According to a recent study, 2 oz. of walnuts per day may slow breast cancer growth. Walnuts contain many potent chemicals such as the anti-inflammatory omega 3 fatty acids, phytosterols, polyphenols, carotenoids, and melatonin. In this study, human breast cancer cells were implanted into mice until the tumors reached 3-5mm in diameter. Then one group was given the human equivalent of 2 oz of walnuts and that significantly slowed the growth of the cancer cells. While the walnuts did not kill the cancer, it is certainly a good idea to consider walnuts in an overall breast cancer prevention or treatment strategy. Please note that if you have insomnia, taking walnuts in the evening might be helpful given the melatonin content. Also melatonin has been shown to slow breast cancer as well, perhaps that is one of the mechanisms.
 
Nutr Cancer. 2008;60(5):666-74