Soy Substances Slow Prostate Cancer Growth In Animals

The finding suggests that substances known as soy isoflavones are probably responsible for the anti-cancer effect.

“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.

However, Clinton cautions against taking soy phytochemicals or dietary supplements enriched with soy isoflavones at this time.

“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 theCancer 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.

“Our results do suggest that including soy-based foods in the diet is reasonable for a number of health benefits. For example, consumption of soy protein has been shown to reduce blood cholesterol levels,” he said.

“The bottom line is that proper clinical studies need to be done to identify the benefits and potential side effects of these compounds.”

The study, which was led by Clinton and involved a team of researchers, was published in the September issue of the Journal of Nutrition.

It tested the ability of two types of soy extract, soy protein isolate and soy phytochemical concentrate, to inhibit 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 divided into six groups. All the animals had been injected with a human prostate-cancer cells, which grow to form a tumor under the skin.

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.

For example, 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.

The researchers also attempted to learn how soy isoflavones retarded prostate tumor growth. They identified three possible mechanisms.

First, tumors receiving the highest levels of soy phytochemical concentrate had a lower rate of cell proliferation and 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.

This work was supported by grants from the National Institutes of Health and the Harvard Clinical Nutrition Research Center.