By Janet Raloff

Web edition: March 19, 2001

Asian women tend to have much lower breast-cancer rates than their Western counterparts--unless they move to Europe or North America. Then the cancer’s incidence in these women begins to match local norms.

This observation has suggested that something about the Western way of life, probably diet, promotes cancer--or that something about Eastern diets inhibits the development of breast malignancies. Strong support for the latter comes from a recent study by researchers at the University of Texas Medical Branch in Galveston and Weizmann Institute of Science in Rehovot, Israel.

The study showed that when women incorporated a large serving of soy into their diet each day, their blood concentrations of two female sex hormones plummeted dramatically. Other studies have linked breast-cancer risk with a high lifetime exposure to these hormones, especially estrogen.

To detect what might be small, subtle changes, the scientists in the most recent trial carefully controlled the diets of the 7 white and 3 black women that they had recruited to participate. Although there weren’t many subjects, such nutritional trials are often small because of their enormous monitoring and data-collection demands.

For at least 3 months, each of the women consumed their normal meals at home and recorded each date of menstruation onset. After that, for another complete menstrual cycle, each woman moved into the metabolic ward of a hospital, where she received meals designed to maintain her current body weight. Allowed to go out and to work during the day, each participant returned at night to eat and sleep at the health center. That’s when each woman also sat down to a nontraditional “dinner” of soy milk--a full 36 ounces of it.

Lee-Jane W. Lu of the Galveston group explains that because this drink doesn't taste like cow’s milk, “we weren’t sure whether anybody would like it.” To ensure that each volunteer downed her entire portion, the researchers saved the day’s soy quota for a time when each women could be watched. In fact, “most of the women initially thought the drink tasted strange," Lu acknowledges. "But by the end, all eventually got to liking it.”

Blood and urine collected from the women daily during this phase of the trial demonstrated that the soy triggered a large change in hormones that persisted throughout the menstrual cycle. Production of estrogen, the primary female sex hormone, dropped by 25 percent. Concentrations of progesterone, a hormone that plays a major role in pregnancy, dropped by 45 percent.

Phytoestrogens

Soy milk is relatively rich in genistein and daidzein, a pair of plant estrogens, or phytoestrogens. Like the body’s own estrogen, phytoestrogens bind to and activate specific receptors on cells throughout the body. However, compared to the body’s own estrogen, the phytoestrogens are very weak. So are their biological impacts, and that might explain any benefits they offer.

During her childbearing years, a woman’s production of estrogen is far higher than a man's, and far higher than at later periods in her own life. Consuming large amounts of weak phytoestrogens can prevent some natural estrogen from reaching its receptors, thereby diminishing a woman’s exposure to this more potent hormone.

Because estrogen can promote and fuel the growth of many breast cancers, diets rich in phytoestrogens might effectively starve precancerous tissues, diminishing a woman’s cancer risk. Or at least that’s the hypothesis that many researchers have begun probing, because of the observation that especially low breast-cancer rates seem to characterize societies that consume a lot of phytoestrogens. Soy-based tofu, tempeh, and soy milk are common in many Oriental cuisines.

Other factors might also diminish cancer risk in these populations, including relatively small meal portions or different proportions of fiber and proteins. However, Lu’s team concludes, “Because decreased levels of [estrogen and progesterone] may reduce breast cell proliferation and breast cancer risk, the results of this study have implications for breast cancer prevention.”

Recipe: Fruit-flavored smoothie

Want a fruity alternative to the soy drink Lu’s team offered? Try this quick recipe, courtesy of the Indiana Soybean Board. It is based on soy milk, and the fruit adds some fiber and more than a dash of flavonoids--plant pigments that studies shown can fight some of the chemical changes that have been linked with fostering chronic disease.

Cranberry Raspberry Smoothie

* 1 cup vanilla soy milk

* 1/2 large banana

* 2 tbs frozen cranberry juice concentrate (undiluted)

* 1/4 cup frozen raspberries

Place all ingredients in blender and puree until smooth. Serve immediately or refrigerate.

Yield: 1.5 cups. Serving size: 1.5 cups. Per serving: 272 calories, 4 g fat, 0.5 g sat fat, 11 g protein, 48 g carbohydrate, 110 mg sodium.