Increased intake of soy isoflavones may reduce the risk of localized prostate cancer by up to 50%, according to a Japanese study, which also found high intake of soy compounds may worsen advanced prostate cancer.
The study published in a recent issue of Cancer Epidemiology Biomarkers & Prevention was the first prospective study to establish an inverse association between isoflavones and localized prostate cancer in a Japanese population, according to the researchers.
Many early studies have demonstrated a preventative effect of soy isoflavones against prostate cancer in animal models, but the results from epidemiologic studies are inconsistent.
One study conducted by Jacobsen BK and colleagues from the Institute of Community Medicine, University of Tromso, Norway found that those with high consumption of soy milk were at reduced risk of prostate cancer.
The study published in the Dec. 1998 issue of Cancer Causes Control involved 225 incident cases of prostate cancer among 12, 395 California Seventh-Adventist men who reported their status of soy milk consumption in 1976.
The researchers found consuming soy milk more than once a day was associated with a 70 percent reduced risk of prostate cancer. The association held true even after other factors were included in the analysis.
Another study by Lee MM and colleagues from the Department of Epidemiology and Biostatistics at the University of California - San Francisco also showed that consumption of soy foods and isoflavones was associated with reduced risk of prostate cancer.
The study published in the July 2003 issue of Cancer epidemiology, biomarkers & prevention found those with their intake of tufu, genistein, or daidzein individually in the highest tertile were more than 40% less likely to have prostate cancer than those with their intake in the lowest tertile.
In the current study of 32,509 Japanese men ages 45 to 74, the researchers from The Japanese National Cancer Center analyzed data from 307 cases of diagnosed prostate cancer between 1995 and 2004 among which 74 cases were advanced, 220 cases were organ localized and 13 cases were unclassified in terms of their stages.
Overall, consumption of genistein, daidzein, miso soup, and soy food were not associated with reduced risk of total prostate cancer, but was linked with reduced risk of localized prostate cancer.
The association was stronger among men who were at 60 or older in whom the prostate cancer risk was reduced by consumption of by isoflavones and soy food in a dose-responsive matter. That is, the more soy food they ate, the lower their prostate cancer.
The results showed that highest quartile intake of genistein, daidzein, and soy food was each associated with a 50% reduction of localized prostate cancer.
Soy consumption is higher in Asia than in the West. And soy consumption was inverse associated with risk of prostate cancer. Because of this, many researchers believe the association between say and cancer may be real.
However, many of those studies associating soy food consumption and lower risk of prostate cancer did not intend to reveal a causal relation between the two. It is well known that in the countries where men eat high amounts of soy people also eat much less animal foods such as meat and dairy products.
One possibility that can not be excluded is that the seemingly protective effect of soy food against prostate cancer may be actually result from the low consumption of meat and dairy or other risk factors.
Further studies are needed to clarify how soy products would affect the risk of prostate cancer.