23 Mar 2010 --- Scientists in California are reporting for the first time that walnuts — already renowned as a rich source of omega-3 fatty acids that fight heart disease reduce the size and growth rate of prostate cancer in test animals. They described their findings at the 239th National Meeting of the American Chemical Society (ACS), being held here this week.

"Walnuts should be part of a prostate-healthy diet," said Paul Davis, Ph.D., who headed the study. He is with the University of California-Davis. "They should be part of a balanced diet that includes lots of fruits and vegetables."

More than 190,000 men in the United States will get a diagnosis of prostate cancer in 2010, making it the most common non-skin cancer. It claims about 27,000 lives annually. Evidence suggests that diet is among the largest factors that influence a man's risk for developing prostate cancer. Studies suggest that tomatoes and pomegranate juice, for instance, may reduce the risk.

Davis and colleagues note that walnuts are a rich source of healthful substances, including omega-3 fatty acids found in more expensive foods like salmon; gamma tocopherol (a form of vitamin E), polyphenols, and antioxidants. The scientists recently showed that walnuts could help fight heart disease by reducing levels of endothelin, a substance that increases inflammation of blood vessels. This effect was in addition to walnuts reducing levels of "bad" cholesterol (low-density lipoprotein cholesterol, or LDL) in the blood. Knowing that people with prostate cancer have elevated levels of endothelin, the scientists decided to test whether eating walnuts could be beneficial in prostate cancer.

"We decided to use whole walnuts in the diet because when a single component of a food linked to cancer prevention has been tested as a supplement, that food's cancer-preventative effects disappear in most cases," Davis said.

The scientists fed lab mice genetically programmed to develop prostate cancer the equivalent of about 2.5 ounces of walnuts per day equivalent to 14 shelled nuts for 2 months. A control group of mice got the same diet except with soybean oil. The walnut-fed mice developed prostate cancers that were about 50 percent smaller than the control mice. Those cancers also grew 30 percent slower.

The scientists reported that the walnut-fed mice had lower levels of insulin-like growth factor-1. High levels of the protein may increase the risk of developing prostate cancer in the first place. In an effort to understand what walnuts were doing, the scientists used gene chip technology to look for changes in gene levels in the tumor itself as well as the mouse's liver. They found that walnuts also had large, beneficial effects in both tumor and liver on genes that have been shown to be involved in controlling tumor growth, the scientists noted.