Research has raised some questions about soy and breast cancer. Studies of Chinese and Japanese women indicate that they have one fifth the rate of breast cancers of U.S. women and that they consume from 10 to 50 grams per day of soy protein in contrast to 1 to 3 grams per day consumed by Americans. This translates to 20 to 80 milligrams of isoflavones consumed per day by Asian women compared to U.S. women whose intake is less than 5 milligrams per day. Epidemiology gives only indications of what may be. Research must answer the question of whether or not there are cause and effect relationships.
Soy products contain five known classes of anti-cancer agents, including phytoestrogens.
Natural soyfood sources of phytoestrogens are recommended as part of a therapeutic and prophylactic diet instead of isoflavone supplements, because the foods contain all the potential anti-cancer agents ingested by apparently protected populations of women with lower rates of breast cancer. Prepared supplements and pills may contain much more than the dietary intake of populations with a lower incidence of breast cancers. Isoflavones are currently the most intensively researched soy phytochemical with respect to breast cancer, although there is a growing body of literature supporting protease inhibitors as anticarcinogens. Even though phytoestrogens have been shown to inhibit human breast cancer cell proliferation in vitro and breast cancer development in animal models, little data exists regarding the effects on the normal human breast. Research results show both positive and negative effects from treatment of cancer cells with phytoestrogens.