Even among people who have never had cancer, some large population studies suggest a human benefit from high levels of phytoestrogens in the diet, while animal studies have noted that extremely high amounts of phytoestrogens may actually promote breast cancer and uterine cancer.
Dr. Otis Brawley, chief medical officer for the American Cancer Society says for a person at average risk of breast cancer, it's not whether soy is beneficial, it’s the amount and type of phytoestrogens you are getting in your diet.
“There is a difference between high levels of natural phytoestrogens in the diet and the very high doses in dietary supplements. It is very reasonable for a person at average risk of breast cancer to eat a moderate amount of soy products, such as tofu, soy butter, soy nuts, and soy burgers, as part of an overall well-rounded healthy diet,” he said.
In the current issue of Cancer Prevention Research, a journal of the American Association for Cancer Research, Seema A. Khan, M.D., professor of surgery at the Robert H. Lurie Comprehensive Cancer Center of Northwestern University found soy supplements had no protective effect against breast cancer. In premenopausal women, the supplements actually worsen cancer cell growth.
“Simply put, supplements are not food. Although soy-based foods appear to have a protective effect, we are not seeing the same effect with supplementation using isolated components of soy, so the continued testing of soy supplements is likely not worthwhile,” said Khan in a written statement.
A 2009 study conducted by Vanderbilt University and the Shanghai Institute of Preventive Medicine, and published in the Journal of the American Medical Association, found breast cancer survivors, ages 20-75, who ate moderate amounts (15.3 grams) of naturally occurring soy protein daily -- about the amount found in three-quarters of a cup of edamame -- had lower cancer recurrence and mortality than their study counterparts whose soy intake was less than 5.3 grams daily.