Walnuts have the highest total phenolic content, flavonoid content, and antioxidant activity of all nuts commonly consumed in the U.S. Walnuts are a good source of alpha-linolenic acid (ALA, an omega-3 fatty acid), β-sitosterol, ellagic acid, pedunculagin, melatonin, and urolithin A, all of which have proven or suspected chemopreventive properties. Walnuts are also a dietary source of copper. Walnut consumption is associated with reduced risks of coronary vascular disease, type 2 diabetes, and blood clot formation, and has been shown to lower cholesterol.
Despite their high caloric and fat content, adding a moderate amount of nuts to the diet has been shown not to result in weight gain. Walnut extract has been shown to reduce the proliferation of liver cancer and colon cancer cells in a dose-dependent manner. Ellagic acid has been found to inhibit cell growth and induce apoptosis (programmed cell death) in cervical cancer cells.
Breast cancer-related effects of eating walnuts
Consumption of ALA is associated with reduced breast cancer risk. Walnuts are the best source of this plant-derived omega-3 fatty acid. ALA is converted during metabolism into eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which have been reported to have chemopreventive effects in numerous studies. For example, they have been shown to reduce lung metastasis in a mouse model of breast cancer and to inhibit the early stages of HER2/neu overexpressing (HER2+) breast cancer. However, the conversion of ALA can be blocked in the presence of vegetable oils with high omega-6 fatty acid content such as corn oil, safflower oil, soybean oil, sunflower oil, and grape seed oil. Therefore, walnuts and walnut oil should be consumed separately from such oils.
Walnut extracts and walnut compounds have been shown to inhibit the growth and proliferation of both hormone receptor positive (ER+/PR+) and triple negative (ER-/PR-/HER2-) breast cancer cells. Walnut consumption has been shown in several studies to significantly reduce breast tumor incidence and size in laboratory mice. For example, one study reported that a diet including walnuts reduced the size of tumors produced by injecting triple negative breast cancer cells into mice.
The melatonin in walnuts protects against breast cancer in several ways, for example by reducing aromatase activity within the breast, thereby reducing estrogen production. Melatonin has also been found to be effective in reducing tumor growth, cell proliferation, and angiogenesis in an animal model of triple negative (ER-/PR-/HER2-) breast cancer.
Walnuts should be consumed in moderation since they contain some copper (approximately 0.45 mg per ounce), which can promote angiogenesis. While copper is a vital nutrient, women with breast cancer should not exceed the RDA (recommended daily allowance) of approximately 0.9 mg.
Refined walnut oil contains essentially no copper. Cold pressed walnut oil is made from nuts that are dried and then cold-pressed, preserving their nutrient content. Refined cold pressed oil is best since refining helps eliminate copper. Walnut oil should be refrigerated and is best used at room temperature (e.g. in salad dressings) rather than in frying.
Pecans are related to walnuts but do not have the favorable omega-3 fatty acid and phenolic profile of walnuts.
Below are links to recent studies concerning this food. For a more complete list, including less recent studies, please click on walnuts.