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.

坚果含有多种成份如：omega-3脂肪酸，抗氧化剂和植物脂醇类，这些都可能降低疾病风险。

每天喂食与人同等的两盎司（56.7克）核桃的老鼠，其患的肿瘤不仅数目更少且肿块体积也更小。该研究发现是在美国肿瘤研究协会的年度会议上提出的。马歇尔大学医学学院的研究人员伊莱恩·哈德曼博士说尽管该研究在老鼠身上发现核桃有益防癌，但今后同样会应用到人类身上；她还说：“我们知道健康的日常饮食总的来说是可以预防各种慢性疾病的。”“现在很明显，核桃是一种健康食品，是可以降低乳癌发病率的。”

此前所作的研究就已表明每顿饭后吃几颗核桃可有助于修复油腻食物对动脉造成的伤害。

坚果一直被人认为富含可减缓动脉硬化、保持动脉畅通的复合物。

在最新的一项研究中，研究人员给老鼠同时喂食了标准食物和带核桃的食物。结果显示喂食了核桃的老鼠，肿瘤发生数较少，肿块体积也较小；即便真的出现了，肿瘤也要经过较长的一段时间才会长大。分子分析师说这是omega-3脂肪酸关键性作用的结果，但也不排除其他成份的贡献。