Soy Products & Breast Cancer

​​​ The History

The concern about soy and breast cancer first began with the fact that soy contains isoflavones (a type of plant hormone that resembles human estrogen). The main isoflavones in soy are genistein and daidzein. These are plant substances which can act like the hormone estrogen. Since estrogen is a factor in estrogen-receptor positive breast cancer, it was thought that soy could be harmful.

There have been many research studies which have produced conflicting results. Some studies looked at how breast cancer cells reacted when they were exposed to soy isoflavones. Other studies looked at the effect of soy intake on animals. Based on the results of this type of research, many health professionals were cautious and recommended that breast cancer survivors avoid soy. However, the breast cancer rates tended to be lower in countries where people ate soy foods as a regular part of the diet. Clearly, more information was needed.

​​​ Newer Findings

In the past few years, some studies involving large groups of breast cancer survivors have shed more light on this issue.

A study in China looked at the association between soy consumption and risk of breast cancer recurrence in over 5,000 breast cancer survivors. The study found lower risk of recurrence in women who consumed more soy. This was true for women with both estrogen-receptor positive and negative breast cancers. This does not mean that soy lowered the risk of recurrence. The women in the study who ate more soy also tended to eat more fish and vegetables and to exercise more than the other women. These other healthy lifestyle habits may have had a beneficial effect.

A study in the United States showed similar results. The Women’s Healthy Eating and Living (WHEL) study examined the effect of eating soy foods on breast cancer recovery in over 3,000 breast cancer survivors. It found that soy intake was not related to a second breast cancer incident. A second incident included recurrences and new breast cancer diagnosis. This held true regardless of hormone receptor status.

The World Cancer Research Fund and American Institute of Cancer Research report for breast cancer survivors states that there is some (limited) evidence of links between better survival after breast cancer and eating foods containing soy.

​​​ The Bottom Line

Women who have had breast cancer and want to include soy foods in their diet can do so safely. Foods made from soy are a good source of protein.

However, if you have a history of breast cancer you should avoid taking concentrated isoflavone (plant hormone) supplements in pill or powder form. These are often marketed as a natural way to relieve menopausal symptoms. Their safety has not been determined.

​​​ Examples of Soy Foods

• Edamame: Fresh, green soybeans, usually salted and boiled or steamed in the pod. Used as a snack.
• Miso: Fermented paste made of soybeans, salt and rice. Used as a condiment or a base for miso soup.
• Soy beverage (soymilk): Milk-like drink made from soybeans. Used as a substitute for cow's milk.
• Tempeh: Fermented whole soybean cake. Used as a meat substitute.
• Tofu (bean curd) – firm: Coagulated soybean beverage, the texture of cheese. Used as a meat substitute.
• Tofu – silken: Coagulated soybean beverage, the texture of custard. Used in making spreads and desserts.​