Genistein has been used in parts of the world as a complementary therapy for cancer. Studies in Japan have shown some benefit of the extract in inhibiting the growth of cancer cells and promoting their death.
Patients with less advanced prostate cancer who want to avoid the side effects of prostate cancer treatment, such as erectile dysfunction and incontinence, sometimes choose to undergo watchful waiting instead. The cancer's progress can be monitored by measuring blood levels of PSA, which can reflect the progression of the cancer.
Reporting their findings at the 2003 meeting of the American Urological Association, Ralph deVere White, MD, of the University of California-Davis School of Medicine, and colleagues recruited 62 men with biopsy-proven prostate cancer and elevated PSA levels.
Patients took daily supplemental amounts of genistein during the study period. Forty-nine men completed the study, nine of whom chose watchful waiting for their disease. The remaining men underwent surgery, radiation, or hormone therapy.
The researchers found that the dietary supplement did not help reduce PSA levels in men who had undergone surgery, radiation, or hormone therapy for their prostate cancer.
However, in six of the nine watchful waiting patients, the researchers found that genistein reduced their six month PSA levels to before-treatment levels.
As for why a difference existed between the groups, patients undergoing watchful waiting may have had more low-risk disease than those undergoing treatment, the authors suggest. Alternatively, in watchful waiting patients, genistein may become concentrated in the prostate tissue (which is removed when patients are treated). "But further research is needed to determine this," they note.
"This study must be interpreted cautiously because the numbers of men enrolled are small," says deVere. "However, the findings do stimulate us to do a larger, placebo-controlled trial in patients who are on watchful waiting."
Jin-Rong Zhou, PhD, a soy expert with Harvard Medical School in Boston, tells WebMD that taking an individual component of soy, such a genistein, might be less effective than increasing the overall soy intake in the diet.
Dietary sources of soy include tofu, soy beans, and soy milk.
"Some basic research is still required to identify the active fractions of soy and their optimal doses for patients with specific disease stages," he says. He also notes that several studies suggest that increased consumption of soy is related to a decreased prostate cancer risk, but studies using different fractions or supplements of soy have had contrasting results.
No side effects have been associated with increased soy intake, although people should avoid taking "super-mega doses," of it, he advises. "And unless a particular soy supplement has been verified in animal and clinical studies, I'd not recommend taking supplements," he says.
The study was funded by Amino Up Chemical Co. Ltd of Sapporo, Japan, a genistein supplement manufacturer.