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Researchers Continue to Explore Possible Benefit of Soy in Helping to Prevent Breast Cancer (dateline December 26, 2000)

While research on soy and breast cancer risk is relatively new, a small study presented at the meeting in Hawaii of the International Chemical Congress of the Pacific Basin Societies offers some promise that maintaining a diet rich in soy may help reduce the risk of breast cancer. The study involved 120 Asian women and found that the women who consumed the largest amounts of soy were the least likely to develop breast cancer. However, the researchers maintain that the relationship between soy and breast cancer risk is largely unknown and warrants further study. In fact, some physicians believe certain ingredients in soy (phytoestrogens) may stimulate cell growth and suggest that women with a family or personal history of breast cancer limit their intake of soy products until researchers are able to better understand the effects of soy on breast tumors.

In a study presented by Adrian Franke, PhD of the Cancer Research Center of Hawaii in Honolulu, researchers studied urine samples from 120 Asian women. Half of the women had been diagnosed with breast cancer and half were disease-free. On average, the levels of isoflavones (an ingredient found in some soy products) were 60% lower among the women who had breast cancer than in the women who were cancer-free, suggesting that maintaining a diet rich in soy helps protect against breast cancer.

Researchers believe that soy may lower breast cancer risk because in Asian countries where the consumption of soy is high, the incidence of breast cancer is much lower than in Western countries such as the United States, where soy intake is relatively low. The Hawaiian study is one of the first studies to examine soy and breast cancer risk in humans (as opposed to animals). However, the Hawaiian researchers say that the results of their study could be "causal" (coincidental) and require further investigation in larger studies.

Currently, researchers are unsure of the exact effect of soy on breast cancer risk. Several animal studies and a few small human studies have shown that soy foods that contain phytoestrogens may offer some protection against breast cancer. Phytoestrogens are natural chemicals that act like weak estrogens in the body. Isoflavones are one type of phytoestrogen, and are believed by some researchers to be responsible for the reduction in breast cancer risk. Isoflavones contain several compounds, including genistein and daidzin.

Some researchers believe that 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 cell reproduction, some researchers believe that higher amounts of estrogen in the body may increase a woman’s risk of breast cancer. Because phytoestrogens found in soy foods may block estrogen from reaching estrogen receptors, pre-menopausal women who include soy in their diets may decrease their risk of breast cancer.

However, researcher are less certain about the effects of soy in post-menopausal women. Some small studies have suggested that soy may provide post-menopausal women with many of the same benefits as hormone replacement therapy (HRT) in helping to relieve menopausal symptoms, such as hot flashes and vaginal dryness. Researchers are also exploring whether soy can help protect against heart disease and bone loss (osteoporosis).

While researchers continue to investigate the benefits of a diet rich in soy, 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.  In fact, 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 have a clearer understanding of the effects of soy on the body. Women should talk to their physicians about diet and other lifestyle factors that may help reduce their chances of cancer.

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