It surprises me how many people are still afraid to eat soy, including some nutritionists. No doubt, reading random online information about soy can be scary and confusing. Who would eat a food that purportedly causes breast cancer, thyroid disorders and even brain damage? But this information is hype and largely unfounded. I’ve followed research on soy foods and breast cancer for the past 15 years, and the evidence is convincing that soy, especially whole soy foods, is safe.
The questionable ingredient in soy is its naturally occurring isoflavones. Soy isoflavones are plant or “phyto” estrogens that have a similar chemical structure to the human hormone estrogen. However, the phytoestrogens in soy are much weaker and may not act exactly like the human form. Yet the concern remains—are they similar enough to advance certain hormone-driven diseases like breast and prostate cancers, or might they instead offer a kind of health protection by blocking the human hormones from their usual action?
Adding to the confusion are the different types of soy foods available. Whole soy food staples native to the Asian diet for thousands of years include tofu, tempeh, miso and edamame, the popular buttery tasting beans served in Japanese restaurants. But what about the next generation of processed soy foods like vegan protein bars and powders or veggie burgers and other meat alternatives, many of which are made with soy protein isolates?
What is soy’s effect on breast cancer?
Whether soy intake affects cancer risk is not entirely clear, though recent research leans towards either a protective effect or no effect on breast cancer. Animal studies show mixed results, with isoflavones both stimulating and not stimulating the growth of tumors.
In human studies, especially when observing large groups of people, the results are more consistent. A review of the three largest observational studies of breast cancer in women, including the Shanghai Breast Cancer Survival Study, found that 9514 breast cancer patients followed for a mean of 7 years had a reduced risk of death, breast cancer-specific death, and breast cancer recurrence when comparing the groups with the highest and lowest isoflavone intakes. The review also suggested a greater protective effect of soy intake from cancer recurrence in women taking tamoxifen, a cancer drug that blocks the action of estrogen.
Another study found that soy isoflavones may prevent angiogenesis in breast cancer cells. Angiogenesis is the growth of new blood vessels that is the lifeline of a tumor.
American diet and microbiota
The soy foods in an American diet are likely different than in an Asian diet. The large observational studies on soy and breast cancer usually include Asian women who have the highest soy intakes in the world. They eat one to three servings of whole soy foods a day. Many of them begin eating soy as children. Asian women tend to carry less body fat than American women.
Do soy foods offer similar benefits for Americans who eat a diet higher in refined carbohydrates, saturated fat and red meat? Do processed soy products that contain soy protein isolates but not the naturally occurring phytochemicals provide any cancer protection? We aren’t sure of these answers yet but growing research points to the role of microbiota, microorganisms such as bacteria, that live in our digestive tract. The unique type and amount of microbiota that you and I have depends directly on what we eat.
These specific microbiota may affect how easily soy isoflavones are broken down into their active form called equol. Asian women who have eaten soy their entire lives may house different bacteria than American women. Estimates suggest that only 25%-35% of U.S. Caucasians are able to convert isoflavones to equol, compared with 40%-60% of Asians who eat soy daily. So simply adding a few servings of tofu a week into the diet may not reap similar health benefits in an American woman as her counterpart in China.
Soy supplements may also not be worthwhile. A randomized controlled study found that women with early stage breast cancer who were given high dose soy supplements in the form of soy protein powder had an increased growth in breast cells as compared with women given milk protein powder. More cell growth is believed to increase the risk of tumor growth. Keep in mind though that they were given 52 grams of soy protein daily, the equivalent of about four veggie burgers, more than the average person would eat.
Other health benefits of soy
Regardless of cancer prevention, whole soy foods are a good source of protein and fiber and are low in carbohydrate, with a low glycemic index. They contain heart-healthy polyunsaturated fats including omega-3, along with B vitamins and trace minerals. For these reasons, many people who wish to reduce meat intake are turning to soy foods.
Soy foods, particularly in their whole form and when included as part of a plant-rich diet, appear nutritious and safe to eat in moderation for women with breast cancer. The American Institute for Cancer Research defines a moderate amount as 1-2 servings daily. One serving equals 1 cup soy milk, 1/2 cup edamame or 1/3 cup tofu. One may wish to limit the amount of processed soy foods or supplements containing isolated soy protein. Other preventive lifestyle factors important for women with breast cancer include maintaining a healthy weight and getting regular daily exercise.
- Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women. Nechuta et al., Am J Clin Nutr 2012;96:123–32.
- The Effects of Soy Consumption before Diagnosis on Breast Cancer Survival: the Multiethnic Cohort Study. Conroy et al., Nutr Cancer. 2013; 65(4): 10.1080/01635581.2013.776694.
- Soy and Breast Cancer: Focus on Angiogenesis. Varinska et al., Int J Mol Sci. 2015 May; 16(5): 11728–11749.
- Consumption of soybean, soy foods, soy isoflavones and breast cancer incidence: Differences between Chinese women and women in Western countries and possible mechanisms. Fen-Jin Hea, Jin-Qiang Chenb, Science Direct. Sep-Dec 2013, Vol.2(3): 146-161.
- The Effects of Soy Supplementation on Gene Expression in Breast Cancer: A Randomized Placebo-Controlled Study. Shike et al., J Natl Cancer Inst. 2014 Sep; 106(9): dju189.