Pomegranate juice appears to trigger a process that genetically programs cancer cells to die.

In addition, prostate-specific antigen, or PSA, the traditional marker used to test for prostate cancer, was lowered substantially in mice that were fed pomegranate juice.

"This work adds to a growing body of evidence that diet and lifestyle factors may have an important influence on prostate carcinogenesis," said Howard Parnes, a physician at the National Cancer Institute.

Antioxidant-Rich Juice

Pomegranate extract would be a good compound to study in a human clinical trial, said Parnes, chief of the prostate and urologic cancer research group in the division of cancer prevention.

He noted that several other dietary substances including selenium, vitamin E, lycopene (a tomato-based compound), green tea and soy protein already are being tested in clinical trials funded by the cancer institute, which is part of the National Institutes of Health.

The UW study is the first published research to show that the antioxidant-rich juice of pomegranates might retard the growth of prostate cancer tumors.

However, researchers cautioned that the findings, published in the Proceedings of the National Academy of Sciences, represent only laboratory and animal research. Before pomegranates can be recommended, the fruit's effectiveness will have to shown in a human clinical trial.

May Prevent Heart Disease

Still, pomegranates have been studied extensively in recent years, with earlier findings suggesting that the fruit may be beneficial in preventing a variety of conditions such as heart disease, cancer and even erectile dysfunction.

Earlier this month, a clinical trial involving 45 heart disease patients showed that drinking one glass of the juice a day for three months improved blood flow to the heart. It is believed that pomegranate juice inhibits the build-up of plaque in arteries.

"Their heart disease began to reverse," said senior author Dean Ornish, founder of the Preventive Medicine Institute and a clinical professor of medicine at the University of California, San Francisco.

Ornish's study, which was published in the American Journal of Cardiology, was funded by a foundation associated with a pomegranate juice company.

Ornish, who also has done research on the effects of diet and lifestyle on prostate cancer, said the UW study suggests that a prostate cancer human trial is needed. He noted that pomegranate juice is a low-risk, low-cost substance.

"The only side effects of pomegranate juice are good ones," he said. "If it were a drug, people would be rushing to study it."

Genetically Programs Cancer Cells to Die

For the UW study, pomegranate juice first was tested to see if it would inhibit the growth of human prostate cancer cells in a lab dish.

"We found the cells grew slower [by 50 percent to 60 percent]," said senior author Hasan Mukhtar, a professor of dermatology in the UW Medical School.

Next, they tested pomegranate juice on mice in which prostate cancer cells had been injected.

Each day, the mice were fed water that had been supplemented with either 0.1 percent or 0.2 percent pomegranate juice. That is roughly the equivalent of a person drinking either 8 or 16 ounces of pomegranate juice a day, Mukhtar said.

Compared with mice that drank just water, there was a significant slowing of the cancer cells in the mice that got the highest concentration of pomegranate juice. In addition, PSA levels dropped.

Mukhtar said pomegranate juice appears to trigger a process that genetically programs cancer cells to die.

Mukhtar's study was sponsored by the US Public Health Service. His early work, but not Tuesday's study, has received funding from a pomegranate juice company.

Rich in Phytochemicals

Early this year, researchers at the University of California, Los Angeles, showed that drinking pomegranate juice substantially increased the time that it took for PSA levels to double in a group of men with prostate cancer who had undergone surgery or radiation.

The study, which was sponsored by a pomegranate juice company and presented at a urology meeting, involved 48 men. By drinking 8 ounces of pomegranate juice a day, their PSA doubling time increased to 37 months, compared with 15 months before they began drinking the juice.

"The quicker the doubling time, the faster the spread of the cancer," said co-author Allan Pantuck, an assistant professor of urology at UCLA.

Pomegranate juice has intrigued researchers because it is rich in so-called phytochemicals, substances found in plants that might have healthy properties.

Pomegranate juice has more antioxidant activity than red wine and green tea, substances that also show promise in disease prevention.

The fruit is rich in two kinds of antioxidants -- polyphenols, which give the fruit its color, and tannins, a bitter substance in plants that discourages consumption by animals and pathogens. Packaged pomegranate juice has a dry, tart taste.

Gary Stoner, a professor of internal medicine at Ohio State University, said he was not surprised by the findings in the UW study.

"There are a lot of strong antioxidants [in pomegranates]," said Stoner, who does research into natural substances that prevent cancer.

Stoner, who was not part of the study, said the results provide a basis for doing a small clinical trial with prostate cancer patients.