Walnuts have been linked to heart disease prevention
Eating walnuts may help to reduce the risk of developing breast cancer, research suggests.
The nuts contain ingredients such as omega-3 fatty acids, antioxidants and phytosterols that may all reduce the risk of the disease.
Mice fed the human equivalent of two ounces (56.7g) of walnuts per day developed fewer and smaller tumours.
The US study was presented to the American Association for Cancer Research annual meeting.
Researcher Dr Elaine Hardman, of Marshall University School of Medicine, said although the study was carried out in mice, the beneficial effect of walnuts was likely to apply to humans too.
She said: "We know that a healthy diet overall prevents all manner of chronic diseases."
"It is clear that walnuts contribute to a healthy diet that can reduce breast cancer."
Previous research has suggested eating walnuts at the end of a meal may help cut the damage that fatty food can do to the arteries.
It is thought that the nuts are rich in compounds that reduce hardening of the arteries, and keep them flexible.
In the latest study mice were either fed a standard diet, or the walnut-based diet.
The animals fed walnuts developed fewer tumours, and those that did arise took longer to develop and were smaller.
Molecular analysis showed that omega-3 fatty acids played a key role - but other parts of the walnut contributed as well.
Anna Denny, a nutrition scientist at the British Nutrition Foundation, said evidence for nuts reducing the risk of heart disease was currently stronger than it was for their anti-cancer properties.
She said: "Although nuts are high in fat (and thus calories), the fatty acids in nuts are predominantly 'good' unsaturated fatty acids.
"Other additional components of nuts that may contribute to a reduction in heart disease and cancer risk include fibre and 'bioactive' compounds.
"Among the many bioactive compounds found in nuts are phytosterols and flavonoids.
"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 to show that eating walnuts could prevent 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."