Brazil nuts (Bertholletia excelsa) are a significant source of copper, fiber, magnesium, manganese, thiamin, and unsaturated fatty acids. They are also a good source of vitamin E and zinc. Brazil nuts contain far more selenium than any other nut (or other food). Their phytochemical content appears to be low compared to most other nuts.
In addition, Brazil nuts incorporate a higher fraction of saturated fats than other nuts. Most of the recommendations for consuming Brazil nuts are based on their uniquely high selenium content.

Cancer-related effects of eating Brazil nuts

Epidemiological studies isolating 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. Unfortunately, these have had widely varying and inconsistent results, as described below.

Selenium and cancer

The Selenium and Vitamin E Cancer Prevention Trial [SELECT], a major U.S. study, reported 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), as well as the relatively high initial levels of selenium in the enrolled men of the SELECT study, may have contributed to the 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 reported that plasma selenium concentrations were 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.
Taken together, these study findings imply that while a threshold base level of selenium is necessary to help prevent cancer, increasing selenium intake above this threshold is not necessarily beneficial and could actually promote cancer.

Breast cancer-related effects of eating Brazil nuts

Higher intakes of tree nuts have been linked to lower breast cancer risk, and ER- breast cancer in particular. However, not all studies are in agreement and Brazil nuts are also different enough from other nuts that this finding might not hold for them.
There are no available epidemiological studies that isolate the possible impact of Brazil nut consumption on breast cancer risk for U.S. or European populations. Nor are there many animal studies concerning Brazil nut intake and mammary tumor development. Any conclusions regarding the breast cancer-related effects of Brazil nut consumption depend for the most part on the micronutrient content of the nuts.

Animal studies

A 2023 study reported that mice given either selenium-rich Brazil nuts or selenomethionine supplementation both had reduced tumor volume in comparison to mice fed a selenium-adequate diet.
A 1993 study found that rats fed a diet including ground Brazil nuts experienced some protection against carcinogen-induced mammary tumors, a result which the authors ascribed to the selenium content of the nuts.

Selenium and breast cancer

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.
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 breast cancer risk). A 2022 Swedish study reported that selenium appears to be associated with reduced breast cancer risk only in women with a particular genetic makeup. This could explain the inconclusive results reported in epidemiological studies.
On the other hand, several studies have reported that selenium can enhance the anti-cancer activities of Adriamycin, Herceptin 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.

Copper and breast cancer

Brazil nuts should be consumed in moderation since they contain relatively high levels of copper (approximately 0.49 mg per ounce), 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. While copper is a vital nutrient, women with breast cancer should not exceed the Recommended Dietary Allowance (RDA) of approximately 0.9 mg.

Bottom line

One Brazil nut incorporates between 68 and 96 micrograms of selenium, which is above the RDA of 55 micrograms daily. Although the optimal level for breast cancer patients and survivors may be somewhat higher than the RDA, intake should fall well below the Tolerable Upper Intake Level (UL) of 400 micrograms per day for selenium. Note that one Brazil nut also contains approximately one-tenth of the copper RDA. Therefore, the selenium rather than the copper content is the limiting factor in determining a safe level of Brazil nut consumption for most people.

Additional comments

Selenium, which accumulates in the body, is a mineral that is essential to health but needed only in small amounts. As noted above, the RDA for selenium is 55 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 could potentially contribute to breast cancer risk.
Below are links to recent studies concerning this food and its main components. For a more complete list of studies, please click on brazil nuts