Studies have shown Asian women have a breast cancer rate one-fifth that of Western women, and some research suggests the lower rates are linked to consumption of soy, a staple of the Asian diet.

Soy is high in phytoestrogens, natural chemicals found in plants that act like weak forms of the estrogens naturally produced in a woman's body. It is especially high in the phytoestrogen, isoflavone, which contains two important compounds -- genistein and daidzin.

Understanding the Effects of Phytoestrogens
The effects of phytoestrogens on a woman's body are not fully understood, but it is believed they compete with estrogen to fit into estrogen receptor sites in different parts of the body, including breast tissue. Because estrogen stimulates cell reproduction, it may be that the more exposure a women has to estrogen, the greater her breast cancer risk. In premenopausal women, phytoestrogens may have an estrogen-reducing effect because they attach to estrogen receptor sites, thus tricking the body into producing less estrogen.

The effect of phytoestrogens is less clearly understood in postmenopausal women, whose estrogen levels are naturally lowered. Although there is some evidence to suggest consumption of soy reduces breast cancer risk in premenopausal women, there is no evidence that consumption of soy reduces postmenopausal breast cancer risk. Some doctors and nutrition experts believe phytoestrogens may actually boost estrogen levels in postmenopausal women.

"Because the chemical structure of phytoestrogens is very similar to estrogen, they may be interpreted by the body and some tissues as estrogen and may be just as likely to increase the risk of breast cancer as to reduce it. And super-concentrated pharmacological doses of phytoestrogens are probably detrimental. You're playing with fire because it could go either way. It could do more harm than good," said Jeanne Petrek, MD, a surgeon and breast cancer specialist at New York's Memorial Sloan-Kettering Cancer Center.

Also unknown are the effects of phytoestrogens on women who already have breast cancer, women at risk for a recurrence of breast cancer, or those at risk for developing it because of genetic factors. However, it is thought that increasing isoflavonoid intake may stimulate cell growth in these cases as well, and should therefore be avoided. "If one has any risk for breast cancer, taking isoflavonoids may not be good," said Abby Bloch, PhD, RD, chair of the nutrition and physical activity advisory committee for the American Cancer Society (ACS).

Trying to Find a Connection
Currently, no clinical trials have shown eating specified amounts of soy products over time reduces the risk of breast cancer. Animal studies and in vitro (test tube) studies of cancer tissue point to a connection between isoflavonoids and reduced breast cancer risk, but no causal relationship has been established.

"There is no clear-cut evidence on the advantages or disadvantages of increased levels of isoflavonoids in the prevention of breast cancer," said Kathryn Hamilton, MA, RD, clinical nutrition manager at Memorial Sloan-Kettering's Breast Oncology Department. "Until we figure out what kind of estrogens fuel breast cancer, no one should be self-prescribing or self-diagnosing."

Although the link between diet and breast cancer is not yet clearly understood, specialists agree there are health benefits associated with consuming a diet rich in naturally occurring plant chemicals found in soy products such as tofu and in the fiber of whole grains, berries, fruits, vegetables, and flaxseed. Whether and how such a diet contributes to cancer prevention is not yet understood. Human trials are needed, particularly on the effects of isoflavonoids on girls and young women, in whom they may have the greatest impact on future health.