Soy slows growth of prostate cancers.

Men who eat soy-rich diets face a lower risk of deadly prostate cancer, epidemiological studies have indicated. Animal experiments now suggest how soy defends the prostate. The legume induces suicide among cancer cells and limits their spread, scientists report.

In a pair of 6-month-long studies, Goran Hallmans of the University of Umea in Sweden and his colleagues implanted cancer cells under the skin of rats and hairless mice. Then, they fed the rodents diets deriving one-third of their calories from protein. Some animals got protein from soy, which is rich in plant estrogens known as isoflavones. Whole-grain rye, which contains large amounts of lignans--another family of plant estrogens--provided the protein for others. The final group consumed milk casein, an estrogenfree protein.

Both the soy- and rye-based diets reduced the growth of tumors, compared with the growth of cancers in caseinfed animals. Moreover, only the soy and rye diets induced apoptosis--or natural, programmed death--in the implanted tumor cells, notes coauthor Herman Adlercreutz of the University of Helsinki.

Ordinarily, tumor cells' failure to undergo normal aging and apoptosis contributes to their uncontrolled growth. Adlercreutz says that his team's data represent "the first time it has been shown diet can induce apoptosis."

Jin-Rong Zhou of Harvard Medical School in Boston described related data from mice. His team had placed human cancer cells into the animals' prostates. As tumors began to grow, some mice received diets rich in soy or supplemented with genistein, soy's primary isoflavone. The rest ate casein-based chow.

After 10 weeks, Zhou reports, tumors in casein-fed mice were about twice as big and twice as likely to spread as those in animals fed the soy or genistein.

Zhou notes that most primary cancers aren't lethal; it's their dissemination to other organs that kills. His data indicate that soy retards the cancers' growth and spread at least in part by inhibiting the body's production of blood vessels to supply nutrients to the prostate tumors.

Both Adlercreutz and Zhou reported their findings Monday in Washington, D.C., at the Third International Symposium on the Role of Soy in Preventing and Treating Chronic Disease.

Soy has garnered plenty of media attention for hints that it might cut the risk of breast cancer, says conference chairman Mark Messina, a Port Townsend, Wash., nutritionist. "I think we can now make the case that the prostate data are more impressive," he notes.