Soy And Prostate Cancer

For some time there has been some controversy over the use of soy to prevent prostate cancer. Soy has a long history and has been increasingly the topic of research related to prostate health.

This paper will discuss some of the evidence in favor of its use to treat prostate cancer and some of the evidence that indicates its misuse in the treatment of prostate cancer and even the potential for its relation to increasing the risk of prostate cancer.

To begin, we must first understand some of the history around soy and why it has been thought to be related to prostate cancer as a potential treatment or preventative dietary supplement. Soy has been consumed in Asia, starting in China, for hundreds of years. However, it was not until the grain was fermented that it became a part of the daily Chinese diet in the form of miso, tempeh, and natto(1). This is due to the multiple enzyme inhibitors found in unfermented soy beans, such as phytates and trypsin inhibitors.

Consumption of excess unfermented soy may lead to mineral deficiencies. Furthermore, soy may not be consumed in the amounts as is commonly thought. The Cornell study found that legume consumption in China varied from 0 to 58 grams per day with an average of about 12.41 grams(2).

Even if two-thirds of the 58 grams/day was consumed daily that would only be 40 grams and less than 3 tablespoons and an average consumption of 9 grams, which is less than two teaspoons. One study found that legume intake is more associated with reduced prostate cancer risk than isoflavone intake(7).

Men in Asia are much less likely to get prostate cancer than men in the Western world (3), but they are also more likely to have other types of cancer such as stomach and esophageal. Soy may be one of the many factors; however, it should be noted that soy variety currently consumed was only first reported 60 years ago, which is Glycine max(4). The ancestor of Glycine max is Glycine soya.

Glycine max is noted for its high isoflavone content and little is known of content of its ancestor. Glycine max has been a very successful cultivar, partly due to its high isoflavone content and other naturally occurring toxins which act as a natural pesticides. Furthermore, the levels of soy isoflavones varies widely among different crops of Glycine max.

A recent study by the UK government found no link between isoflavones in foods and their positive health benefits(4). Another study found that localized prostate cancer is assocaited with a reduced risk with a diet containing isoflavones, specifically, genistein, daidzein, miso soup, and soy food. More research is needed to differentiate the effects of different forms of soy and isolated isoflavones.

There seems to be no question that soy affects the prostate. Orally administered isoflavones can be found as glucaronides in prostate tissue after administration (6). Another study found that orally administered isoflavones can decrease PSA levels in the blood and concluded that more research is needed but there is evidence that soy supplements have biological activity in the prostate. The FDA is currently not allowing companies to make claims that soy can prevent cancer but many companies are implying exactly that in their advertising(1).

In my opinion, research is focusing on specific isoflavones isolated from soy and in larger studies is not determining the isoflavone content in participant diets so that there is a disconnect between the epidemiologic evidence suggesting that soy in the diet may prevent prostate cancer and the actual potential treatments that are being studied. The isoflavone content in mature whole soy beans may be 5.1 ug/g protein and 2.0 ug/g protein in soy milk, but as already discussed this varies depending on the crop(8). Thus, any future FDA recommendation in terms of prostate cancer prevention related to isoflavone content should take this into account.

Another problem using large population studies studying prostate cancer risk and diet is that certain food combinations may affect participant outcomes. One study found that combining soy protein and tomato products to increase blood levels of lycopene was effective at decreasing PSA levels in patients undergoing conventional prostate cancer treatment(9). Dietary soy and tea combinations showed potentially synergistic effects on reducing leptin and testosterone concentrations in male rats(21), which may be due to their efficacy in alleviating metabolic disorders.

Soy protein isolates were found to reduce proliferating cell nuclear antigen (PCNA), epidermal growth factor receptor, B-cell non-Hodgkin lymphoma-2 (Bcl-2), and Bcl-2-associated X protein (Bax), but not PSA in another study(10). In older Scottish men, one study found that isoflavones had no association between prostate cancer risk dietary intkae but dietary lignans were protective(11). Thus, the evidence is mixed as shown by the variance of outcomes in multiple studies. Further research on the mechanism of action is warranted.

Prostate cancer is thought to be related to an increase in DHT possibly due to overconversion of testosterone to DHT by 5 alpha-reductase, which is inhibited by soy isoflavones. This has been shown in one study using rats where DHT was reduced by dietary consumption of soy flour and purified soy isoflavones(12). However, environmental oestrogens have been associated with the pathogenesis of prostate cancer in humans(13).

The greatest hazards assocaited with oestrogens and prostate hyperplasia may start during development where the prostate is most sensitive to hormones. This may cause the prostate to be hypersenstive to estrogen later in life(14). Effects in rodents phytoestrogens had similar results to DES exposure in higher doses(17). When genistein was given to mice via maternal milk or fortified feed there was a dose dependent increase in prostate and other tissues(18).

Doses of genistein given to puppies that were high enough to negatively affect sexual development were low enough to be achieved dietarily in humans(20). Another study found that isoflavones may reduce SHBG but increase DHT levels(15). Again, epidemiologic evidence suggests that diets rich in phytoestrogens may reduce prostate cancer risk but there is no direct evidence that phytoestrogens are beneficial(16). In another study, men consuming tofu instead of meat showed lower testosterone to estrogen ratios and changes in other hormone levels(19).

In conclusion, the negative effects of soy consumption as isolates does not warrant its use as a treatment for prostate cancer with current research inconclusive of its efficacy. Epidmiological evidence suggests that soy is associated with decreased risk for prostate and breast cancer, but by this same logic we would associate it with an increased risk for cancer of the esophagus,stomach, pancreas, liver, and thyroid(1).

The potential hazards of soy causing endocrine disorders (16) warrant more a more intensive search for alternatives to soy in the treatment of prostate cancer. One cannot ignore the commercial pressures that are driving the intense search for the use of soy in all sectors of the economy.

A holistic approach to prostate cancer treatment should not include soy, in my opinion, due to the many hazards discussed in this report as well as the inconsistent changes in prostate cancer markers from study to study, which has also been discussed.


Learn More About Naturopathic Philosophy

Learn More About Naturopathic Philosophy