Soy and breast cancer

Dr. Aedin Cassidy and colleagues ¹⁸ at the Dunn Clinical Nutrition Centre, Cambridge, UK, showed that when a group of pre-menopausal women consumed 45mg of soy isoflavones from textured vegetable protein each day, menstrual cycle length significantly increased by one to five days. (This level of isoflavones is similar to the amount that would be obtained from two cups of soy drink per day)

As a result, the menstrual cycles of these women became closer in length to those of many Asian women. The longer menstrual cycle in Japanese women (typically 32 days) compared to Western women (28 days), is thought to contribute to reducing risk and lower rates of breast cancer experienced by Japanese women ¹ . Indeed, it was concluded from this study that the changes in menstrual cycle length observed with soy consumption are likely to be beneficial in relation to breast cancer protection ¹⁸ as, over a lifetime, it means the female body is less exposed to oestrogen.

Studies in rats (which are a well-accepted model for human breast cancer) have shown that administering genistein early in life reduces the susceptibility to developing mammary cancer after exposure to a carcinogen later in life ¹⁹ , ²⁰ . Based on these findings, it has been suggested that the protection Asian women have against breast cancer may be related to their traditional consumption early in life, of soy foods containing isoflavones. Interestingly, it has also been found that when Asian women migrate to western countries (and adopt a western diet and lifestyle), their protection against breast cancer diminishes within two to three generations ²¹ , ²² .

It is believed that a high level of the body's own oestrogen increases the risk of developing breast cancer. Some scientists have questioned whether isoflavones could stimulate certain types of breast cancer cells to grow in women, ²³ , ²⁴ since isoflavones have a similar structure to oestrogen. These questions relate specifically to the type of breast cancer known as 'oestrogen receptor positive breast cancer', accounting for 80% of all cancers, where the cancer is stimulated to grow in the presence of oestrogen.

Laboratory tests have shown that under certain artificial experimental conditions, isoflavones can stimulate breast cancer cells to grow in the test tube ²³ , ²⁴ . However, these conditions are not reflective of what happens in the human body and therefore do not apply to women. There is no evidence when isoflavones are consumed through the diet, that they stimulate the growth of breast cancer in women. On the contrary, a higher intake of isoflavones and greater blood and urine concentrations of isoflavones have been linked to a reduction in breast cancer risk ¹ .