|Brazil nuts & selenium: "How much is too much?" A weighty question.|
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)|
"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)|
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).
"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.
Signs of early selenosis: White-brownish discoloration of the nail (Schuh. 2007)
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)
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).
- 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)
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.
|Brazil nuts in bad stored conditions infected by Aspergillus flavus. (Freita-Silva. 2011)|
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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