Diets rich in whole walnuts or walnut oil may slow down prostate cancer growth, a new study has claimed.
Both walnuts and walnut oil reduced cholesterol and increased insulin sensitivity in mice, researchers at the University of California Davis found.
The walnut diet also reduced levels of the hormone IGF-1, which had been previously implicated in both prostate and breast cancer.
"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," said lead scientist and research nutritionist Paul Davis.
A previous study found that walnuts reduced prostate tumour size in mice; however, there were questions about which parts of the nuts generated these benefits.
Researchers used a mixture of fats with virtually the same fatty acid content as walnuts as their control diet.
The mice were fed whole walnuts, walnut oil or the walnut-like fat for 18 weeks.
While the walnuts and walnut oil reduced cholesterol and slowed prostate cancer growth, in contrast, the walnut-like fat did not have these effects, confirming that other nut components caused the improvements - not the omega-3s.
"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," Davis said.
"It's becoming increasingly clear in nutrition that it's never going to be just one thing; it's always a combination," said Davis.
While the study does not pinpoint which combination of compounds in walnuts slows cancer growth, it did rule out fibre, zinc, magnesium and selenium.
In addition, the research demonstrated that walnuts modulate several mechanisms associated with cancer growth.
"The energy effects from decreasing IGF-1 seem to muck up the works so the cancer can't grow as fast as it normally would," Davis said.
"Also, reducing cholesterol means cancer cells may not get enough of it to allow these cells to grow quickly," said Davis.
In addition, the research showed increases in both adiponectin and the tumour suppressor PSP94, as well as reduced levels of COX-2, all markers for reduced prostate cancer risk.
Although results in mice do not always translate to humans, Davis said his results suggest the benefits of incorporating walnuts into a healthy diet.
The study was published in the Journal of Medicinal Food.
