Researchers at UC Davis and other
institutions have found that diets rich in whole walnuts or walnut oil slowed
prostate cancer growth in mice. In addition, both walnuts and walnut oil
reduced cholesterol and increased insulin sensitivity. The walnut diet also
reduced levels of the hormone IGF-1, which had been previously implicated in
both prostate and breast cancer. The study was published online in the Journal
of Medicinal Food.
“For
years, the United States government has been on a crusade against fat, and I
think it’s been to our detriment,” said lead scientist and research
nutritionist Paul 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.”
Davis and colleagues have been investigating
the impact of walnuts on health for some time. A previous study found that
walnuts reduced prostate tumor size in mice; however, there were questions
about which parts of the nuts generated these benefits. Was it the meat, the
oil or the omega-3 fatty acids? If it was the omega-3 fats, the benefit might
not be unique to walnuts. Since the fatty acid profile for the soybean oil used
as a control was similar, but not identical, to walnuts, more work had to be
done.
In the current study, 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. The results replicated those from the previous study. 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.”
While the study does not pinpoint which
combination of compounds in walnuts slows cancer growth, it did rule out fiber,
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.”
In addition, the research showed increases
in both adiponectin and the tumor suppressor PSP94, as well as reduced levels
of COX-2, all markers for reduced prostate cancer risk.
Although results in mice don’t always
translate to humans, Davis said his results suggest the benefits of
incorporating walnuts into a healthy diet. Other research, such as the PREDIMED
human study, which assessed the Mediterranean diet, also found that eating
walnuts reduced cancer mortality.
Still, Davis recommends caution in diet modification.
“In our study the mice were eating the
equivalent of 2.6 ounces of walnuts,” he said. “You need to realize that 2.6
ounces 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.
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