By Marji McCullough, ScD, RD
EDITOR'S NOTE: Dr. McCullough added the following statement 4/8/14 in response to questions related to sources of isoflavones:
Research on soy and cancer is highly complex, controversial, and evolving.
When concerns about soy are raised, they generally focus on findings from rodent models of cancer which tend to use isolated soy compounds like soy protein isolate or high doses of isoflavones (compounds found in soy). However, soy is metabolized differently in humans than it is in mice and rats, so findings in rodents may not apply to people. (See: http://www.ncbi.nlm.nih.gov/pubmed/16614407 for more on this.)(Setchell, AJCN, 2011). There is no evidence in the medical literature that soy protein isolate is bad for humans, compared to other forms of soy. Soy protein isolate is often used as a supplement in randomized studies of the effects of soy on health and none of these studies have shown harm.
Most of the studies suggesting benefits of soy consumption in people have measured how much soy foods people are eating, including tofu, soybeans, and soy milk. These foods are more commonly eaten in Asian countries. In the U.S., purified forms of soy are used in the food supply, including in energy bars and soy hot dogs. The few US studies that have measured these forms of soy do not suggest harm.
More research is needed to understand the relationship between specific forms of soy and doses of isoflavones on cancer risk and recurrence. We also need to learn more about childhood exposure to isoflavones and risk of cancer. Until more is known, if you enjoy eating soy foods, the evidence indicates that this is safe, and may be beneficial (but note that miso, a fermented soy product, is high in sodium.) It is prudent to avoid high doses of isolated soy compounds found specifically in supplements, as less is known about their health effects. As for other "hidden" sources of soy proteins, the evidence to date does not suggest harm or benefit. However, if you are concerned about these products, you can choose to avoid them.
Before writing a blog about soy and breast cancer, I took an informal poll of a few friends to get a sense of what women believe about soy. I asked them, "What do you know about eating soy food? Is it good for you? Not good for you?" (I didn't even mention breast cancer.) The responses I got were, "I think it acts like estrogen in the body"; "Consuming any soy products increases the risk of breast cancer"; "I don't eat it a lot because I heard something negative but I can't remember what it was;" and "I've heard you should only have it in moderation." Apparently, people are hearing that soy may not be good. But what's the truth? In this blog I'll walk you through what we know and what we don't know about soy and breast cancer, and give you some practical tips on eating soy. More...