Walnuts slow prostate cancer growth

They’re no cure and active constituent(s) remain unknown

SAN FRANCISCO A new study suggests that mice with prostate tumors should say “nuts to cancer.” Paul Davis of the University of California, Davis, hopes follow-up data by his team and others will one day justify men saying the same.

PARKED...AND MAKING MONEY: Even in the San Diego convention center, this hybrid car was able to help out the grid, Janet Raloff reports in her latest blog. Credit: T. Siegfried
HEART HEALTHY TOO UC Davis researchers tested these nuts on prostate cancer after walnuts proved beneficial to the heart. iStockPhoto

For years, this nutritionist had been studying heart benefits of walnuts. Most nuts – in sensible quantities – can benefit the heart. But among walnuts’ special attributes were their ability to fight inflammation, an underlying cause of much heart disease, and to allow vessels to dilate as needed, which should limit unhealthy blood pressure hikes. But inflammation plays a role in many cancers. And Davis notes that walnuts can tinker with production of endothelin, a protein not only related to blood pressure control but also to helping regulate prostate growth.

So Davis and his colleagues decided to test walnuts in a mouse model of prostate cancer.

The line of rodents they used are genetically programmed to spontaneously develop prostate cancer. When fed what is for mice a normal quantity of fat – five percent of calories – the tumors grow slowly.  But bump the fat content of their diet up to a whopping 20 percent of calories and tumor growth mushrooms. Except if that 20 percent of fat calories comes from walnuts, Davis reported this week at the American Chemical Society spring national meeting.

Mice getting the high-fat walnut diets developed the same type of tumors seen in all animals from this cancer-prone strain. Their cancer just grew slowly, similar to the rate seen in animals downing a low-fat diet.

The researchers began supplementing the diet of some 8-week-old rodents with ground up walnuts. The rest got low- or high-fat diets where the source of that fat came from soybean oil. In all other respects, the animals dined on similar chow.

But 18-weeks into the feeding trial, those in the walnut group were exhibiting a tumor mass 30 to 40 percent smaller than in animals on the high fat diet. Six weeks after that, at the end of the experiment, the walnut group was still trending toward having a somewhat reduced tumor mass, relative to the high-soy-oil group, although the difference was no longer statistically significant, Davis reported.

He notes that “We saw reductions in several different mediators [of cancer growth] that are present in the bloodstream.” He was referring to insulinlike growth factor-1, or IGF-1, and tissue plasminogen activator – “both of which, when they’re high, are bad prognostic indicators of prostate cancer.” Indeed, he said, the reduction in these chemicals correlated with the lower prostate-tumor growth in the walnut-supplemented animals.

Davis’ group also used molecular probes to see if they could uncover biochemical signatures differentiating prostate tissues in the two high-fat groups. This gene-chip technology identifies which genes are turned on or off. And comparisons of the two groups turned up a host of differences that the scientists are now poring over. One characteristic change that has already emerged is a reduction of gene activity for IGF-1 in the walnut-supplemented mice.

When asked what constituents of walnuts might be slowing tumor growth, Davis said “I suspect it’s a combination of things.” He pointed to the omega-3 fatty acids, such as alpha linolenic acid, together with minerals, other trace nutrients – perhaps even some of the proteins. Other nuts might also prove beneficial, he allowed, but he has no plan to study them since he’s funded through the University of California by the state’s Walnut Board.

How many walnuts would a man have to consume for his intake to be roughly equivalent to what the mice downed? About 500 calories worth, Davis says – the energy content in a “MacDonald’s big fries.” Studies by others have indicated that additional foods might also fight prostate cancer, including tomatoes, pomegranates,selenium-rich foods and tea. “And I heard through the grapevine,” Davis adds, “that pistachios are being examined [for preventing prostate cancer].”

When someone asked Davis if he would recommend walnuts to men concerned about prostate cancer, he said “I don’t think it will do any harm.”

But remember not to expect a cure. At best, just a slowing in the growth of any cancers. And achieving that could be great, he noted, since most men will – if they live long enough – eventually develop prostate cancer. The goal, he says, is to find foods that might help ensure that when it comes to this malignancy, men “die with it, not of it.”

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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