In addition, an herbal product that was yanked off the market earlier this year also might be effective in fighting the disease.
As reported in the Nov. 6 issue of the Journal of the National Cancer Institute, research with garlic and other members of the allium group of vegetables showed men who consume more than 10 grams (one-third an ounce) of alliums a day were about half as likely to develop prostate cancer as men who ate 2 grams a day.
"This is only the second study showing that allium vegetables are associated with reduced (prostate cancer) risk," said lead researcher Ann W. Hsing, an epidemiologist at the National Cancer Institute in Bethesda, Md.
"The point is not that people should increase their intake of garlic and not pay attention to other components of their diet," Hsing told United Press International. "The best advice is still a balanced diet, regular exercise and to try to avoid obesity -- to have a healthy lifestyle."
Allium vegetable consumption is significantly higher in China than in the United States, she added.
Hsing interviewed 238 men with prostate cancer and 471 men without the condition about their diet five years prior to the interview, in order to avoid reporting changes in diet due to the development of the disease. She recommends further, prospective studies to follow men over time to determine which ones develop prostate cancer and whether there is an association with garlic and other alliums.
"There is a growing confidence that prostate cancer is effectively a disease of diet and lifestyle in our developed world," oncologist Bill Nelson, of the Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins University in Baltimore, told UPI.
For example, prostate cancers are historically rare among Asians who live their entire lives in Asia, Nelson said, but Asians who immigrate to the United States often develop a the same risk for the disease as American and western European men.
Allium vegetables are rich in flavonols and organosulfur compounds, which have been shown to fight tumors in laboratory studies.
"It may be more practical to focus future research on identifying the active anti-cancer components in these vegetables and using concentrated forms ... in future clinical trials for prostate cancer prevention and treatment," urologist William Aronson of the Jonsson Comprehensive Cancer Center at University of California, Los Angeles, told UPI.
A second study appearing in JNCI focused on a patented product called PC-SPES, which consists of eight herbs, seven of them from China. Although some studies have shown PS-SPES can reduce prostate cancer indicators -- even when the disease has metastasized -- this is the first study to identify the mechanism at the cellular level, researcher Peter Nelson told UPI.
"Some component in PC-SPES inhibited the synthesis of structures within cells called microtubules that are involved with the cell's support scaffolding structure," said Nelson, an oncologist at the Fred Hutchinson Cancer Research Center in Seattle. "Inhibiting the synthesis results in cancer cell death," he said.
However, the study also found PC-SPES can interfere with the therapeutic effects of paclitaxel, a commonly used drug for prostate cancer, so researchers concluded PC-SPES and paclitaxel should not be taken together.
Although the eight-herb mixture -- which includes chrysanthemum, licorice, saw palmetto and panax pseudo-ginseng -- has been shown to be effective, researchers still want to know how it works.
"Ultimately, what needs to be done is separate all the active PC-SPES components ... and then test them for anti-cancer activity," said H. Phillip Koeffler, chief of hematology-oncology at Cedars Sinai Medical Center in Los Angeles and also a member of the Jonsson Comprehensive Cancer Center. "Then the components would need to be combined in different mixtures to see which works best."
PC-SPES, which is associated with some increase in blood clotting, was taken off the market earlier this year when low levels of prescription blood thinners and certain hormones were found in the product. Other manufacturers now are producing a variation claimed to be similar in effect. Nelson said he would like to see PC-SPES come back on the market in an uncontaminated version.
(Reported by Joe Grossman, UPI Science News, in Santa Cruz, Calif.)