Asian populations have a lower incidence of hormone-related diseases, such as breast cancer, uterine cancer, and prostate cancer, than Westerners do. It has been suggested that soy consumption is one reason for this difference in disease incidence. Women who were born in Asia but migrated to the United States likewise have a lower risk of breast cancer, possibly due to their early exposure to soy. But obviously soy is only one of many factors that influence cancer risk, and now we know that it is many contributing factors that make a diet cancer-protective.
It is now clear that soy intake during adolescence, a time when breast tissue is most sensitive to environmental stimuli and carcinogenesis, may reduce the risk of breast cancer later in life. Recent articles in Cancer Epidemiology and The American Journal of Clinical Nutrition reported that soy consumption during childhood and teenage years reduced the risk of breast cancer in adulthood by 60 percent and 40 percent, respectively.
Soybeans are rich in isoflavones, a type of phytoestrogen. Phytoestrogens are plant substances that are chemically similar to estrogen and since higher estrogen levels promote breast cancer, some people predicted that soy would too. Now we know that the phytoestrogens in soy actually block the effects of the body's estrogen. Despite myths propagated on the Internet, the most recent and reliable clinical studies support a strong protective effect of minimally processed soy foods against breast cancer.
In 2006, a meta-analysis in the Journal of the National Cancer Institute examining data from eighteen studies on soy and breast cancer that were published between 1978 and 2004 concluded that soy overall has a protective effect. Again in 2008, another meta-analysis in the British Journal of Nutrition compiling data from eight studies (which were not included in the 2006 meta-analysis) also concluded that soy consumption decreases breast cancer risk. These effects were dose-dependent - a 16 percent reduced risk for each 10 milligrams of soy isoflavones consumed daily.
Soy has protective effects even after a diagnosis of breast cancer. A new study of breast cancer survivors has shown that premenopausal breast cancer survivors who consumed more soy had a 23 percent reduced risk of recurrence.
Soy provides protection against other hormonal cancers as well. A meta-analysis of studies on soy consumption and prostate cancer found a 31 percent decrease in prostate cancer risk with a high consumption of soy foods.
Soy has also been shown to be protective against endometrial and ovarian cancers. Soy products such as tofu and soy milk can be useful in moving toward a plant-centered diet with less saturated fat, less animal protein, more plant protein, and more fruits and vegetables. In the United States, the majority of our soy intake, which is very low compared to that of Asian countries, is consumed via soy-based additives or isolated soy protein in processed foods.
Please note that the most healthful soy foods are those that are minimally processed - these include edamame, tofu, unsweetened soy milk, and tempeh. You should be aware that soy nuts and other processed soy products do not retain much of the beneficial compounds and omega-3 fats that are in the natural bean. The more the food is processed, the more these beneficial compounds are destroyed. Minimally processed soy foods are a beneficial addition to a healthy diet. I do not recommend consuming large quantities of soy products in the hopes of reducing cancer risk, however. A healthy diet should include a variety of beans, all of which have beneficial anticancer compounds, and not a disproportionate share of calories from soy. I always recommend the consumption of a broad variety of phytochemical-rich foods to maximize one's health. Beans are no exception - try to include various different types of beans, including soybeans.
Processed foods, because of their low nutrient levels, high amounts of salt, acrylamides, and other toxic additives, should not be considered healthy. Vegetarians and vegans who eat tofu-turkey, soy burgers, soy ice cream, soy hotdogs, soy cheese and other soy-derived processed foods on a regular basis are certainly not eating a healthy diet. Isolated soy protein is a heavily processed food with the natural micronutrients lost in processing. The key to good health is to eat unprocessed foods, because their nutrient-per-calorie density is high.
(Joel Fuhrman, M.D., ‘Super Immunity’)