ENFIELD, AUSTRALIA — Evidence that soy products may help reduce the risk of prostate cancer development and progression emerged in a study published in Urology, a medical journal.

Subjects in the study showed a marked reduction in prostate-specific antigen (P.S.A.) levels in patients with prostate cancer who incorporated soy into their diets.
The study took place at Monash University in Australia. A team led by Fabien S. Dalais of Monash University wrote "Effects of a Diet Rich in Phytoestrogens on Prostate-Specific Antigen and Sex Hormones in Men Diagnosed with Prostate Cancer."

Participants in the study experienced a significant improvement in P.S.A. levels after a month, with the drop averaging 13%. Doctors widely use P.S.A. levels to screen for prostate cancer and for tracking the disease after diagnosis.

Prostate cancer is the leading cancer in men in Australia. It is the second-deadliest cancer for men in the United States. The study noted that considerable differences exist worldwide in rates of prostate cancer, with the incidence and mortality rates particularly low in China and Japan.

"Studies of Chinese and Japanese men migrating to the United States found that the men adopt a greater prostate cancer incidence pattern within two generations," the study said. "It has been suggested that the phytoestrogens contained in soybeans, the isoflavones, and, to a lesser extent, the phytoestrogens in linseed, the lignans, could be responsible for this difference."

Subjects in the Australia study were divided into three groups. Each subject was asked to eat four bread slices per day provided as part of the study. One group ate bread with added heat-treated soy grits. A second group ate bread with the soy grits plus linseed. The third group was a control group eating regular wheat bread.

The random, double-blind, placebo-controlled study enrolled participants diagnosed with prostate cancer and scheduled for radical prostatectomy. In a prestudy visit, the subjects gave blood and urine samples. They gave samples a second time one day before surgery. Of the 28 patients who completed the study, significant differences (-13%) were observed between the heat-treated soy grits and the control wheat bread group.

"The favorable change in P.S.A. levels observed in the H.T. soy group compared with the control group suggests an inhibition in prostate cancer cell growth," the authors said.

At the same time, they observed a number of limitations in the study. Changes in the biomarkers "are only suggestions and do not necessarily translate into a reduced incidence of the disease or improved outcome from treatment," the authors said.

They warned that the results should be "interpreted with extreme caution" and suggested further study.

"There is a need to examine these effects on in vitro markets of prostate cancer, such as the proliferation index," they said. "Longer duration studies with a larger number of subjects would be worthwhile to confirm the potential beneficial results."

Demonstrating health benefits specific to men could broaden the market for soy products. To date, a large part of the marketing emphasis for soy has related to health and wellness issues associated with women. Specifically, makers of soy products have emphasized the potential benefits of soy in helping prevent osteoporosis and breast cancer and reducing menopausal symptoms.