A just-published research study by Urological Sciences Research Foundation may help explain a quandary that has perplexed medical scientists for decades: why is prostate cancer so much more common in the West than in Asia?

Diet has long been chief suspect for the difference. The new study compared dietary influences on prostate tissues of Japanese men who spent their life in Japan versus other Japanese men who spent their life in the United States. The Western diet, relatively rich in animal fat and poor in soy, was found to exert cancer-causing influences that could be traced directly into prostate tissues.

"This study is important because it's among the first to look directly at tissues, in an attempt to link diet and cancer," commented Alan Partin of Johns Hopkins University. The side-by-side tissue comparison appears in the medical journal Urology and is the product of a five-year collaboration between U.S. and Japanese scientists.

Diet long has been thought to influence development of prostate (and other) cancers that are common in Western countries and rare in Asia. As much as a 10-fold greater occurrence of prostate cancer in the U.S., compared to Japan and other Asian countries, has been observed. When Asian men migrate to Western countries and adopt a Western lifestyle, the protection begins to disappear within one generation.

Now a linkage between diet and cancer appears to be explained at the tissue level. "Oxidative damage from saturated fat (in Western diet) and a protective effect from soy phytoestrogens (in traditional Japanese diet) are possible mechanisms," the authors write. The authors speculate that if further research confirms these findings, "... dietary modification would become an important public health issue and interest in nutrition-related treatment methods might evolve."

"Cancer can be caused by both hereditary and environmental factors," according to principle author Leonard S. Marks, MD, of UCLA and Urological Sciences Research Foundation (USRF). Marks said, "We chose to study prostate cancer in men with the same genetics (all of Japanese descent), but with differing diets -- one Eastern and one Western -- to see if dietary differences translate into differing tissue effects." This does seem to be the case.

The mechanism of cancerous change appears to be different, determined by dietary influences on how fat is handled by the prostate tissues and how certain enzyme systems known as caspases operate within the tissues. Despite fundamental genetic homogeneity of the two groups, the genetic material (DNA) in the prostate cells was different between the two groups, suggesting the possibility of a gene-nutrient interaction.

The Japanese-American men in the study were all born in Los Angeles, Calif., or Hawaii, and their body composition was measurably different from those native Japanese men who had spent their lives in the Nagoya, Japan area. Tissue and dietary data were gathered by Marks and colleagues in Los Angeles with the help of area urologists George Yamauchi and Yuichi Ito, and in Nagoya, Japan, by Dr. Munekado Kojima. Tissue studies were performed at Johns Hopkins University and dietary studies at the UCLA Center for Human Nutrition.

The study was completed under the auspices of the UCLA Office for Protection of Human Subjects and was supported by grants from The Prostate Cancer Foundation of Santa Monica, Calif., and the Pardee Foundation of Midland, Mich.