Soy and Breast Cancer

Recently, soy foods and products have generated much discussion in the health care industry and among the public. Researchers have just recently begun to investigate the possible benefits soy may offer against breast cancer and other conditions. This section discusses the current research on soy. Women who are considering adding soy to their diets to help prevent breast cancer or other conditions should talk to their physicians, especially if they have a personal or family history of breast cancer or other medical problems.

Terminology: Soy, Phytoestrogens, Isoflavones

Many soy foods are rich in phytoestrogens, natural chemicals that act like weak estrogen (a hormone found in the female body). Isoflavones are one type of phytoestrogen. Isoflavones contain several compounds, including genistein and daidzin. Some researchers believe that isoflavones can alleviate menopausal symptoms and help prevent breast cancer in some women. Phytoestrogens are found in soybeans, flaxseeds, black cohosh, alfalfa spouts, and other plants.

Does Soy Help Prevent Breast Cancer?

Research on soy is relatively new. Currently, researchers are unsure of the exact effect of soy on breast cancer. Several animal studies and small human clinical trials have shown that soy foods that contain phytoestrogens (more specifically, isoflavones) may offer some protection against breast cancer.

Researchers believe phytoestrogens found in soy may help protect against breast cancer because phytoestrogens compete with estrogen in the body to bind to estrogen receptors on cells. Since estrogen triggers breast cell reproduction, some researchers believe that a higher amount of estrogen in the body may increase a woman’s risk for breast cancer. Because phytoestrogens found in soy foods may block estrogen from reaching estrogen receptors, pre-menopausal women who include soy in their diet may decrease their risk of breast cancer. Though research on the effects of soy in pre-menopausal women shows some promise, it is preliminary and needs to be confirmed in large clinical trials.

Researchers are less certain about the effects of soy in post-menopausal women. Some small studies have shown that soy may provide post-menopausal women with many of the same benefits as hormone replacement therapy. Soy may reduce hot flashes, vaginal dryness, and other menopausal symptoms. Soy may also protect against bone loss (osteoporosis) and heart disease and possibly reduce the risk of diabetes and kidney disease.

Whether soy can help protect against breast cancer in post-menopausal women is unclear at this time. Some researchers believe that soy consumption may actually increase the risk for breast cancer in post-menopausal women because the chemical structure of plant estrogens (phytoestrogens) is very similar to estrogen. Jeanne Petrek, MD, surgeon and breast cancer specialist at Memorial Sloan-Kettering Cancer Center in New York, recently told the American Cancer Society that phytoestrogens could possibly be misinterpreted by the body as estrogen and increase the risk of breast cancer in post-menopausal women. ⁽¹⁾

What is the Effect of Soy on Breast Cancer Patients and Survivors?

At this time, researchers are not certain what effect soy may have on breast cancer patients or survivors. However, some researchers believe that phytoestrogens found in many soy foods may stimulate cell growth and could actually lead to a higher risk of breast cancer recurrence. 

Some physicians feel that women with estrogen receptor-positive breast cancers or those who are taking the drug tamoxifen for breast cancer should limit their intake of soy products containing phytoestrogens until researchers are able to better understand the effects of soy on breast tumors. In fact, many physicians advise women with a strong personal or family history of breast cancer not to consume phytoestrogens.

What are Some Other Benefits of Soy?

Researchers have also been exploring other possible benefits of soy. Again, the research on soy is relatively new and the effects have not been confirmed in large human clinical trials. A few recent studies have shown that soy foods containing phytoestrogens may mimic the benefits of hormone replacement therapy. In addition to relieving menopausal symptoms such as hot flashes, vaginal dryness or other menopausal symptoms, phytoestrogens may help prevent heart disease and osteoporosis, a degenerative bone disease. Preliminary research shows that soy may also help protect men against prostate cancer.

In a study conducted at the Stanford University Medical Center in California, women who were given a soy-based diet lowered their total cholesterol levels by 9% and LDL cholesterol levels ("bad cholesterol") by 13% compared to women who did not include soy in their diets. The FDA allows soy food labels to state that 25 grams of soy protein daily may help reduce the risk of coronary heart disease by lowering blood cholesterol levels. Phytoestrogens were also shown to protect against menopausal symptoms, osteoporosis, and decrease the risk for breast cancer in the study.

Researchers believe that phytoestrogens found in many soy foods may provide multiple benefits to women because they help balance hormone levels in the body. A lack of the hormone estrogen has been shown to increase the risk for osteoporosis and heart disease. However, too much estrogen may contribute to breast or uterine cancer because some cancer cells depend on estrogen for survival.

Presently, the general consensus is that women at high risk of cardiovascular disease could benefit from a soy diet, especially if they do not have a family or personal history of breast or uterine cancer. However, because the benefits of soy need to be confirmed in larger clinical trials, physicians do not generally recommend soy as the only treatment or means of protection against heart disease, osteoporosis, or menopausal symptoms (such as hot flashes, vaginal dryness, etc.).

Soy Content of Selected Foods

The FDA has recently granted approval to the use of soy food labels that contain a phrase equivalent to: "25 grams of soy protein daily can reduce the risk of coronary heart disease by lowering blood cholesterol levels."

Source: U.S. Soyfoods Directory

The United States Department of Agriculture (USDA) has also collaborated with Iowa State University to create a database that catalogues the amounts of genistein, daidzin, and total isoflavones found in foods.

Additional Resources and References

• The July 15, 1999 American Cancer Society’s News Today report, "Effects of Soy on Breast Cancer Risk Still Unknown," is available at http://www.cancer.org/docroot/NWS/content/NWS_1_1x_Soy_and_Breast_Cancer.asp/
• The May 3, 2000 Imaginis.com report, "Phytoestrogens (Soy) Show Similar Benefits of Hormone Replacement Therapy And May Help Prevent Breast Cancer," is available at http://www.imaginis.com/breasthealth/news/news5.03a.00.asp
• The February 1, 2000 Imaginis.com report, "New Research Shows Soy May Prevent Breast Cancer," is available at http://www.imaginis.com/breasthealth/news/news2.01.00a.asp
• Several abstracts from studies on soy presented at the third international symposium (Oct.-Nov. 1999) on "The Role of Soy in Preventing and Treating Chronic Disease," are available on the U.S. Soyfoods Directory website at http://soyfoods.com/

Updated: September 10, 2007