Epidemiology studies indicate that some populations have lower incidences of cancer (1
), the challenge is to identify the foods that contribute to reducing cancer risk given the complex compositions of whole foods. Another challenge in all dietary studies is to determine whether it is the addition of a beneficial component or the subtraction of a detrimental component that provided the benefit against cancer.
The primary aim of this study was to assess the effects of walnut consumption on mammary gland cancer risk. The results presented herein indicate that walnut consumption could significantly alter expression of multiple genes and mammary gland cancer development. Walnut consumption by both mother and offspring, as would naturally occur when walnuts are consumed as part of the usual diet of a population, did significantly reduce mammary gland cancer development and reduced multiplicity in this transgenic mouse. Consumption of walnut after weaning, as might occur when children migrate to a different environment or choose to add walnut to their diet, also slowed mammary gland cancer development and showed a 40% reduction in tumor incidence and a 44% reduction in multiplicity compared to mice not exposed to walnut. The next question is: What component of walnut was effective at slowing carcinogenesis?
Our first hypothesis was that the increased omega 3 content and decreased omega-6 content of the diet-reduced carcinogenesis. Long-chain omega-3 fatty acids have been shown to slow breast cancer growth in multiple animal studies (7
) and have been proposed for cancer prevention (30
). Conversely, omega 6 fatty acids, especially linoleic acid as found in corn oil, have been shown to increase carcinogenesis (31
). In order to maintain balanced fat in the diet, if one species is increased (α
-linolenic) then another (linoleic) has to decrease. We had data from another recently completed study (14
) that provided information that indicated that increasing the α
-linolenic content contributed to reduced mammary gland cancer risk. After doing the calculations we realized that the canola oil and walnut diets contained the same amount of omega-3 in the form of α
-linolenic acid. The canola oil containing diet did significantly suppress tumorigenesis compared to the corn oil containing diet; however, the walnut diet resulted in an additional significant suppression of tumorigenesis. Thus, some component of the diet, in addition to the increased α
-linolenic content (or reduced linoleic acid), was functioning to suppress carcinogenesis.
There are reports that β
-sitosterol can suppress cancer cell growth (34
) and walnuts contain a significant amount of β
). However, in assessing the β
-sitosterol composition of the diets (using composition of the diet and Ref. 15), we found that the β
-sitosterol content for the corn oil diet was 968 mg/kg; for the canola oil diet was 413 mg/kg; and for the walnut diet was 71 mg/kg, thus the total β
-sitosterol content of the walnut diet was less than that of the corn oil diet. Most of the references for benefit of β
-sitosterol against cancer do not use breast cancer cells, which might be stimulated by binding of β
-sitosterol to the estrogen receptor, but use other cell types that are not usually considered estrogen dependent. Interestingly, we did find 1 paper (9
) indicating that β
-sitosterol may increase breast cancer cell growth in estrogen responsive cells; this would agree with our data. The tumors formed by this C3(1)/SV40 TAg model have been well characterized (16
). Green et al. reported that
Atypia of the mammary ductal epithelium develops at about 8 wk of age, progressing to mammary intraepithelial neoplasia (resembling human ductal carcinoma in situ) at about 12 wk of age with invasive carcinomas at about 16 wk in 100% of female mice. The tumors appear hormone responsive at early stages, invasive carcinomas are hormone independent, which corresponds to loss of ERα
during progression. (16
) (p. 1020)
Tumors were evident sooner in the mice that consumed the corn oil diet, perhaps in response to the β-sitosterol stimulation of ERα. At later times, when tumors became invasive and should be ERα-, the growth of tumors of mice that consumed the corn oil diet was not as influenced by corn oil diet. More research will be needed to decide this question.
Assessment of vitamin E in the diets did provide a lead to an additional active component that was common to the diets. We found the gamma tocopherol is associated with slowing cancer cell growth (38
) whereas alpha tocopherol did not have benefit against cancer and may block some of the activity of gamma tocopherol. Assessing the tocopherol content of the diets revealed that for the corn oil diet, alpha tocopherol = 14.3 mg/kg and gamma tocopherol = 0; for the canola oil diet, alpha tocopherol = 17.46 mg/kg, gamma tocopherol = 27.34 mg/kg; for the walnut diet, alpha tocopherol = 1.77 mg/kg and gamma tocopherol = 22.9 mg/kg. The changes of alpha and gamma tocopherol are clearly in the direction that according to the work of others would indicate benefit against cancer. Studies could be devised to test this question.