Soy and breast cancer risk

Q: "I have a history of breast cancer (DCIS).  I am trying to eat a healthy diet, and am unsure if I can safely drink soy milk or eat soy products, as I heard this might increase my risk of cancer recurrence.  Is soy safe to consume with a history of breast cancer?"

A: You are not alone in feeling perplexed about this issue! So let me break it down.  Soy contains isoflavones, or plant chemicals, that weakly resemble estrogens found in humans.  Since certain cancers are estrogen-sensitive (like breast cancer and cancer of the uterus), patients like yourself are often concerned about eating soy-based products. Here's the important thing- isoflavones have both pro-estrogen and anti-estrogen effects.  This means that unlike human estrogens, isoflavones can block hormone-sensitive receptors in breast tissue and potentially decrease breast cancer risk.  In addition, isoflavones provide benefits like anti-oxidants and fiber and are low in saturated fats.  A recent large study of over 9,000 invasive breast cancer survivors found that daily consumption of at least 10mg of soy was associated with a 25% decrease in breast cancer recurrence, most strikingly among those with hormone-negative tumors and those with hormone-positive tumors taking  a drug called Tamoxifen (Nechuta et al., Am Jour. Clinical Nutrition. July 2012; 96). Additional support for soy comes from the 2012 American Cancer Society's Nutrition and Physical Activity Guidelines for Cancer Survivors, which  states that "Current evidence does not suggest that consuming soy foods is likely to have adverse effects on risk of recurrence or survival".  However, soy should be consumed through food intake, rather than supplements, as currently no evidence supports their use. So in short, moderate consumption of soy can be safely consumed, with potential beneficial effects, in breast cancer survivors.