Ask the Expert: Soy

The Physicians Committee

Ask the Expert: Soy

Q: Are the phytoestrogens in soy foods helpful or risky for cancer survivors? Do estrogens in soy increase breast cancer risk? What about the estrogens in soy for men and boys?

A: Soy products, such as miso soup, tofu, and edamame, contain very weak plant estrogens called phytoestrogens that hinder the body's natural estrogen from attaching to cells. (The prefix "phyto" simply means "plant.") Normally, estrogens hook onto tiny receptor proteins in your cells that allow them to change the cell's chemistry.

Think of it this way: An estrogen molecule is like a jumbo jet that attaches to the Jetway of an airport. It discharges passengers into the terminal, which is suddenly a busy, noisy place. Phytoestrogens, being weak estrogens, are like small, private planes with few passengers and no cargo, yet they still occupy the Jetway after landing. When phytoestrogens occupy the cell, normal estrogens cannot. Plant estrogens do not eliminate all of estrogen's effects, but they do minimize them, apparently reducing breast cancer risk and menstrual symptoms.

For men and boys, the phytoestrogens in soy do not appear to have any effect on hormone levels and have not been shown to affect sexual development or fertility. Research studies show that men consuming soy have less prostate cancer and better prostate cancer survival.

In Asia, where tofu, soymilk, and other soy products are commonly consumed, not only is the population healthier overall, but cancer and heart disease are much rarer than in the United States and Europe, and longevity is greater. As these populations differ in other ways—Asians eat much less meat and dairy products and generally exercise more, but also smoke more cigarettes and eat more salt—researchers have simply attempted to tease out the effects of soy itself. Also, it’s possible that the more processed soy products such as veggie burgers and veggie hot dogs are not as beneficial as the less processed soy products such as tofu and tempeh traditionally consumed in Asia. In general, the less processed your diet is, the more nutrient-dense it will be. Thus, replacing processed soy products such as veggie burgers and veggie hot dogs with tofu, tempeh, beans, and lentils may provide you with a more nutrient-dense diet.

Research findings once indicated soy may be harmful for women with a history of estrogen-receptor positive breast cancer. Many of these findings were based on animal models that are not applicable to human health research. Today, researchers agree that whole soy products are safe for women who have had estrogen-receptor positive breast cancer. Up to three servings per day of soy products such as soymilk, tofu, or tempeh are fine for these women. Soy products do not appear to have any effect on women who have had estrogen-receptor negative breast cancer. However, soy foods can interfere with the effectiveness of certain cancer medications, so speak to your oncologist or physician before adding more soy to your diet.

Like all foods, soy has its advantages and disadvantages. Soybeans are rich in essential omega-3 fatty acids, but tend to be higher in total fat than other beans. Many soy products derive roughly half their calories from fat, while black beans, pinto beans, or other varieties are only about 4 percent fat. Also, soy extracts, such as genistein, may not have the same beneficial effects as products made with the whole bean. In fact, it’s best for breast cancer survivors to avoid concentrated soy supplements and protein powders until further research is conducted.

However, it’s also to remember that a vegan diet of beans, vegetables, grains, and fruits does not have to include soy products to be nutritionally complete. Soy products make convenient and tasty substitutes for meat and other unhealthy foods that people, quite rightly, are looking to avoid.  However, the benefits of complete protein and soluble fiber can easily be found in an array of plant foods. 

In human research studies, soy products have been shown to lower serum cholesterol levels, in part due to their rich content of soluble fiber, and the isoflavones also play a role in bone formation. Soy products have been shown to reduce estrogen activity, at least in premenopausal women, which, over the long run, reduces cancer risk. The evidence is not as clear for postmenopausal women, but up to three servings of whole soy products are safe to consume for postmenopausal woman with previous history of breast cancer.

A handful of individuals and organizations have taken an anti-soy position and have questioned the safety of soy products. In general, this position latches to statistically insignificant findings, understates how powerfully the research refutes many of the main anti-soy points, and relies heavily on animal research studies, which are medically irrelevant to human health. 

Brzezinski A, Debi A. Phytoestrogens: the "natural" selective estrogen receptor modulators? Eur J Obstet Gynecol Reprod Biol. 1999; 85:47-51.

Duncan AM, Underhill KE, Xu X, Lavalleur J, Phipps WR, Kurzer MS. Modest hormonal effects of soy isoflavones in postmenopausal women. J Clin Endocrinol Metab. 1999; 84:3479-3484.

Ham JO, Chapman KM, Essex-Sorlie D, et al. Endocrinological response to soy protein and fiber in midly hypercholesterolemic men. Nutr Res. 1993; 13:873-884.

Kurzer MS. Hormonal effects of soy in premenopausal women and men. J Nutr. 2002;132:570S-573S.

Messina M, Barnes S. The role of soy products in reducing risk of cancer. J Natl Cancer Inst. 1991; 83:541-546.

Strom BL, Schinnar R, Ziegler EE, et al. Exposure to soy-based formula in infancy and endocrinological and reproductive outcomes in young adulthood. JAMA. 2001; 286:807-814.

Caan BJ, Natarajan L, Parker BA, et al. Soy Food Consumption and Breast Cancer Prognosis. Cancer Epidemiol Biomarkers Prev. 2011;20(5):854-858. Epub 2011 Feb 25.

Q: Is there an alternative to soy/tamari sauce if folks want to steer clear of soy based products?

A: When it comes down to it, there really isn't very much soy in soy sauce, especially since people typically do not use large quantities at a time. But, some substitutes are seasonings such as Spike, Mrs. Dash, or try other asian seasonings that do not have soy such as ume boshi plum juice or mirin.

Q: Does soy interfere with thyroid function?

A: Certain foods, perhaps most notably soy, have been linked to the development of hypothyroid problems, but this is still controversial.  Along with soyfoods, millet, and cruciferous vegetables such as cabbage, kale, kohlrabi, rutabaga, cauliflower, mustard greens, radishes, broccoli, Brussel sprouts, contain goitrogens (thyroid suppressants) and/or progoitrogens (thyroid stimulants) which, though they probably do not adversely affect persons with optimal thyroid function, may affect those with thyroid conditions if they are frequently consumed. Lightly steaming these foods, for the most part kills these thyroid-supressing substances.

Other dietary factors that may inhibit thyroid function include:

1. Synthetic and genetically modified animal products
2. Vitamin and mineral deficiencies, most notably Vitamin A, B, zinc, iron, and selenium
3. Excess intake of polyunsaturated fats

The Cancer Project recommends a plant based diet that is rich in vitamin and minerals, and low in fat. We have many recipes on our website that do not contain the foods listed above.