Researchers say components of the walnut, not its omega-3 content, are conferring anti-prostate cancer health benefits.
The study, which is published in the Journal of Medicinal Food, notes that previous studies have suggested intake of tree nuts is linked with reduced cardiovascular disease risk factors as well as cancer.
A 2013 study, for example, showed that consuming walnut oil boosts blood vessel functioning and eating whole walnuts helps "good" cholesterol transport and remove extra cholesterol from the body more effectively.
Researchers from this latest study, led by scientist and research nutritionist Paul Davis, note that they have been assessing the health impacts of walnuts for quite a while.
One of their previous studies showed that walnuts reduced prostate tumor size in mice, but there were questions about which parts of the nuts were responsible for this effect - was it the meat, the oil or the omega-3 fatty acids? If it was the omega-3s, then the benefits could be found in any food type containing them.
"For years, the US government has been on a crusade against fat, and I think it's been to our detriment," says Davis. "Walnuts are a perfect example. While they are high in fat, their fat does not drive prostate cancer growth. In fact, walnuts do just the opposite when fed to mice."
Components of walnut, not omega-3s, are conferring benefits
To further examine which part of the walnut is responsible for its health effects, the team used a mixture of fats with the same omega-3 fatty acid content as walnuts for their control diet.
Fast facts about prostate cancer in the US
- Aside from non-melanoma skin cancer, it is the most common cancer among men
- In 2011, 209,292 men were diagnosed with the disease
- That same year, 27,970 men died from prostate cancer.
Mice were then fed either whole walnuts, walnut oil or the control fat diet for 18 weeks.
Results revealed that the walnuts and walnut oil lowered cholesterol and slowed prostate cancer growth, but the control fat diet did not, suggesting it is other components of the walnut, not the omega-3s, that are conferring these benefits.
"We showed that it's not the omega-3s by themselves, though, it could be a combination of the omega-3s with whatever else is in the walnut oil," says Davis. "It's becoming increasingly clear in nutrition that it's never going to be just one thing; it's always a combination."
The research also showed that walnut intake adjusts certain mechanisms linked to cancer growth, decreasing insulin-like growth factor 1 (IGF-1) - a growth hormone previously implicated in prostate and breast cancer - "so the cancer can't grow as fast as it normally would," notes Davis.
Another added benefit is that the reduced cholesterol deprives cancer cells of it, meaning they are not able to grow as quickly.
Walnut consumption was also linked to increases in adiponectin and the tumor suppressor PSP94 and decreases in COX-2, which are all markers for a reduced risk of prostate cancer.
'Unrealistically high walnut consumption levels not needed for benefits'
The researchers say their results suggest incorporating walnuts into a healthy diet could confer these health benefits, though they do caution that studies in mice do not necessarily translate to humans.
Still, the team says "walnut's beneficial effects on human cancer do not require unrealistically high consumption levels," which means we do not need to dramatically increase intake to get results.
"In our study, the mice were eating the equivalent of 2.6 oz of walnuts. You need to realize that 2.6 oz of walnuts is about 482 calories. That's not insignificant, but it's better than eating a serving of supersized fries, which has 610 calories.
In addition to the cancer benefit, we think you also get cardiovascular benefits that other walnut research has demonstrated."
It should be noted that this study was supported by research grants to the University of California-Davis from the American Institute for Cancer Research and the California Walnut Board. The authors state, however, that neither organization had any input into their study data analysis or the contents and conclusions of their research.
Medical News Today recently reported on a study published in the Journal of Alzheimer's Disease that suggested a walnut-enriched diet slows progression of Alzheimer's in mice.