Eating a modest amount of walnuts -- about 2 ounces, or two handfuls, a day -- may protect against prostate cancer, U.S. researchers suggest. Senior author Russel Reiter of the School of Medicine at The University of Texas Health Science Center San Antonio and colleagues injected immune-deficient mice with human prostate cancer cells. Within three to four weeks, tumors typically start to grow in a large number of these mice, Reiter said. The study compared the mice that ate walnut-enriched diet versus a non-walnut diet. The study, published in the journal Cancer Investigation, found 3-of-16 mice eating the walnut-enriched diet developed prostate tumors, compared with 14-32 mice, or 44 percent, on the non-walnut control diet. Also, the final average tumor size in the walnut-fed animals was roughly one-quarter of the average size of the prostate tumors that developed in the mice eating the control diet, Reiter said. \"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,\" Reiter said in a statement. \"We were absolutely surprised by how highly effective the walnut diet was in terms of inhibition of human prostate cancer.\" The mice consumed a diet typically used in animal studies, except with the addition of a small amount of walnuts pulverized into a fine powder. \"The walnut portion was not a large percentage of the diet,\" Reiter said. \"It was the equivalent to a human eating about 2 ounces, or two handfuls, a day, which is not a lot of walnuts.\"