Q. Is soy helpful or harmful for breast-cancer survivors and those at risk for getting the disease?

A. Recommendations regarding soy for breast-cancer survivors and those at risk for developing breast cancer are conflicting. There is concern over the possibility that soy could actually stimulate breast tumor growth in some women.

Many types of breast cancer need estrogen to grow. Substances in soy called isoflavones are plant estrogens, which are weaker than a woman's natural estrogen. Isoflavones bind to estrogen receptors in the breast. This can block estrogen from entering breast cells and triggering tumor formation. Sometimes, however, isoflavones can actually stimulate tumor growth. This difference may depend on the type of estrogen receptors in a woman's breast.

Reports of soy preventing breast cancer come mainly from Asian populations, who have eaten soy foods since childhood. For those who begin eating soy after being diagnosed with breast cancer, the effects could be different.

Most research showing soy to have harmful effects used isoflavone supplements rather than whole soy foods. Whole soy foods include edamame, tofu, miso, soy milk, soy flour and natto. Supplements often contain higher doses of isoflavones than traditional Japanese diets. Supplements don't contain all of the biologically active substances found in whole soy foods. These naturally occurring substances seem to work better together than each alone in reducing cancer risk.

Although more study is needed, current knowledge suggests that moderate amounts of whole soy foods as part of a balanced diet do not adversely affect women at risk for breast cancer. High breast-cancer survival rates among Japanese women point to the likelihood that the benefits of soy foods outweigh their potential harm. Soy foods are also helpful in decreasing heart-disease risk.

The American Cancer Society recommends that "breast cancer survivors consume only moderate amounts of soy foods as part of a healthy plant-based diet, and should not intentionally ingest very high levels of soy products."

Since this issue is still unresolved, you should discuss this with your oncologist. As a precaution, women who have, or are at high risk for, estrogen-dependent breast cancer may want to limit soy foods to two or three servings per week.

Soy is only one piece of the breast-cancer puzzle. Eating lots of fruits and vegetables, limiting animal fat, exercising and limiting alcohol can reduce your risk of breast cancer and its recurrence.

Amy Tousman is a registered dietitian with the Health Education Center of Straub Clinic & Hospital. Hawai'i experts in traditional medicine, naturopathic medicine and diet take turns writing the Prescriptions column. Send questions to: Prescriptions, Island Life, The Advertiser, P.O. Box 3110, Honolulu, HI 96802; islandlife@honoluluadvertiser.com; or fax 535-8170. This column is not intended to provide medical advice.