A study published in the Journal of Nutrition suggests that colon cancer may be a hormone-responsive cancer, and that soy protein can not only help prevent its occurrence but can have a very positive effect on the number and size of tumors that do occur. In this study, female laboratory animals whose ovaries had been removed to limit their sources of estrogen were exposed to an agent that causes colon tumors, then fed five different diets designed to compare the effects of specific ingredients, and followed for a year.
Diet One contained milk protein, and Diet Two, soy protein with no isoflavones; both these diets were free of any kind of estrogen. The remaining three diets contain soy protein plus an estrogenic component. Diet Three contained soy protein and the isoflavone, genistein, a phytoestrogen found in soy. Diet Four contained soy plus a mixture of soy-derived isoflavones including genistein, and Diet Five contain estrone, a naturally occurring human estrogen.
While the diet containing estrone was the most effective in preventing colon cancer, all the soy/estrogen diets were also protective and even those animals given soy protein with no estrogen-like factors that did develop colon cancer had fewer and smaller tumors compared to mice given milk protein.
Lead researcher, Ruth MacDonald, professor of food science at the University of Missouri, is now trying to determine how soy compounds protect against colon cancer. Until this work is done, however, she notes that not only is soy protein thought to also be helpful in the prevention of heart disease, but ¡°the good news is that there are many ways to add soy to your diet now, and we know of no harmful side-effects to eating soy protein.¡±

In addition to its fiber, soy's isoflavones also contribute to its protective effect against breast and prostate cancer. Research involving nearly 22,000 Japanese women¡ªthe Japan Public Health Center-Based Prospective Study on Cancer and Cardiovascular Diseases¡ªshows that a diet high in isoflavone-rich soy foods, particularly miso, is linked to a significantly lower risk of breast cancer. The women, who ranged in age from 40 to 59 years, filled out a dietary questionnaire that included questions about soy consumption and were followed for 10 years. Whether pre- or postmenopausal, women who reported eating three or more cups of miso soup per day had a 40% lower risk of developing breast cancer compared to women who reported consuming less than one cup per day. Women with the highest intakes of isoflavones¡ªcompounds in soyfoods that can bind to estrogen receptors in the body and block out human estrogen, thus lessening its effects¡ªhad a 54% lower risk of developing breast cancer compared to those whose intake of isoflavones was lowest.
A variety of soyfoods commonly eaten in the Asian diet contain isoflavones including tofu, miso, soymilk, soy sauce, soy flour, green or dried soybeans, soybean sprouts and a fermented soy food called natto. Almost 75% of the women reported eating miso soup daily, and of these 34% ate three or more cups of miso soup per day. More than 45% reported eating soy foods other than miso daily. The average intake of isoflavones among participants in this study was calculated to be about 700 times higher than that of Caucasians in the United States.

In addition to genistein, another isoflavone found in soybeans called daidzen has also demonstrated protective action against prostate cancer. A study published in Cancer Epidemiological Biomarkers that evaluated 398 Chinese men (men with prostate cancer as well as residential community matched controls) found that those consuming the most tofu had a 42% lower risk of developing prostate cancer compared to those consuming the least. When researchers checked the relationship between the soy isoflavones, genistein and daidzen, which are found not only in tofu, but in soy beans and other foods made from them, those consuming the most genistein were found to have a 47% lower risk for prostate cancer, while those consuming the most daidzen had a 44% lower risk.

Soy foods may also reduce risk of endometrial cancer. Research conducted in Shanghai and published in the British Medical Journal suggests that eating soy foods may be one reason Asian women have the lowest incidence in the world of endometrial cancer (cancer affecting the lining of the uterus).
The study included over 800 women with endometrial cancer, aged 30-69 years, and a disease-free, matched control group of over 800 women. For five years, all subjects answered food frequency questionnaires. When the data was evaluated, a significant inverse association was found between frequency of eating soy foods and endometrial cancer risk. Compared to women who ate the least soy foods, endometrial cancer risk was reduced from 7% to 15% to 33% as soy food intake increased.
Two reasons soy may help prevent endometrial cancer:
Soybeans contain the isoflavones genistein and daidzein, which, although about 1,000 times less potent than human estrogen, can bind to estrogen receptors in the body, blocking out human estrogens and providing a much more gentle estrogenic effect.
Plus, minimally processed soy foods are rich in dietary fiber, which has also been shown to lower estrogen levels.