Studies have not established the effect of Brazil nuts on breast cancer
While all tree nuts generally are good sources of unsaturated fatty acids, numerous phytochemicals, and fiber, the phytochemical content of Brazil nuts appears to be low compared to walnuts and other healthful nuts. However, 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 the latest available study findings here.
The March 2009 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 2009 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 published in December 2008 found that plasma selenium concentration was not associated with prostate cancer risk. Another major U.S. population study published in 2008 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 published in 2007. A significant inverse relationship between serum selenium in recent smokers and the risk of a colon cancer precursor was found in a 2006 study. An outright deficiency in selenium was found to accelerate prostate cancer progression in a mouse model in another 2006 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 2007 U.S. study found little association between levels of selenium in breast tissue and breast cancer risk. A 2005 study found 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 tamoxifen. Selenium levels typically decline after breast cancer diagnosis, but this appears to be a result of the disease itself, according to one study. Based on the available evidence, our conclusion is that smokers and carriers of BRCA1 or BRCA2 might benefit from having their serum selenium levels checked, with appropriate follow up under medical supervision. Assuming a sufficient level of selenium in the diet, consuming Brazil nuts in moderation is not likely to be helpful or harmful for those with, or at risk for, breast cancer.
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.
People with a history of oxalate kidney stones should limit or avoid Brazil nuts, since consumption of these nuts has been shown to increase urinary oxalate excretion.
Note that while we are continually searching for new evidence concerning this food, there is not much interest in it among breast cancer researchers so few recent studies are available.