"The study strongly supports the need for clinical trials to assess the potential role of soy in reducing the incidence and progression of human prostate cancer," said Steven Clinton, associate professor of internal medicine at Ohio State University.

In view of Asian use of soy foods, they are widely accepted as safe. This week US newspapers are carrying ads urging the public to add soy foods to their daily diet to protect against heart disease. Prostate cancer specialists often recommend dietary soy and and also supplements, particularly genistein.

"Our results do suggest that including soy-based foods in the diet is reasonable for a number of health benefits," Clinton said. "For example, consumption of soy protein has been shown to reduce blood cholesterol levels," he said. But Clinton cautions against taking soy phytochemicals or dietary supplements enriched with soy isoflavones until tests have been run.

"Soy phytochemicals are potent compounds. Plants use them to regulate genes during growth and development and as poisons to ward off predators," said Clinton, who is also director of the Cancer Prevention and Control Program at Ohio State's Arthur G. James Cancer Hospital and Richard J. Solove Research Institute. "Some of these chemicals may even enhance or promote the growth of some other types of tumors."

The study tested the power of two types of soy extract -- soy protein isolate and soy phytochemical concentrate -- to hold back the growth of human prostate cancer cells transplanted into mice.

Soy protein isolate is a high quality protein made from whole soy; it contains a very small amount of isoflavones. Soy phytochemical concentrate is an extract of soy that is rich in isoflavones (the amount is about 85 times greater than that found in soy protein isolate).

The study involved 48 mice. All the animals had been injected with a human prostate-cancer cells, which grow to form a tumor under the skin. The mice were divided into six groups according to diet:

• Three of the groups were fed soy protein isolate as their protein source.
• The remaining three groups, which formed the second arm of the study, received casein (milk protein) as their protein source.
• One group in each arm was also fed an amount of soy phytochemical concentrate at a level equivalent to 0.2 percent of their diet
• A second group in each arm was fed an amount equal to 1.0 percent of their diet.

After three weeks, the volumes of the tumors were compared in all the groups. "We found that as the amount of isoflavones went up, the amount of tumor growth went down," said Clinton.

Mice fed soy protein and no phytochemical concentrate had tumors that were 11 percent smaller than did the control mice that received casein only. Mice that received the highest level of isoflavones -- those fed soy protein plus 1 percent phytochemical concentrate -- had tumors that were 40 percent smaller than those in the casein-fed controls.

As well as finding out that soy isoflavones do retard prostate tumor growth, the researchers attempted to find out how it does so. They identified three possible mechanisms.

First, tumors receiving the highest levels of soy phytochemical concentrate had a lower rate of new cell growth (cell proliferation) and a higher numbers of cells undergoing programmed cell death, or apoptosis.

Second, there was a lower number of blood vessels in these tumors. This may reflect the ability of isoflavones to inhibit the growth of new blood vessels in the tumor (that is, they have antiangiogenic properties).

Third, animals fed isoflavones had lower levels of insulin-like growth factor 1, a hormone that stimulates the growth of prostate cancer.

Nutr 1999 Sep;129(9):1628-35Soybean phytochemicals inhibit the growth of transplantable human prostate carcinoma and tumor angiogenesis in mice. Zhou JR, Gugger ET, Tanaka T, Guo Y, Blackburn GL, Clinton SK.

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