I sometimes emphasize to my patients that when we talk about soy, we are talking about 'just a bean.'
First, the present study used very large amounts of soy protein isolate (51.6 g/d) that contained very high levels of genistein (98.44 mg/d) and diadzein (16.51 mg/d). The consumption of nearly 100 mg of genistein from dietary sources would be extreme. What are considered “high soy‒intake diets” in previous studies did not reach such levels. For example, in the Shanghai Breast Cancer Survival Study, the highest quintile of intake was less than 15.41 g soy protein and over 62.68 mg total isoflavones daily.1 The Women’s Healthy Eating and Lifestyle Study considered high consumption to be 16.33 mg to 86.9 mg per day of total isoflavones as the highest intake group.2 The current study may be relevant to isolates of isoflavones (ie, supplements and fortified foods) but dietary amounts, even in high soy consumers, are unlikely to reach the levels studied.
- Evidence suggests that isoflavones in excessive amounts (100 mg/d) are contraindicated and may carry some risk.
- Soy foods—in moderation—have been shown to be neutral or beneficial in all of the studies that tracked soy intake and breast cancer recurrence.
- Shu XO, Zheng Y, Cai H, et al. Soy food intake and breast cancer survival. JAMA. 2009;302(22):2437-2443.
- Caan BJ, Natarajan L, Parker B, et al. Soy food consumption and breast cancer prognosis. Cancer Epidemiol Biomarkers Prev. 2011;20(5):854-858.
- Kaczor T. The effects of soy consumption on breast cancer prognosis. Nat Med J. 2012;4(11). http://naturalmedicinejournal.com/journal/2013-01/effects-soy-consumption-breast-cancer-prognosis. Accessed February 28, 2015.