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.

堅果含有多種成份如：omega-3脂肪酸，抗氧化劑和植物脂醇類，這些都可能降低疾病風險。

每天喂食與人同等的兩盎司（56.7克）核桃的老鼠，其患的腫瘤不僅數目更少且腫塊體積也更小。該研究發現是在美國腫瘤研究協會的年度會議上提出的。馬歇爾大學醫學學院的研究人員伊萊恩·哈德曼博士說盡管該研究在老鼠身上發現核桃有益防癌，但今后同樣會應用到人類身上；她還說：“我們知道健康的日常飲食總的來說是可以預防各種慢性疾病的。”“現在很明顯，核桃是一種健康食品，是可以降低乳癌發病率的。”

此前所作的研究就已表明每頓飯后吃幾顆核桃可有助于修復油膩食物對動脈造成的傷害。

堅果一直被人認為富含可減緩動脈硬化、保持動脈暢通的復合物。

在最新的一項研究中，研究人員給老鼠同時喂食了標準食物和帶核桃的食物。結果顯示喂食了核桃的老鼠，腫瘤發生數較少，腫塊體積也較小；即便真的出現了，腫瘤也要經過較長的一段時間才會長大。分子分析師說這是omega-3脂肪酸關鍵性作用的結果，但也不排除其他成份的貢獻。