Walnuts may inhibit prostate cancer, study says

By Jasminee Sahoye

A study is examining the potential health benefits of walnuts and walnut oil.
A study is examining the potential health benefits of walnuts and walnut oil.

As efforts are being made this month for Movember to raise funds for prostate cancer and men’s health, a new study done with mice by researchers at the University of California-Davis suggests diets rich in walnuts or walnut oil could slow prostate cancer growth.

Prostate cancer is one of the leading types of cancer in males (above 60 years). It is a malignant tumour that starts in the prostate gland and usually progresses at a slow rate.

It generally causes no symptoms in its early stages but as the malignancy spreads, it causes urinary problems (pain or difficulty during urination) along with sexual dysfunction. In most cases, prostate cancer is detected incidentally in the early stages by means of a screening test followed by a biopsy for confirmation.

The study in the Journal of Medicinal Food and led by scientist and research nutritionist Paul Davis, notes that the researchers have been assessing the health impacts of walnuts for quite a while.

One of their previous studies showed that walnuts reduced prostate tumour 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 U.S. 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.”

The team in further examining which part of the walnut is responsible for its health effects used a mixture of fats with the same omega-3 fatty acid content as walnuts for their control diet.

Mice were fed 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 confer these benefits.

The study could not identify which compounds in walnuts slow the cancer growth but researchers have ruled out fibre, zinc, magnesium and selenium.

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,” says Davis.

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.

Davis says in their 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.”

Another study published in the Journal of Alzheimer’s Disease suggested a walnut-enriched diet slows progression of Alzheimer’s in mice.

While walnut has been discovered to lower cholesterol, it appears that consuming whole walnuts and walnut oil also reduces the risk of cardiovascular disease in other ways.