Diet and Nutrition

Soy and Breast Cancer: Does a Link Exist?

January 11, 2017   /
Jason Machowsky, MS, RD, CSCS
Nutrition411 Staff

For the past 15 years, soy and soy-based foods (soy milk, tofu, and soy flour) have gained considerable attention in regard to their rich nutrient composition (vegetable protein, fiber, B vitamins, calcium, iron, zinc, and phytochemicals, such as isoflavones), and studies have looked at their potential health benefits when consumed as part of a balanced diet. 

In fact, in 1999, the United States (US) Food and Drug Administration (FDA) approved the nutrition label claim that “consuming 25 grams (g) of soy protein per day may reduce the risk of heart disease” based on research about soy’s potential to lower cholesterol. Other research has suggested that soy consumption also may relieve hot flashes associated with menopause, and lower the risk of osteoporosis and cancers of the prostate and breast.

Press coverage about these benefits has grown (primarily from studying Asian populations), and so has soy consumption by the general public, particularly in the US. Along these lines, many new soy-derived products were created, including supplements, foods with soy protein isolate, and isoflavone-enriched foods, which try to isolate and capitalize on the compounds responsible for soy’s possible health benefits. However, recent studies have shown that some of these compounds, predominantly isoflavones, could potentially increase a person’s risk for breast cancer.

What are isoflavones?
Isoflavones, a phytochemical of the flavonoid group, have a limited distribution in nature, with dietary-relevant amounts found only in the soybean. Soybeans have three types of isoflavones: genistin (50% soybean isoflavone content), daidzin (40%), and glycitin (10%). These isoflavones are relatively inactive compounds until converted to their biologically active forms (genistein, daidzein, and glycitein) by fermentation or bacteria within the digestive tracts of humans or animals. 

Bearing similar chemical structures to 17β-estradiol, a key compound in estrogen replacement therapy, soy isoflavones are shown to produce estrogenic effects within the body. However, these soy isoflavones result in weak estrogenic effects, about 100 to 10,000 times weaker than 17β-estradiol. Nevertheless, these mild estrogenic influences may possibly increase risk of breast cancer. Evidence shows that long-term exposure to endogenous estrogens, such as 17β-estradiol, may promote certain types of breast cancer by increasing breast cell reproduction.

Yet, evidence also exists suggesting that soy consumption has protective effects against breast cancer. The initial focus of soy on breast cancer is attributed to:

  • Historically low breast cancer rates in Asia, where soy foods are an important dietary component
  • Early epidemiologic data showing inverse associations between soy intake and breast cancer risk
  • Research demonstrating the potential for isoflavones, mainly genistein, to exert antiestrogenic effects in high-estrogen environments (premenopausal women, high body mass index (BMI)/body fat, etc):
    • Genistein is shown to inhibit cancer cell growth and differentiation.
    • Isoflavones may have antioxidant activity, stimulate the immune system, and impede tumor growth.

What do studies show?
When comparing the studies and their findings, it is difficult to ignore the fact that most studies showing a cancer-protective effect for soy are prospective or case-control studies evaluating breast cancer incidence, while those showing a cancer-causing relation are typically weaker studies (correlational, in vitro, and animal models), usually looking at specific aspects of breast cancer risk (mammary tissue, implanted tumor cells, etc), rather than actual human breast cancer rates directly. Furthermore, a significant review of 18 studies looking at soy intake and breast cancer risk revealed modest, yet significant, protective effects between significant soy intake and breast cancer risk.

However, you need to use caution when choosing the type of soy foods that you purchase, because you cannot make general conclusions from some of these findings. Many questions have arisen regarding the impact of both the quantity and quality of soy consumed across populations. For example, daily isoflavone intake from traditional soy foods of older Japanese adults ranges from 25-50 milligrams (mg), while isoflavone levels in Western-based high-soy diets are considerably less (1-10 mg), primarily because of soy-food processing. While each gram of soy protein in soybeans and traditional Asian soy foods contains about 3.5 mg of isoflavones, processing may reduce soy protein isoflavone content by up to 80%.

More studies on the potential negative effects of processed soy foods are needed to determine if a true relationship exists and if it is possible to make generalizations from these findings based on research in different populations. 

How much soy is safe to eat?
It appears that women consuming a diet that includes a moderate amount of unprocessed or minimally processed soy products (eg, edamame, tofu, and miso) are most likely safe from harm and may even gain some health benefit. Soy milk is a highly debated food, because some people consider it a highly processed soy food, while others consider it minimally processed. However, isoflavone-enriched foods or supplements (ie, isoflavone powder, pills, or foods with a large amount of soy-protein concentrate/isolate [check the nutrition labels]) are perhaps not as ideal, as is the case for heavily processed soy foods, because it appears that their effect on health is negligible at best, and may be even harmful. 

In the end, two old rules of thumb apply best: 

  • Always try to have the “whole food” (ie, the less processing, the better)
  • Have everything in moderation

Based on the FDA recommendation of 25 g of soy protein/day, this is approximately equivalent to:

  • 3½ cups (C) soy milk (1 C provides 7 g of soy protein)
  • 1½ C edamame (½ C provides 8 g of soy protein)
  • ⅝ C tofu (¼ C provides 10 g of soy protein)
  • 3 C of Kashi® GOLEAN® cereal (1-1/4 C provides about 10 g of soy protein)
  • 2½ Clif® Bars (one bar provides about 10 g of soy protein)

References and recommended readings

Allred CD, Allred KF, Ju YH, Goeppinger TS, Doerge DR, Helferich WG. Soy processing influences growth of estrogen-dependent breast cancer tumors. Carcinogenesis. 2004;25(9):1649-1657. doi:10.1093/carcin/bgh178.

Clif Bar website. Accessed March 7, 2016.

Hu SA. Risks and benefits of soy isoflavones for breast cancer survivors. Oncol Nurs Forum. 2004;31(2):249-63. doi:10.1188/04.ONF.249-263.

Jaceldo-Siegl K, Gatto N, Beeson L, Fraser G. Intake of Soy Isoflavones Reduces Breast Cancer Incidence among Women in North America. The FASEB Journal. 2015;29(1 Supplement), 406-5. Accessed March 7, 2016. 

Kashi GoLean Original Cereal website. Accessed March 7, 2016.

Messina M, McCaskill W, Lampe JW. Addressing the soy and breast cancer relationship: review, commentary, and workshop proceedings. J Natl Cancer Inst. 2006;98(18):1275-1284. doi:10.1093/jnci/djj356.

Messina MJ, Wood CE. Soy isoflavones, estrogen therapy, and breast cancer risk: analysis and commentary. Nutr J. 2008;7:17. doi:10.1186/1475-2891-7-17.

Shike M, Doane AS, Russo L, et al. The effects of soy supplementation on gene expression in breast cancer: a randomized placebo-controlled study. J Natl Cancer Inst. 2014;106(9). doi:10.1093/jnci/dju189.

Shu XO, Jin F, Dai Q, et al. Soyfood intake during adolescence and subsequent risk of breast cancer among Chinese women. Cancer Epidemiol Biomarkers Prev. 2001;10(5):483-488. Accessed March 7, 2016.

Soy. American Institute for Cancer Research website. Accessed March 7, 2016.

Soy + women’s health. Soy Connection United Soybean Board website. Accessed March 7, 2016.

Trock BJ, Hilakivi-Clarke L, Clarke R. Meta-analysis of soy intake and breast cancer risk. J Natl Cancer Inst. 2006;98(7):459-471. doi:10.1093/jnci/djj102.

Wu AH, Koh WP, Wang R, Lee HP, Yu MC. Soy intake and breast cancer risk in Singapore Chinese Health Study. Br J Cancer. 2008;99(1):196-200. doi:10.1038/sj.bjc.6604448.