Brazil Nuts & Selenium: Are You Nuts If You Have More Than 1/Day? Putting the Toxicity Issue into Perspective

Brazil nuts & selenium: "How much is too much?" A weighty question.
"Isn't it dangerous to consume Brazil nuts on a regular basis?" This is probably one of the most common questions I have been getting in relation to posts about selenium.

In fact, the issue of selenosis (=toxically high selenium levels) from the intake of brazil nuts resurfaced only a couple of days ago in the comment area of one of the blogposts and since I am a lazy bast*** and unwilling to answer this question time and again, I decided to try and settle the issue once and for all by writing this post about the health hazardous effects of Brazli nuts...

Well, ok - maybe they are not so health hazardous, after all

Despite the fact that the evidence from human studies is not exactly extensive, the studies we do have would suggest that going nuts on brazil nuts is much safer than the hilarously high selenium content of Brazli nuts from certain regions in (you guessed it) Brazil would suggest (see table 1)

Now the data in table 1 alone should tell you that the question "how much is too much" is not an appropriate question to be asked. After all, you could eat almost 20x more bulk-shelled nuts in from the Secor study than shelled nuts from Autazes (Brazil).
Table 1: Mean, minimal and maximal selenium content (in µg/100g)
Instead of figuring out whether a handful of Brazil nuts a day is already too much, it does therefore appear to be more prudent to check whether eating plenty of Brazil nuts will bring about any general health problems. Luckily, Brazil nuts do not just originate from the Amazonian regions in Brazil, but are also traditionally consumed there.

"Toxically high" selenium levels in the blood and perfect health!?

In this respect the inhabitants of the Tapajós River region are a particularly interesting community to study, as their naturally high selenium levels in the blood (142 and 2447 μg/L in whole blood; Lemire. 2006, 2009; Pinheir. 2005) comes exclusively from local dietary selenium sources, such as Brazil nuts (Berthollethia excelsa), domestic chicken, game meat and certain fish species (Lemire. 2010) and not from drinking water, or industrial sources. Interestingly, even here...
Figure 1: Location of the study area
"[t]here are significant variations in Se status between villages and seasons, and the highest Se levels are among persons who consume large amounts of Brazil nuts, particularly during the Brazil nuts season, from December to April, when the mature nut capsules fall from the trees." (Lemire. 2011)
Accordingly the object of a 2011 study by Lemire and other researchers from Université Laval, the Université du Québec à Montréal, the Universidade de Brasília, the Universidade Federal do Rio de Janeiro and Universidade de São Paulo was to evaluate dermal signs and sentinel symptoms of Se toxicity in relation to plasma and whole blood biomarkers of Se status in the communities along the Tapajós River.
Figure 2: Precentage of study subjects showing hair- (left) or nail-related (right) symptoms of selenium toxicity (Lemire. 2011)
As you can see in figure 1 there is actually no difference as far as the occurrence of common hair- and nail-related signs selenium toxicity are concerned. The same was true for the occurance of garlic breath or dermal signs of selenium toxicity, such as a general irritation or mycosis on the body, which occured in ~1/4 of all subjects irrespective of selenium status and a whole host of neurological problems ranging from disorientation, tingling hands, tiredness or pain in the extremities, hand tremors, muscle twitches and camps, joint pain, memory problems, headaches or depression. Accordingly, the researchers conclude:
Signs of early selenosis: White-brownish discoloration of the nail (Schuh. 2007)
"In this study population, there were no apparent cutaneous or breath-related signs, or sentinel symptoms (gastro-intestinal disorders andmotor or sensory deficits) of selenosis, despite highSe body burdens in some individuals.

On the contrary, these findings are consistent with other results in this study population showing positive associations between Se status and motor performances and near visual acuity and a lower revalence of age related cataracts among those with elevated Se." (Lemire. 2011)
This is in fact surprising, after all, the selenium levels of some of the study participants were almost 3x higher than those in a 2007 study by Schuh & Lappe who report cardiometabolic changes in response to supplemental selenium at blood levels as "low" as 350 μg/L (Schuh. 2007).

Not so fast, though...

Now, despite the fact that there is no evidence of seleno-toxicity in any of the indigenous inhabitants of the Amazon delta with "toxic" (according to Western standards) selenium levels in the blood, it is by no means sure that the ingestion of several hundred grams of high (or even "low") selenium brazil nuts is save for you. I mean what if there was something that distinguishes you from the average Amazonian that makes you more susceptible to the negative effects of selenium?
  • The epigenetic / hormesis hypothesis: One of the potential downsides of high selenium intake that is commonly overlooked, is the development of insulin resistance and diabetes. For someone who subscribes to black/white painting of the blogosphere this may be difficult to understand, but in view of the fact that selenoprotein P and hepatic gluconeogenesis share common metabolic pathways and selenium compounds such as glutathione peroxidases interfere with insulin-regulated molecular pathways, it is actually only logical that the overconsumption of selenium may lead to the development of diabetes.

    A whole foods diet is unquestionably the most natural anti-cancer agent and if it contains not just veggies and fruit, but also selenium containing foods like Brazil nuts, its anti-skin cancer effects are going to be even more pronounced. At least that's what a recent study  from the Huntsmen Cancer Institute in Salt Lake City did suggest (learn more).
    Moreover, the fact that this process is related to the expression of certain enzymes could also mean  that someone who has been exposed to high selenium levels during his fetal development may be capable to deal with much higher amounts of selenium in his diet than the average Westerner whose mother was rather selenium deficient than -toxic during her pregnancy (ATSDR. 2003). Studies that were conducted on Inuit who are not yet following the McDonald's diet, would support this hypothesis. Despite dietary selenium intakes of up to 3,500µg/day, their diabetes rate (3%) is comparatively low (Chateau-Degat 2010). In fact, Mark P. Mattson even speculates that the chronic exposure to increasing amounts of selenium could induce a hormetic reaction that may be at the bottom of the many positive effects that are ascribed to this potentially toxic mineral (Mattson. 2008)
  • The mercury hypothesis: Another potential confounding variable of which I would hope that it is not something you are affected by is the high amount of mercury both the inhibants of the Amazonas and the Inuits are exposed to. High dietary selenium would after all not only offset the mercury toxicity, the high intake of mercury, on the other hand, would also bind tons of selenium and thus reduce, as Khan & Wang suggest the "excess" of selenium and thus inhibit or reduce its potentially toxic effect (Khan. 2009). 
  • Selenium and mercury make a perfect match. One inhibits the toxicity of the other - if they come in the right ratios (learn more)
    The organic vs. inorganic hypothesis: In view of the fact that the upper uptake levels are largely based on selenium toxicity that was observed as a consequence of the exposure to inorganic forms of selenium it is no wonder that third potential confounding factor that may influence the toxicity of selenium is the form of the non-metal you are exposed to. In view of the increasing number of reported cases of selenosis that occurred in response to the ingestion of obviously organic, supplemental selenium, the validity of this hypothesis has been questioned.

    It should also be noted that the most commonly sold over-the-counter supplements (even the high(er) quality ones) contain selenomethionine, which will - much contrary to selenocysteine - accumulate in the in plasma proteins and erythrocytes, where it replaces the existing methionine. From an evolutionary perspective this may be considered advantageous. After all, the protein-bound selenium may serve (hypothetically) as a storage pool. With the continuous supply of exuberant amounts of selenium it could however become a serious thread to your health. 
As you can see, there are still a couple of questions to be answered and hypothesis to be evaluated before the "Brazil Nuts Kill" myth is fully busted.

Brazil nuts in bad stored conditions infected by Aspergillus flavus. (Freita-Silva. 2011)
Bottom line: The assumption that handful of Brazil nuts eaten once or twice a week will kill you rather sooner, than later is still totally flawed, regardless of whether or not you are "selenium adapted" or not...

And in case you still die, I'd suggest you tell your relatives to check not the selenium but rather the aflotoxin content of your Brazil nuts. It is after all about as, if not much more probable, that the carcinogenic "exrements" of the fungi that are accumulating on the nuts, when they are not appropriately handled by the manufacturers are going to kill you than the "innocent" selenium that's buried beneath the mold (Freitas-Silva. 2011).

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