My friend has a history of breast cancer. She was telling me that she needed to avoid products with soy protein isolate. Can you explain why?

Your question is excellent. Let’s start by explaining what we are talking about. Soy protein isolate is the protein removed from defatted soy flakes. It does not have a flavor and is added to processed foods such as shakes, energy bars, and cereals to boost the protein content. It’s available as a plain powder sold in canisters in health foods stores and natural supermarkets.

Soy has chemical structures that look a bit like estrogen. That is where the word “phytoestrogen” comes from. In some types of breast cancer, the tumors are “fed” by estrogen. Since soy kind of looks like estrogen, there was concern that it would act like estrogen in the body and further the growth of breast cancer. But, the phytoestrogens found in soy are not the same thing as estrogen. There have been several large studies looking at soy intake of thousands of women for many years. The research on soy and breast health has looked at soy foods, not supplements. These observational studies show that women who regularly eat soy have lower breast cancer risk than women who do not eat soy. Some of these studies also suggest that breast cancer survivors who consume soy foods have a lower risk of breast cancer recurrence compared with women who do not eat soy foods.

To review, studies have looked soy intake in the form of whole foods like tofu, tempeh, soymilk, and edamame. These foods are natural, healthy forms of soy. Soy protein isolate is the concentrated soy protein that is added to processed soy bars and other products. We have documented research on the safety of whole soy foods but we lack evidence either way about the impact of consuming a high number of products enriched with soy protein isolate. Soy foods are a healthy option, while soy pills and isoflavone-enriched powders, and a high consumption of foods with soy protein isolate may not be. If you’re a woman concerned about breast health, choose healthy, whole soy foods, and limit consuming too many processed forms of soy. If you are a breast cancer survivor and would like to consume whole soy foods, I recommend consuming a moderate amount each day (1 to 2 servings).

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