A cup of walnuts holds 117 percent of the fat you need in a day. This fact prompted Paul Davis, a professor at UC Davis (no relation), to approach the California Walnut Board for funding. Davis (the man, not the university) wanted to feed the nuts to lab mice.
The research nutritionist had seen studies that connected fatty diets and prostate cancer. And he knew federal guidelines recommend we limit our fat intake to less than 30 percent of our calorie intake. But, he says, he’s also “convinced, given the epidemiological evidence, that the Mediterranean diet, which is high in nuts and high in fat, is one of the best ones around.” He was confounded by the disconnect.
So he took mice that had been genetically engineered to develop the disease and divided them into three groups. One batch ate a high-fat diet that included 155 grams per day of ground walnuts. Another group was fed a comparable amount of fat in the form of soybean oil; while a third received a low-fat diet that also included soybean oil. All other nutrients were identical.
The mice were killed at 9, 18, and 24 weeks; upon examination, researchers found that prostate tumors in the walnut-fed mice were smaller, and tumor growth rate was much slower, than in the other two groups. The walnut-fed mice also had lower levels of “insulin-like hormone growth factor 1,” which has been linked to cancer.
The finding, reported in January in the British Journal of Nutrition, is the first to spell out the effects of walnuts on prostate cancer in animals. The benefits are not due to a particular chemical compound in walnuts, he adds. “It’s a combination of effects” from a whole piece of food.
“It’s not the magic bullet,” he says, “but it’s part of the magic fusillade.”