have significantly lower rates of prostate cancer.
These variances reflect a host of differences between the two populations, including screening, genetic and environmental influences. Epidemiologic studies have tried to isolate the impact of genetics and environment by examining the incidence of prostate cancer between men of Chinese decent living in either China or the US. Interestingly, those living in the United States had a ten-fold higher incidence of cancer. Similar studies have found identical trends in Japanese men suggesting a significant role of environment in the disease process.
Diet is one such environmental factor which has been presumed to pay a key role in reducing the disease burden in Asian countries. In particular it has been hypothesized that the high intake of soy in these countries may be responsible for the lower risk of prostate cancer. Soy is a unique dietary source of isoflavones, such as genistein and daidzein. The plant utilizes these compounds to naturally ward of fungal infections, while in humans they act like the hormone estrogen and have been found to inhibit tumor cell growth.
Genistein, in particular, has been targeted as a potent anticancer compound. In cell culture studies, researchers have shown that genistein inhibits prostate cancer cell growth and also induces cellular death. Additionally, studies have demonstrated that genistein enhances the ability of radiation to kill prostate cancer cells. Similarly, rodents feed a diet high in soy were less likely to develop tumors and those that did develop prostate cancer, tended to have less aggressive disease. Specifically, these findings were attributed to a substantial increase in apoptosis, cell death, and decrease in tumor vessel development.
Large population studies have also illustrated a strong relationship between soy consumption and reduced prostate cancer mortality. More targeted controlled studies, however, have been inconsistent regarding the effects of soy on prostate cancer. Notably, these studies are limited by small sample sizes and short durations. Regardless, the American Cancer Society recommends eating soy to reduce ones risk and the National Cancer Institute has been actively investigating isoflavones since the early 1990’s.
A high soy diet has many potential benefits. In addition to its potential chemopreventative effects on prostate cancer it can also help reduce cholesterol and heart disease. Replacing 40 grams of meat with protein can reduce LDL or “bad cholesterol” by up to 10 percent. At the same time, substituting fatty protein sources with soy can help promote weight loss and curb obesity, another risk factor for prostate cancer. For instance, exchange a beef burger for one composed of soy adds 12 grams of soy protein to your diet while reducing the calorie content by more than 50 percent.
While there are many benefits associated with soy consumption, recent research has also demonstrated some potential risks. Since isoflavones can act like the hormone estrogen, they can disrupt ones natural hormone production. In women, this can result in abnormal ovulatory cycles interfering with one’s fertility. Additional, some advocated for decreased soy consumption in children as the high levels of estrogen-like compounds can upset normal sexual development.
As a physician, I recommend moderate soy intake to maximize health benefits. Additionally, choosing natural sources of soy such as soy beans, soy milk, or tofu are preferable to supplements or processed foods. Older individuals, especially those with high cholesterol, may experience the most benefits including improved bone and cardiovascular health, and perhaps a decreased risk of tumor development. On the other hand, women who are pregnant or nursing should be cautious regarding soy consumption as it may not be the best option for their babies. Lastly, it is important to clarify that while soy may prevent prostate cancer it is not a treatment. Even if you consume a diet rich in soy products it important to get screened for prostate cancer and if diagnosed it is critical to partner with an urologist to develop a comprehensive treatment plan that best fits your goals.
