New research indicates that eating a modest amount of walnuts can protect against prostate cancer. A walnut-enriched diet resulted in prostate cancers that were fewer and smaller.

Immune-deficient mice were injected with human prostate cancer cells. Within three to four weeks, tumors typically start to grow in a large percentage of these mice. The study found that 3 of 16 mice (18%) eating the walnut-enriched diet developed prostate tumors, compared with 14 of 32 (44%) on the nonwalnut control diet. Notably, the final average tumor size in the walnut-fed animals was roughly one-fourth the average size of the prostate tumors that developed in mice eating the control diet.

“We found the results to be stunning because there were so few tumors in animals consuming the walnuts and these tumors grew much more slowly than in the other animals,” said study senior author Russel Reiter, PhD, of The University of Texas Health Science Center San Antonio. “We were absolutely surprised by how highly effective the walnut diet was in terms of inhibition of human prostate cancer.”

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The study, published in Cancer Investigation (2013; doi: 10.3109/07357907.2013.800095), involved feeding the mice a diet typically used in animal studies, except with the addition of a small amount of walnuts. The walnuts were pulverized into a fine powder to prevent the animals from eating only the walnuts. The walnut portion of their diet was equivalent to a human eating about 2 ounces, or two handfuls, a day, which is not a lot of walnuts.

Previous research by the study’s co-author showed fewer and smaller tumors among walnut-fed mice injected with human breast cancer cells.

“The data to date suggest that using walnuts on a regular basis in the diet may be beneficial to defer, prevent, or delay some types of cancer, including breast and prostate,” said Reiter.