The most recent findings on soy and isoflavones were presented in Washington DC on the occasion of the 9th International Symposium on the Role of Soy in Health Promotion and Chronic Disease Prevention and Treatment (October 16-19, 2010). A major part of the topics was dedicated to safety and health benefits in menopausal women, and to the issue of soy and breast cancer.
Prof. Dr. Xiao Ou Shu (Vanderbilt University, Nashville, USA) reported on practical experience in breast cancer patients exposed to isoflavones (Shu 2010). Breast cancer is one of the most common cancer types in the US. The 5-year survival rate is approximately 89 %, and the rate of recurrence after 3 years is 10-15 % (Nasr et al. 2009). Despite such successes breast cancer therapy is far from being optimal, as it goes along with considerable adverse effects. I.e., 63-69 % of postmenopausal breast cancer patients experience ovarian failure within the first year of treatment.
Isoflavones act at the estrogen receptor beta (ER-β). Different potential mechanisms make isoflavones an interesting subject in cancer therapy: isoflavones inhibit neoangiogenesis and DNA topoisomerases, and they are bioavailable in breast tissue after soy food intake (Bolca et al. 2010). In fact, as with estrogens the blood levels of isoflavones and their tissue concentration correlate rather poorly.
As isoflavones are considered as “phyto-estrogens” the most important question is whether isoflavones might have a direct impact on estrogen levels and production, and whether they may interact with the efficacy of tamoxifen. Both aspects would call for caution in breast cancer patients. These considerations have already been addressed in clinical settings, but to date no negative impact of isoflavones on the levels of sexual hormones or on the efficacy of tamoxifen was found (Guha et al. 2009).
In her studies Shu observed the association of soy food intake after diagnosis of breast cancer with total mortality and cancer recurrence as well as with menopausal symptoms. Her findings were based on the Shanghai Breast Cancer Survival Study, a large population-cohort study of 5.042 breast cancer survivors (Boyapati et al. 2005). The study involved the assessment of the effects of soy exposure on recurrence, mortality, quality of life, bone loss, depression, cognitive decline, cardiovascular disease and histopathology of breast tissue. Shu found that soy food intake, measured by soy protein and/or soy isoflavone intake, was inversely correlated with mortality and cancer recurrence (Shu et al. 2009). The hazard ratio associated with the highest quartile of soy protein intake was 0.67 (95 % CI 0.52-0.84) for recurrence compared with the lowest quartile. The multivariate adjusted mortality rates were 13.1 and 9.2 %, and the 5-year-recurrence rates were 13.0 and 8.9 %, respectively, for women in the lowest and highest quartiles of soy protein intake. Higher isoflavone exposure of breast cancer patients clearly improves the prognosis.
The improved prognosis was evident among women with ER(-) and with ER(+) breast cancer, and was also found in users and non-users of tamoxifen. Soy protein and isoflavones both correlated with the time of disease-free survival. Best effects were noted with approximately 11 g of soy protein daily, or with 30-70 mg of isoflavones. There was no difference with respect to pre- or postmenopausal women.
The importance of this cohort study lies in the fact that most of the participants were treated with chemotherapeutic agents such as tamoxifen, which sheds a positive light on the safety of isoflavones. Instead of the discussed negative impact a statistically significant improvement of mortality and recurrence rate was found when patients had long-term exposure to isoflavones.
References
Shu XO (2010). Soy food intake among breast cancer patients: Association with survival and menopausal symptoms. 9th International Symposium on the Role of Soy in Health Promotion and Chronic Disease Prevention and Treatment, Washington DC, 16-19 October.
Cancer 