Nutritionists debate soy's health benefits.

The simple soybean is turning into one of nutrition's biggest puzzles.

Scores of presentations at the Experimental Biology '99 meeting this week in Washington, D.C., examined the relationship between soy products and cancer. In almost all cases, the researchers agree, soy seems to be a cancer fighter. However, a soybean component that has been touted for its health benefits is not living up to expectations. For postmenopausal women, the story may be even more complicated.

Reaffirming soy's potential health value, several studies at the conference reported that diets rich in the legume protect rats against colon cancer, breast cancer, and prostate cancer. Moreover, men who ate 39 grams of soy protein each day for 1 year had fewer colon cells in the process of dividing than did men who didn't eat soy, reports Deepa Thiagarajan of Michigan State University in East Lansing. With fewer cells proliferating, the men stand a better chance of avoiding colon cancer, she says.

Soy compounds called isoflavones mimic the hormone estrogen. These soy constituents might disrupt hormone-dependent cancers, such as many prostate and breast cancers, scientists have reasoned (SN: 10/11/97, p. 230).

More than 100 companies sell supplement pills or foods fortified with genistein, one of the most hyped isoflavones, says nutrition consultant and author Mark Messina of Port Townsend, Wash.

In one new study, genistein slowed the growth of human prostate tumors grafted into rats, reports Jin-Rong Zhou of Beth Israel Deaconess Medical Center in Boston.

However, unlike complete soy, genistein alone does not protect mice from breast cancer, says Ruth S. MacDonald of the University of Missouri-Columbia. Even more troubling, Maurice R. Bennink of Michigan State University finds that adult rats fed genistin, which is metabolized into genistein, are more likely to develop colon cancer than those fed standard diets.

The contradictory results may reflect genistein's malleable role in the body, says Martin J. Ronis of the Arkansas Children's Nutrition Center in Little Rock. It can serve as either an estrogen or an antiestrogen, depending on how much natural estrogen is circulating, he suggests.

In a rat study designed to mimic the hormone balance in postmenopausal women, genistein promoted cancers that need estrogen, Clinton D. Allred of the University of Illinois at Urbana-Champaign reports. Fifty to 60 percent of all breast cancers are estrogen-dependent. In the study, researchers removed the rats' ovaries and implanted human breast cancer cells under the skin. When the rats received estrogen supplements, the tumors grew. Tumors in rats given genistein but not estrogen also grew, but less dramatically. Without estrogen or genistein, the tumors shrank.

Timing may explain some conflicting genistein results, says Bill Helferich, also of the University of Illinois. Soy fed to animals early in life protects them against many kinds of cancer, he says. At that stage, soy may affect how cells differentiate. Later on, however, in the absence of other estrogens, genistein or other soy components could stimulate cancer-cell proliferation.

No one knows if genistein or soy helps estrogen-dependent tumors grow in postmenopausal women, Helferich says.

As for whether people should take genistein supplements, most researchers recommend eating soy foods instead, a practice endorsed by the Food and Drug Administration as a way to reduce cholesterol.

"If you're eating soy or soy flour, that's fine," Bennink says. "I wouldn't take pills."

David Heber of the University of California, Los Angeles agrees, "Food is not just a vehicle for delivering genistin."