Mice that consumed approximately 14 shelled walnuts per day for two months exhibited significantly smaller, slower-growing prostate tumors after 18 weeks than did mice that ingested an equal amount of fat from soybean oil instead of walnuts.

Nutritionist Paul Davis, PhD, of the University of California-Davis Cancer Center, and colleagues reported their findings at the 239th National Meeting of the American Chemical Society in San Francisco.

The two groups of mice were genetically programmed to develop prostate cancer. The tumors that developed in the walnut-fed mice were about half the size of those in the other mice, and grew about 30% more slowly.

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The walnut-fed mice also had lower levels of insulin-like growth factor-1, high levels of which may increase the risk of developing prostate cancer.