Selenium is an important mineral for general health and is of particular importance in the prevention of cancer or its relapses.
I knew Brazil nuts are an easy high source of selenium and I wondered “Can people reliably and safely get their selenium from Brazil nuts?”
For those in a hurry, the answer is: It’s possible but not reliably.
Brazil nuts have very random amounts of selenium in them. From close to nothing at all, to 20 times the RDI* (Recommended Daily Intake), the RDI being ~65 microg/g for adults.
Variation of selenium content in Brazil nuts
The following three studies show just how random the content of selenium in Brazil nuts can be:
“The average and standard deviation and range of selenium concentrations in ppm, fresh weight for nuts from [the two] regions were, respectively, 3.06 ± 4.01 (0.03–31.7) and 36.0 ± 50.0 (1.25–512.0).” 
Another study looked concluded that “concentrations were highly variable (median: 13.9 microg/g; range: 0.4-158.4 microg/g). ”
In yet another study, they wanted to compare selenium from nuts and from supplements. But in trying to measure the exact amount of selenium they wanted from nuts they faced “difficulties in analysing individual nuts” because the “first 10 nuts [were] ranging from 0.816 to 1390 microg Se/g”. 
Content variation, a widely ignored reality in nutrition and plant medicine
As I explained in my previous post about green tea, the content of specific compounds in organic products is known to vary greatly, while too often overlooked. The presence and wide usage of nutritional facts databases for fruits and vegetables is a good example of the lack of general understanding of this variation. With variations from nil to 20 times the RDI as is the case for Brazil nuts, you would expect a gigantic red asterisk next to virtually every number, on those websites that give you nutritional information for individual foods. This would remind the general public that the numbers vary, that they are averages of a wide range, that the quantifying was done on a single fruit/vegetable/nut or that the sample may not be representative of the real life consumption. However, such variations are the main reason for inconsistencies in efficiency or safety of herbal products in herbal medicine. Many health-conscious people today would rather get their nutrients from food and heal from using plants rather than from supplements and drugs respectively. The good thing however about (reliable brands of) supplements is that unlike in foods and plant medicine, the dosage of active ingredient is measured, consistent and reliable to some extent.
Questioning the reliability and safety of Brazil nuts as a selenium source
Given the tremendous variations reported about selenium, it is fair to address the following questions:
- How does the variation in selenium content affect blood selenium?
- Is there a risk in having even a single small intake of the nuts highest in selenium?
- Supposing we vary sources, does nuts selenium (and therefore blood selenium) average to healthy levels?
This is all to answer the question: Are Brazil nuts a safe and reliable method of ensuring a healthy intake of selenium?
Note to readers: The study annotated “” is led by a French team that has published several papers on selenium and Amazonian populations and may be the first most relevant direction for further study on the subject.
UPDATE (2018): Bio-variations in food are common, often wide, and are generally compensated for by our body’s generally great ability to regulate absorption depending on the varying concentration of nutrients in our diet. It is uncertain to which extent that applies to selenium.
I am due for some peer-reviewed reading on selenium toxicity, and plan to interview the French research team that are experts on selenium. It will be interesting to hear their take on selenium from natural foods.
In the meantime, my current position is that Brazil nuts are generally considered safe. For those who can eat nuts, it may be sensible to limit them to 1 Brazil nut a day. They undoubtedly provide more selenium than a diet without them. Many soils worldwide are deficient in selenium, New Zealand soils are for sure. As with all sensible positions, I reserve myself the right to update this position in light of new research or information gained on this matter.
* Not all reference intakes are created equal. Some are built on largely insufficient data if you care to dive deep in the 1000+ pages reference documents. Some also do not apply at all to people on exclusively or nearly exclusively plant-based diets.
 “Selenium content of Brazil nuts from two geographic locations in Brazil”, (Jacqueline C. Chang & al., 1994)
 “Elevated levels of selenium in the typical diet of Amazonian riverside populations”, Sci Total Environ. 2010 Sep 1;408(19):4076-84. doi: 10.1016/j.scitotenv.2010.05.022.
 Brazil nuts, an effective way to improve selenium status (Thomson & al, 2008)
 Selenium – New Zealand Nutrition Foundation