According to a new study eating walnuts may help reduce the risk of breast cancer due to the compounds they contain.

Walnuts contain ingredients such as omega-3 fatty acids, antioxidants and phytosterols that may all reduce the risk of the disease. Previous research indicated that the nuts are rich in compounds that reduce hardening of the arteries, and keep them flexible and has suggested eating walnuts at the end of a meal may help cut the damage that fatty food can do to the arteries.

The study involving mice specially created to develop tumors were fed the human equivalent of two ounces (56.7g) of walnuts per day and they developed fewer and smaller tumors than the mice that ate a regular diet. Molecular analysis showed that although omega-3 fatty acids played a key role, other parts of the walnut contributed as well.

Researcher Dr Elaine Hardman, of Marshall University School of Medicine, said, "These laboratory mice typically have 100 percent tumor incidence at five months; walnut consumption delayed those tumors by at least three weeks."

She added that "It is clear that walnuts contribute to a healthy diet that can reduce breast cancer." Although the study was conducted on mice Hardman said the beneficial effect of walnuts was likely to apply to humans too. "Walnuts are better than cookies, French fries or potato chips when you need a snack," Hardman said. "We know that a healthy diet overall prevents all manner of chronic diseases."

Anna Denny, a nutrition scientist at the British Nutrition Foundation, said nuts were better known for reducing the risk of heart disease than for their anti-cancer properties. "More research is needed before it will be possible to attribute specific health benefits of nuts to specific bioactive compounds because nuts contain a complex mixture of different bioactive compounds."

Josephine Querido, of the charity Cancer Research UK said there was insufficient evidence in favor of walnuts preventing breast cancer in humans. She said, "We know that a healthy balanced diet - rich in fruit and vegetables - plays an important part in reducing the risk of many types of cancer.

"The strongest risk factor for breast cancer is age - 80% of breast cancers occur in women over the age of 50 so attending screening is important.

"Making lifestyle changes, such as keeping a healthy body weight, limiting alcohol intake and taking regular exercise, can also help reduce breast cancer risk."

The US study was presented to the American Association for Cancer Research annual meeting and was funded with matching grants from the American Institute for Cancer Research and the California Walnut Commission.