Studies have not established the effect of Brazil nuts on breast cancer

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Effect of Brazil nuts on breast cancer not established
In general, tree nuts are dietary sources of unsaturated fatty acids, thiamin, vitamin E, zinc, copper, fiber and various phytochemicals. However, the phytochemical content of Brazil nuts appears to be low compared to walnuts and other healthful nuts. Brazil nuts contain the highest percentage of selenium of any food source. Most of the recommendations for consuming Brazil nuts are based on this unique property. Epidemiological studies that isolate the possible impact of Brazil nut consumption on the risk of cancer for U.S. or European populations are not available. However, there have been many studies of the relationship between selenium consumption and cancer risk and these have had widely varying and inconsistent results. We present some of the available study findings here.
The preliminary results of a major U.S. study, the Selenium and Vitamin E Cancer Prevention Trial [SELECT], indicated that neither selenium nor vitamin E, alone or in combination, reduced the risk of prostate cancer. In response, one respected observer commented that the source of the selenium supplement, L-selenomethionine, and the relatively high initial levels of selenium in the enrolled men of the SELECT study, may have contributed to this finding. One Australian study found that selenium levels in the blood were inversely associated with the risk of common skin cancers (basal cell and squamous cell carcinoma). A large European study found that plasma selenium concentration was not associated with prostate cancer risk. Another major U.S. population study found that increasing serum selenium levels were associated with decreases in all-cause mortality up to 130 ng/mL. However, the relationship was not linear and higher serum selenium levels appeared to be associated with increased mortality. Long-term selenium supplementation appears to increase the risk of type 2 diabetes, according to another U.S. study. A significant inverse relationship between serum selenium in recent smokers and the risk of a colon cancer precursor was found in another study. An outright deficiency in selenium was found to accelerate prostate cancer progression in a mouse model in another study. Our tentative conclusion is that these study results imply that while a threshold base level of selenium is necessary to help prevent cancer, increasing selenium intake above this threshold amount is not necessarily beneficial and could actually promote cancer.

Breast cancer-related effects of eating Brazil nuts

One U.S. study found little association between levels of selenium in breast tissue and breast cancer risk. Another study reported that selenium supplementation could reduce the number of DNA breaks typically associated with mutations in BRCA1 carriers. Other studies have found that selenium disrupts estrogen signaling by altering estrogen receptor expression and ligand binding in human breast cancer cells, but it is not clear how these results can be used therapeutically. Numerous population studies have failed to find a negative relationship between risk of breast cancer and pre-diagnosis toenail selenium levels (in fact, a few studies have hinted at a possible positive association between high levels of selenium and the risk of breast cancer).
However, several studies have found that selenium can enhance the anti-cancer activities of Adriamycin and tamoxifen. Selenium levels typically decline after breast cancer diagnosis, but this appears to be a result of the disease itself, according to one study.

Additional comments

Selenium is a mineral that is essential to health but needed only in small amounts. The recommended daily value for selenium is 70 mcg; amounts above 400 mcg can cause selenium toxicity over time (symptoms include garlic breath odor, hair loss, white blotchy nails, irritability, fatigue, gastrointestinal upset, and mild nerve damage.) Most people residing in the U.S. receive the recommended amount of selenium in their diets. This is not the case in some other countries (e.g., parts of China and Russia), where selenium deficiencies can cause osteoarthropathy, enlarged hearts, and mental retardation due to a form of hypothyroidism.
Brazil nuts contain moderately high levels of copper, which could contribute to angiogenesis and metastasis of breast cancer, especialy in women with inflammatory breast cancer (IBC) or triple negative (ER-/PR-/HER2-) disease.
Below are links to recent studies concerning this food. For a more complete list of studies, please click on brazil nuts

Selected breast cancer studies