Is soy linked to breast cancer?

Conditions Expert Dr. Otis Brawley Chief Medical Officer,
American Cancer Society

Expert answer

Dear Curious,

Your neighbor's doctor is concerned about estrogenic stimulation, which can cause breast cancer. Some of the most effective breast cancer treatments involve blocking estrogen stimulation. Soy does have estrogens, but I believe a moderate amount of soy in the diet is still a good idea for most people.

Within the normal diet there are a number of naturally occurring plant-derived compounds called phytoestrogens. Even green beans, peanuts and dates have some phytoestrogen content, but flaxseed, soy and soy byproducts are especially high in phytoestrogens. These compounds are called phytoestrogens as a group, but you may be more familiar with them when they are also referred to by their categories within the phytoestrogen family -- the flavonoids, isoflavones, coumestans or lignans.

The phytoestrogens have some pro-estrogenic activity and some anti-estrogenic activity. The concern is that estrogenic stimulation can in some circumstances promote breast cancer development. Within the past few years, we have come to realize that some post-menopausal estrogen replacement therapies increased risk of breast cancer, and as a result more attention is being paid to all forms of estrogen stimulation.

Even among the soy products, there are differences in terms of amount of phytoestrogen. One survey of foods found that 100 grams of flaxseed had nearly four times as much phytoestrogen as 100 grams of soybeans; 100 grams of soybeans had three times as much phytoestrogen as 100 grams of tofu, and 33 times as much as 100 grams of soy milk.

There is the suggestion that a diet with moderate to high levels of phytoestrogens may decrease risk of cardiovascular disease and prevent osteoporosis. Some population studies also suggest that high phytoestrogen intake might have a preventive effect on breast and endometrial cancers in women, and on prostate cancer in men. The Asian diet is high in soy and this is one of the theories as to why the Asian population has low rates of breast, endometrial, and prostate cancer.

While large population studies suggest a benefit from high levels of phytoestrogens in the diet, animal studies have noted that extremely high amounts of phytoestrogens may actually promote breast and uterine cancer growth.

There is more certainty that phytoestrogens can disrupt the anti-tumor effect of some drugs used to treat breast cancer. As a result, many physicians recommend that women with breast cancer, or at high risk for breast cancer, reduce the amount of phytoestrogen in their diet. Even more physicians do not consider high doses of phytoestrogen safe for women being treated for breast cancer with hormonal therapies (the estrogen receptor blockers or the aromatase inhibitors).

For the person who is at average risk of breast cancer, dose is the key issue in the answer to your question. There is a difference between high levels of phytoestrogens in the diet and the very high doses in dietary supplements. It is very reasonable for a person at average risk of breast cancer to eat a moderate amount of soy products, such as tofu, soy butter, soy nuts, and soy burgers, as part of an overall well-rounded healthy diet.

While moderate consumption of soy-based products is very reasonable, some supplements extremely high in phytoestrogens, especially soy-based isoflavone compounds and flaxseed based lignans, have been promoted as "natural" treatments for hot flashes and other menopausal symptoms. Very well-designed clinical trials show these supplements are no more effective than placebo (sugar pills) at relieving these symptoms. There is good science to suggest these high dose supplements may have negative health effects.