Is it true that eating soybean products can prevent cancer?
Various plants contain a number of different phytoestrogens, substances that behave like a weak form of the human female sex hormone, estrogen. One class of phytoestrogens, called isoflavones, is present in especially large amounts in soybeans, which as a result appear to have greater estrogen-like activity than other plant foods.
Some scientists have proposed that eating soy foods can protect against both breast cancer in women and prostate cancer in men.
They hypothesize that during the premenopausal years the phytoestrogens can prevent the cancer-promoting effects of estrogens on breast tissue cells. They also propose that phytoestrogens may stave off prostate cancer by reducing the production of testosterone, the male hormone known to be required for the development of prostate cancer, or interfering with the effects of testosterone on prostate tissue cells.
These notions are supported by some animal and test-tube studies. The strongest argument for the benefits of phytoestrogens, however, is based on the observation that the rates of breast cancer and deaths from prostate cancer are much higher in the United States than in Japan, China and Korea -- countries where diets are rich in soy foods. Based on reports that menopausal symptoms are relatively uncommon among Japanese women, some scientists also suggest that an ample intake of phytoestrogens may lessen the severity of menopausal symptoms, and even reduce the likelihood of postmenopausal osteoporosis.
It is important to recognize that these supposed benefits of soybeans and other phytoestrogens, while extremely interesting, remain highly speculative. Other than the amount of soy beans in their diets, many differences in the lifestyles of Japanese, Chinese and Korean men and women may explain their relatively low incidence of breast cancer, menopausal symptoms, and death from prostate cancer. Much more research is needed before any cancer-promoting capacity of phytoestrogens is put on a firm footing.
Even if soybeans are proven to prevent cancer, it may be difficult to take full advantage of their benefits because the phytoestrogen content of the major soybean products (tofu, tempeh and miso) probably depends on the variety of soy bean and the methods used in processing them.
Dr. Margolis is professor of medicine and biological chemistry at the Johns Hopkins School of Medicine.