Soy and breast cancer

Dr. Aedin Cassidy and colleagues ²¹ 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 more similar 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 their reduced risk and lower rates of breast cancer1. Indeed, it was concluded from this study that the changes in menstrual cycle length observed with regular soy consumption are likely to be beneficial in relation to breast cancer protection ²¹ as, over a lifetime, it means the female body is less exposed to oestrogen.

Based on the findings from population studies, it seems that a higher level of daily soy food intake in Asian countries may have protective effects against breast cancer. A meta-analysis of 8 studies conducted in high soy consuming Asians showed a significant trend of decreasing breast cancer risk with increasing soy food intake. There was a 29 per cent reduction in risk among those women with the highest intakes (defined as supplying 20 mg or more of isoflavones per day) compared to the lowest level of soy food intakes (5 mg or less of isoflavones per day). In contrast, soy intake was unrelated to breast cancer risk in studies conducted with the 11 low soy consuming western populations whose average highest and lowest isoflavone intakes were, 0.8 mg and 0.15 mg per day, respectively ²² .

However, the timing of introduction to soy may also have some relevance. Earlier studies in rats showed that administering genistein (a soy isoflavone) in early life reduces the susceptibility to developing mammary cancer after exposure to a carcinogen later in life ²³ ^, ²⁴ . Based on these findings, it was suggested that the protection Asian women have against breast cancer may be related to their traditional consumption of soy foods containing isoflavones in early life. Three population studies lend their support to this hypothesis ²⁵ ^, ²⁶ ^, ²⁷ . Women who consumed isoflavones during adolescence were found to be at lower risk of breast cancer in later life than those who did not. In the Los Angeles Asian Breast Cancer Study ²⁶ , soy intake during adolescence exhibited a stronger protective effect on risk than if soy foods were first introduced in adulthood. Interestingly, when Asian women migrate to western countries (and adopt a western diet and lifestyle, where soy intake is significantly reduced), their protection against breast cancer diminishes within two to three generations ²⁸ ^, ²⁹ .

As it is generally 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, ³⁰ ^, ³¹ 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 experimental conditions, isoflavones may stimulate breast cancer cells to grow in the test tube ³⁰ ^, ³¹ In certain rodent models, such as the athymic nude mice, which lacks an immune system and has had its' ovaries removed, isoflavones may also promote the growth of existing or transplanted cancer cells. However, even in rodent models, isoflavones are generally considered 'weak oestrogens' relative to the effects of real oestrogen. These studies have also used high doses (at least 5 times the amount found in traditional Asian diets) and many have directly injected purified isoflavones into the bloodstream, which results in substantially higher levels in the bloodstream than what is possible through dietary intake. These conditions are not reflective of what happens when humans eat soy foods and therefore often do not predict results in humans. There is no evidence to suggest that isoflavones will increase breast cancer risk in healthy women and little clinical evidence they will worsen the prognosis of breast cancer patients. On the contrary, a higher intake of isoflavones and greater blood and urine concentrations of isoflavones in women have been linked to a reduction in breast cancer risk ¹ .

Prof. David Jenkins and his research team at the University of Toronto examined the effect of soy protein containing diets in post-menopausal women on the urinary levels of specific sex hormone metabolites, which are an indicator of oestrogen (hormonal) activity. They found no effect on the levels of these urinary compounds, indicating that the risk of breast cancer was not increased by the consumption of soy foods during this study ³² .