Brazil Nuts: Selenium Overload and Safety

Table of Contents

1. Why a Nut Can Overshoot
2. The Official Limits, by Age
3. How Much Selenium Is in One Nut
4. The Arithmetic: Nuts Against the Limits
5. What Selenosis Looks Like
6. Case Reports and Population Studies
7. Brazil-Nut Trials at High Selenium Doses
8. The Supplement Trials: Cancer and Skin
9. Type 2 Diabetes and High Selenium Status
10. Radium, Barium and Other Elements
11. Mould, Aflatoxin and the Import Rules
12. Tree-Nut Allergy and Ber e 1
13. What Is Not Known
14. Who the Research Raises Concerns For
15. Connections
16. Key Research Papers
17. Featured Videos

Why a Nut Can Overshoot

Selenium is a trace mineral: the body needs it in micrograms (mcg, millionths of a gram). It is built into a set of proteins called selenoproteins, including the glutathione peroxidase enzymes and the deiodinases that activate thyroid hormone. A 2007 review counts 25 human selenoproteins and notes that excess selenium can produce selenosis affecting the liver, skin, nails and hair (Alexander 2007). The gap between "enough" and "too much" is narrow compared with most nutrients, which is why selenium is one of the few nutrients where an ordinary food raises the question at all.

The Brazil nut tree takes selenium up from the soil and stores it in the seed. A study of commercial nuts found the selenium was mainly in organic form, bound into compounds such as selenomethionine, and most concentrated in a ring 1 to 2 mm below the seed's surface (Lima 2019). A review of the agronomy explains that how much a plant accumulates depends on the soil's selenium profile, both the amount and its chemical form (Alcântara 2022). Amazonian soils differ enormously from place to place, so the nuts do too.

The form matters for how the body handles an excess. The U.S. Institute of Medicine (IOM) explains that selenomethionine, the main form of selenium in food, is built into body proteins at random in place of the amino acid methionine, so high intakes produce large rises in tissue selenium. That storage "delays" toxicity and has created the impression that food selenium is less toxic than inorganic selenium; the IOM's assessment is that inorganic selenium has greater immediate toxicity, but that both forms are likely to have similar toxicities under chronic intake (Institute of Medicine 2000, chapter 7). A review of selenium speciation, the analysis of which chemical forms are present, discusses Brazil nuts alongside garlic and selenium-enriched yeast as foods whose particular selenium compounds shape how they are metabolized (Dumont 2006).

Blood tests reflect this. In people eating Brazil nuts, whole-blood and plasma selenium rise together in a straight line, which researchers studying Inuit adults in northern Quebec contrast with the levelling-off of plasma selenium they saw from marine foods, and attribute to the selenomethionine in the nuts (Achouba 2016). The liver's selenium carrier, selenoprotein P, rises with intake until it plateaus once needs are met; a 2022 review notes that a further rise after toxic intake gives information about possible selenosis (Schomburg 2022).

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The Official Limits, by Age

Two kinds of reference value matter here. The Recommended Dietary Allowance (RDA) is the daily intake that covers the needs of nearly all healthy people. The Tolerable Upper Intake Level (UL) is the highest daily intake, from all sources combined, that is not expected to cause harm. The European Food Safety Authority states plainly that a UL "is not a recommended level of intake" and that as intake rises above it, the risk of adverse effects increases (EFSA NDA Panel 2023).

United States and Canada: the Institute of Medicine (2000)

The U.S. values come from the IOM's 2000 Dietary Reference Intakes (Institute of Medicine 2000), the figures the NIH Office of Dietary Supplements selenium fact sheet uses. The table below is taken from the IOM chapter (IOM 2000, chapter 7) and, for the infant and child upper limits, the IOM column of EFSA's comparison table (EFSA NDA Panel 2023).

How the adult UL was reached is worth knowing, because it shows how thin the underlying data are. The IOM based it on hair and nail brittleness and loss, the most frequently reported signs of chronic selenosis. Its key data set was a 1992 re-examination of five people in Enshi, China, previously found to have hair loss and nail sloughing. At first examination the lowest blood selenium among them corresponded to an intake of about 913 mcg a day, taken as the lowest level at which harm was seen. When they were re-examined and described as recovered, their intakes were estimated at about 800 mcg a day, which the IOM took as the no-observed-adverse-effect level. It divided 800 by an uncertainty factor of 2, "to protect sensitive individuals", giving 400 mcg a day. The IOM itself noted the uncertainty: the data set was small, and the subjects might be more or less sensitive than other populations (IOM 2000, chapter 7).

Europe: EFSA's 2023 re-evaluation

In 2023 EFSA's nutrition panel re-assessed the limit and lowered it. It chose alopecia (hair loss) as the critical effect and took its starting point not from the Chinese village studies but from the SELECT trial (described below), where average total intakes of about 330 mcg a day, around 130 mcg from the diet plus 200 mcg from a supplement, for a median of 5.5 years increased the risk of alopecia and dermatitis. Applying an uncertainty factor of 1.3 to that 330 mcg gave an adult UL of 255 mcg a day, including during pregnancy and breastfeeding. Children's limits were scaled down from the adult value by body size (EFSA NDA Panel 2023). The earlier European limit, set by the Scientific Committee on Food in 2000, was 300 mcg a day for adults (EFSA NDA Panel 2023; Alexander 2007).

EFSA's risk assessment names Brazil nuts directly. Its conclusion is that adult consumers in Europe are unlikely to exceed the UL "except for regular users of food supplements containing high daily doses of selenium or regular consumers of Brazil nuts" (EFSA NDA Panel 2023). For comparison, EFSA's 2014 Adequate Intake for adults is 70 mcg a day.

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How Much Selenium Is in One Nut

There is no single honest "selenium per nut" figure. Every source measures something different, and the spread runs across two orders of magnitude. The figures below are the ones the cited studies and databases actually report, with the per-nut value worked out from them where the source gives a concentration. One kernel weighs about 5 g in the USDA database.

A few things follow. First, the lowest and highest per-nut figures in the trials differ about fifty-fold (roughly 26 against 1,261 mcg). Second, the database average is a midpoint, not a description of any particular bag. Third, even inside one batch, individual seeds varied up to eight-fold (Lima 2019). The NIH Office of Dietary Supplements gives an average of 544 mcg per ounce and notes that other analyses vary widely, and Germany's Federal Office for Radiation Protection (BfS) states that because the selenium content "can fluctuate greatly", a targeted selenium supply from eating the nuts is not possible (BfS). EFSA puts the same point in serving terms: 5 or 6 Brazil nuts could contribute "approximately between 100 and 2,000 μg selenium", depending on where in the reported range the nuts fall (EFSA NDA Panel 2023).

Reviews of the nut's composition rank it among the richest food sources of selenium (Cardoso 2017), and the figures above show why that description says nothing about any single nut.

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The Arithmetic: Nuts Against the Limits

The table divides each official figure by the per-nut values above. It is arithmetic, not guidance: it shows how many nuts it would take for the nuts alone to equal each reference value. Two caveats apply to every cell. The limits cover total intake from all sources, and ordinary diets already supply some selenium (EFSA estimated about 130 mcg a day in the SELECT participants' background diets). And no shopper knows which row their bag belongs to.

For young children the limits are lower and the arithmetic tighter. At the USDA average of about 96 mcg, one nut is above the IOM upper limit for ages 1 to 3 (90 mcg) and EFSA's limits for ages 1 to 3 (70 mcg), and is about equal to EFSA's limit for ages 4 to 6 (95 mcg). At about 289 mcg (the Cardoso 2016 nut), a single nut is above the IOM limits for every age up to 13 and above every EFSA child and adolescent limit; at about 1,261 mcg (the Duarte 2019 nut), it is above every limit for every age in both tables.

The table also shows why the trials' outcomes cannot be compared nut-for-nut. "One Brazil nut a day" meant about 289 mcg in one trial and about 1,261 mcg in another; "two nuts a day" meant 53 mcg on average in a third. Studies that report a number of nuts without analysing those nuts give no reliable selenium dose at all.

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What Selenosis Looks Like

Selenosis is chronic selenium poisoning. The IOM lists hair and nail brittleness and loss as the most frequently reported features, with gastrointestinal disturbances, skin rash, garlic breath odour (from selenium compounds breathed out), fatigue, irritability and nervous-system abnormalities also reported (IOM 2000, chapter 7). EFSA's 2023 description is more detailed: brittle, thickened nails with white spots and lengthwise streaks; brittle hair and hair loss; discoloured and decaying teeth; garlic breath; skin lesions; and neurological signs including fatigue, weakness, tingling or numbness in the hands and feet, exaggerated reflexes, pain in the limbs, unsteady gait, paralysis and decreased cognitive function (EFSA NDA Panel 2023). The proposed reason for the hair and nail damage is that selenium takes the place of sulfur in proteins such as keratin, the structural protein of hair and nails, disrupting their structure (EFSA NDA Panel 2023).

The garlic smell has a specific cause. The IOM explains that when large quantities of selenium are ingested, methylated selenium compounds appear in the breath and produce the odour; it also notes that breath selenium has never been linked quantitatively to toxicity and cannot be used to measure it (IOM 2000, chapter 7).

Acute poisoning, from a single large dose, is a different and much rarer event. The IOM describes fatal or near-fatal cases after accidental or deliberate swallowing of gram quantities of selenium, usually in industrial liquids such as gun-bluing solution, followed by severe gut and nervous-system disturbances, breathing failure, heart attack and kidney failure (IOM 2000, chapter 7). These were not food cases.

Each sign has its own page on this site: Selenosis (Selenium Toxicity): Symptoms, Causes, and Risks, with deep dives on hair and nail loss, garlic breath, nerve problems and stomach upset.

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Case Reports and Population Studies

Almost everything known about selenosis in people comes from a handful of populations living on selenium-rich soils and a few individual cases. The findings do not line up neatly, and that disagreement is itself the main finding.

Enshi, China

High rates of selenosis in Enshi, in south China, allowed researchers to study about 380 people with high intakes. EFSA's summary reports 60 cases of selenosis, aged 13 to 70, diagnosed mainly by fingernail changes, and no clinical signs in anyone whose whole-blood selenium was below 1,000 mcg per litre, which corresponded to intakes of about 850 mcg a day. In five people with long-lasting symptoms, blood selenium ranged from 1,054 to 1,854 mcg per litre, and the symptoms disappeared after a change of diet lowered their intake (EFSA NDA Panel 2023; IOM 2000, chapter 7). The same study found prolonged blood-clotting times in 45% of a subset with blood selenium above 1,000 mcg per litre, against 2.7% below it (EFSA NDA Panel 2023).

South Dakota and Wyoming ranchers

A study of 142 ranchers and family members in a region known for selenium poisoning in livestock found an average intake of 239 mcg a day and a highest intake of 724 mcg a day. The IOM reports that no selenosis was found and no attributable changes in enzymes, clotting times or blood counts (IOM 2000, chapter 7). EFSA's later review notes that the same study did observe a rise in the liver enzyme ALT with selenium intake (EFSA NDA Panel 2023).

The Tapájos River, Brazilian Amazon

In 448 people aged 15 to 87 from 12 riverside communities whose traditional diet includes Brazil nuts, chicken, game and certain fish, median whole-blood selenium was 228 mcg per litre, with a range of 103 to 1,500 mcg per litre. Some values surpassed the 1,000 mcg per litre the U.S. Environmental Protection Agency considers toxic. A nurse examined hair, nails, skin and breath, and no signs or symptoms of selenosis were associated with the selenium measurements. The authors concluded there was no evidence of selenosis in that population and called for selenium toxicity to be re-assessed with the chemical form and route of exposure, and co-exposure to mercury, taken into account (Lemire 2012). EFSA, reading the same data, notes that signs of selenium toxicity were reported in 2.2% of individuals with plasma selenium below 328 mcg per litre, and were not consistently present in the higher groups, which included fewer than 15 people (EFSA NDA Panel 2023).

Preschool children fed Brazil nuts

EFSA describes a study of 41 preschool children in the Brazilian Amazon, aged about 2 to 6.5, who received 15 to 30 g of Brazil nuts three days a week as part of a public-health programme, compared with 88 children who did not. The nut group's measured average intake was 155 mcg a day, above the European limits for their ages, against about 44 mcg in the comparison group. A doctor's examination found "no signs of selenosis" in either group (EFSA NDA Panel 2023).

A case from a related nut

EFSA also cites a 2012 case of a 55-year-old woman who ate 10 to 15 paradise nuts a day for 20 days. Paradise nuts are a different tree nut, not Brazil nuts. Her serum selenium was 512 mcg per litre, and she had headaches, dizziness, vomiting, abdominal pain, massive hair loss and greyish discoloration of the fingernails. Her hair began to regrow two months after she stopped (EFSA NDA Panel 2023).

What the panels made of it

EFSA concluded that signs of selenium toxicity have been seen at average intakes below those in the Chinese studies used to set the older limits, that the number of studies is limited, and that individuals seem to vary widely in susceptibility (EFSA NDA Panel 2023). That conclusion, together with the SELECT result, is why Europe's limit is now lower than the American one.

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Brazil-Nut Trials at High Selenium Doses

The controlled trials of Brazil nuts were mostly designed to look for benefits, and a 2022 systematic review of 24 of them (15 non-randomized) found improvements in antioxidant markers in every study that measured them (da Silva 2022). A few, though, gave selenium doses near or above the upper limits, and their results bear on safety.

• About 1,261 mcg a day for two months. In 55 obese women in São Paulo who were not selenium-deficient at the start, one high-selenium nut a day raised every selenium marker and increased the activity of genes for several pro-inflammatory signals (interleukin-6, tumour necrosis factor-alpha and Toll-like receptors 2 and 4). Plasma levels of the inflammatory markers themselves did not change. The authors read the gene changes as a possible pro-inflammatory stimulus from high-selenium nuts (Duarte 2019).
• Six months in type 2 diabetes. In a Brazilian study of people with type 2 diabetes, comparing blood before and after six months of Brazil nuts, serum selenium rose and fasting blood glucose rose significantly; glycated haemoglobin (HbA1c, a three-month glucose average) did not change significantly. Blood cells taken after the six months resisted laboratory-induced DNA damage better (Macan 2022).
• About 347 mcg a day for eight weeks. In a non-randomized controlled trial of 56 women with overweight or obesity on a calorie-reduced diet (46 completed), 8 g of Brazil nuts a day was followed by lower levels of several inflammatory markers than a nut-free diet. The authors noted that the effects seemed to depend on selenium status (Silveira 2024). The dose was below the IOM adult limit and above EFSA's.
• About 289 mcg a day for six months. In older adults with mild cognitive impairment, one nut a day raised blood selenium and red-cell glutathione peroxidase activity; 20 of the 31 people enrolled completed the trial (Cardoso 2016).

None of these abstracts reports cases of selenosis. They were small and short, and most were not designed to detect hair, nail or nerve effects, so the absence of reported cases is not evidence of safety at those intakes. The 2018 commentary on the health concerns of Brazil nut consumption lists selenium toxicity, selenium-induced type 2 diabetes, weight gain, radioactivity, aflatoxins and allergic reactions (Mazokopakis 2018).

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The Supplement Trials: Cancer and Skin

The largest human evidence on high selenium intake comes from cancer-prevention trials that gave selenium tablets, not nuts. They matter here because they tested intakes that a few Brazil nuts can reach, in people who were mostly not deficient.

• SELECT (prostate cancer). The Selenium and Vitamin E Cancer Prevention Trial randomized 35,533 men aged 50 or 55 and over at 427 sites in the United States, Canada and Puerto Rico to 200 mcg a day of selenium as L-selenomethionine, vitamin E, both, or placebo. In the long-term report, 575 men in the selenium group developed prostate cancer against 529 on placebo (hazard ratio 1.09, 99% confidence interval 0.93 to 1.27), a difference that was not statistically significant. Selenium did not reduce prostate cancer; vitamin E alone increased it (Klein 2011). EFSA's review of SELECT's safety data reports a significantly higher risk of alopecia in the selenium group (relative risk 1.28) and higher rates of mild dermatitis (EFSA NDA Panel 2023).
• Nutritional Prevention of Cancer Trial (skin cancer). This double-blind trial gave 200 mcg a day of selenium as selenized yeast, or placebo, to 1,312 people in the eastern United States who had already had nonmelanoma skin cancer. Over the whole blinded period, selenium did not prevent basal cell carcinoma (hazard ratio 1.09, not significant) and was associated with a significantly higher risk of squamous cell carcinoma (hazard ratio 1.25) and of total nonmelanoma skin cancer (hazard ratio 1.17). The authors concluded that selenium supplementation was ineffective at preventing basal cell carcinoma and increased the risk of squamous cell and total nonmelanoma skin cancer (Duffield-Lillico 2003).
• SELENIB (bladder cancer). In 270 UK patients newly diagnosed with non-muscle-invasive bladder cancer, 200 mcg a day of high-selenium yeast did not change the time to recurrence (hazard ratio 0.92, not significant), progression or survival over a median follow-up of 5.5 years (Bryan 2023).

The site's Selenium and Cancer Prevention page covers these trials from the mineral's side. None of them studied Brazil nuts. What they show is that adding 200 mcg a day to an adequate diet did not prevent cancer in these trials, and in one trial was linked to more skin cancer.

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Type 2 Diabetes and High Selenium Status

The diabetes question arose from secondary findings in the same supplement trials. EFSA's 2023 opinion pooled five randomized trials that recorded new cases of type 2 diabetes (including the Nutritional Prevention of Cancer Trial and SELECT). All five found more cases in the selenium groups than in the control groups, and the pooled relative risk was 1.11 (95% confidence interval 1.00 to 1.24). All of them gave 200 mcg a day, corresponding to total intakes of roughly 270 to 340 mcg a day, to people whose blood selenium was already adequate. EFSA rated its certainty in a causal link between selenium intake and type 2 diabetes as moderate, while noting that measures of glucose tolerance and insulin sensitivity in other studies did not support the finding (EFSA NDA Panel 2023).

Brazil nut evidence on this point is sparse. The before-and-after study in people with diabetes reported a significant rise in fasting glucose after six months of nuts, without a change in HbA1c (Macan 2022), and the 2018 commentary lists selenium-induced type 2 diabetes among the concerns about the nut (Mazokopakis 2018). Results run the other way in a different setting: in a 2024 randomized trial in pregnant women with gestational diabetes, whose selenium levels were lower than those of healthy pregnant women, four weeks of 200 mcg of selenium plus diet lowered blood glucose more than diet alone (Yigit 2024). One reading, not tested directly in any of these studies, is that the effect depends on a person's starting selenium status; the authors of one Brazil-nut trial likewise described their results as seeming to depend on selenium status (Silveira 2024). See Type 2 Diabetes for the wider picture.

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Radium, Barium and Other Elements

Natural radium

Germany's Federal Office for Radiation Protection reports that Brazil nuts carry several tens of becquerels per kilogram each of radium-226 and radium-228 (its activity table lists 41 and 46 Bq/kg), so that a kilogram of the nuts can hold about 1,000 times the radium of a kilogram of the average German diet. Its explanation is that the trees grow on weathered, nutrient-poor rainforest soils short of calcium, and take up radium, which is chemically similar to calcium, instead. In the body, radium also behaves like calcium and part of it is laid down in bone (BfS, Natural radioactivity in Brazil nuts).

The BfS dose estimates, in microsieverts (a unit of radiation dose) per year:

The BfS describes moderate adult consumption as without concern. As a precaution, it advises against eating Brazil nuts during pregnancy and breastfeeding, and advises children and adolescents not to eat them, because radium is deposited in growing bone. It adds that selenium status can be improved by supplements without any additional radiation dose (BfS). The site's Radiation and Physical Hazards section covers radiation dose more generally.

Barium

The U.S. Agency for Toxic Substances and Disease Registry (ATSDR) states that some foods, "such as Brazil nuts, seaweed, fish, and certain plants, may contain high amounts of barium", and lists Brazil nuts with legumes, forage plants and mushrooms among the land plants that can concentrate it. It identifies soluble barium compounds, such as barium chloride and barium carbonate, as of greater health concern than insoluble ones, and lists gastrointestinal disturbances followed by low blood potassium, high blood pressure and heart rhythm abnormalities as frequently reported after acute oral exposure to high doses (ATSDR Barium ToxGuide). The ToxGuide does not state which barium compounds occur in Brazil nuts, and none of the human Brazil-nut trials cited here measured barium. Background is on the Barium page.

Other elements measured

A Serbian method study analysing 19 elements in seven kinds of nuts sold locally, Brazil nuts among them, found the toxic elements arsenic, cadmium and lead either undetected or below the limit of detection in all samples (Tošić 2015). A laboratory study measured tellurium, an element chemically related to selenium, at an average of about 4 nanograms per gram of Brazil nut, with about 30% of it released in a simulated digestion (Berenguel 2018); no human data on tellurium from nuts were found.

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Mould, Aflatoxin and the Import Rules

Aflatoxins are poisons made by moulds of the Aspergillus section Flavi group. The U.S. Food and Drug Administration states that consumption of food with high levels of aflatoxins is associated with liver cancer in humans, and names Brazil nuts among the tree nuts most susceptible to aflatoxin-producing moulds (FDA CPG Sec. 570.200). Brazil nuts are gathered from the forest floor in a hot, humid climate, which is why mould is the main food-safety question for them.

What the studies found

• In 110 stored samples from Itacoatiara, Amazonas (55 of nuts and 55 of shells, collected monthly for 11 months), aflatoxin-producing moulds, mainly Aspergillus flavus, were isolated frequently and persisted during storage. No aflatoxin or cyclopiazonic acid (another mould toxin) was detected in any sample; the authors credited the low water activity, meaning the dryness, of the stored nuts, and said the persistence of toxin-producing moulds showed the need for good handling practices (Baquião 2013).
• In 25 samples of Brazil nut oil from northern Brazil, 7 made by local producers by traditional methods from nuts the industry had discarded and 18 factory-made, 7 samples (28%) contained aflatoxins, at up to 50.87 mcg per kg. The authors described it as the first study of aflatoxin in Brazil nut oil (Kluczkovski 2022).
• A genetic survey of 396 Aspergillus section Flavi strains from Brazilian foods, including Brazil nuts, identified two new species, one of them potentially able to make aflatoxins (Silva 2022), a reminder that the mould group involved is diverse.

The legal limits

• United States. The FDA's compliance policy guide for Brazil nuts states that the agency may consider Brazil nuts containing total aflatoxins (B1, B2, G1 and G2) above 20 micrograms per kilogram, or parts per billion, to be adulterated, and sets out the criteria for seizure or import refusal (FDA CPG Sec. 570.200).
• European Union. Commission Regulation (EU) 2023/915 sets separate limits for Brazil nuts (together with hazelnuts). For nuts going on sale to consumers or used as an ingredient, the maximum is 5.0 mcg per kg of aflatoxin B1 and 10.0 mcg per kg of total aflatoxins. For nuts that will still be sorted or otherwise physically treated first, the maximum is 8.0 and 15.0 mcg per kg (Regulation (EU) 2023/915). The EU total-aflatoxin limits (10 and 15 mcg per kg) are half to three-quarters of the U.S. level of 20.
• The 2003 EU import measure. After repeated findings of excessive aflatoxin, the European Commission in 2003 imposed special conditions on imports of Brazil nuts in shell from Brazil: consignments had to carry official aflatoxin certificates, could enter only at named points, and were not released if unacceptably contaminated (Commission Decision 2003/493/EC). That decision was repealed and replaced in 2006.

The mould picture is covered in depth on Mould and Mycotoxins, and the same problem in another crop, including how sorting removes contaminated kernels, on Aflatoxin, Mould and Safe Sourcing.

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Tree-Nut Allergy and Ber e 1

Tree nuts cause IgE-mediated allergy, the antibody type behind fast allergic reactions, often serious and sometimes life-threatening. Reported prevalence of tree-nut allergy is up to 4.9% worldwide. For hazelnut and walnut, several allergens are known and already used in component-resolved testing (blood tests for antibodies to individual proteins), while for Brazil nut only individual allergens have been identified and data on others are missing (Geiselhart 2018). A 2003 review places Brazil nut among the tree nuts less often associated with allergy than hazelnut, walnut, cashew and almond (Roux 2003).

The best-studied Brazil nut allergen is Ber e 1, a major allergen that is very stable to digestion by stomach enzymes in laboratory tests. It does not fully unfold until about 110°C at the acidity of most foods (pH 5 to 7), and probably higher inside the nut itself, so the authors concluded that heat processing is unlikely to denature all of it and that it is eaten mainly in its native form (van Boxtel 2008). A review of plant food allergens places the major Brazil nut allergens in the 2S albumin group of the prolamin protein superfamily, which it describes as stable to heat processing and to digestion (Mills 2004). Roasting therefore cannot be assumed to make the nut safe for someone allergic to it.

A 2024 systematic review of treatments for tree-nut allergy, which included Brazil nut, describes the standard of care as strict avoidance of the culprit nut and symptomatic treatment of accidental reactions. Oral immunotherapy, with or without the antibody drug omalizumab, was the most studied approach and appeared to protect against accidental exposures; omalizumab alone was the only approved alternative for reducing reactions to accidental exposure (Pasioti 2024). See Food Allergy and Anaphylaxis.

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What Is Not Known

• The real selenium dose of any given nut. No study supplies a per-nut figure that holds across batches, regions or even seeds within one batch (Lima 2019; EFSA NDA Panel 2023).
• Whether nut selenium and tablet selenium behave the same at high intakes. The IOM considered chronic toxicity likely similar for food and inorganic forms (IOM 2000, chapter 7); the Amazon study authors argued the form and food context need re-examination (Lemire 2012). The upper limits were not built on Brazil-nut data.
• Who is most susceptible. EFSA notes wide individual variability, and toxicity signs at intakes lower than older limits assumed, from a small number of studies (EFSA NDA Panel 2023).
• Long-term outcomes of regular high-selenium nut eating. The trials ran from eight weeks to six months. The diabetes and skin-cancer signals come from years of 200 mcg tablets, and have not been tested with nuts.
• The importance of radium and barium at ordinary intakes. The radium figures are model estimates from one agency, and no human Brazil-nut study cited here measured barium.
• The full allergen list. Brazil nut allergens beyond the few identified are not yet characterized (Geiselhart 2018).

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Who the Research Raises Concerns For

• Young children. Their upper limits are the lowest, and at the USDA average one nut exceeds the IOM limit for ages 1 to 3. The BfS radium advice also concerns children and adolescents, because of bone growth (IOM 2000; EFSA NDA Panel 2023; BfS).
• Pregnancy and breastfeeding. EFSA's 255 mcg limit applies in pregnancy and breastfeeding, and the BfS advises against Brazil nuts during both as a radiation precaution (EFSA NDA Panel 2023; BfS).
• People who already take selenium. The limits cover total intake. Multivitamins and thyroid or "immune" formulas can contain selenium, and EFSA identifies high-dose supplement users and regular Brazil nut eaters as the two groups likely to exceed the adult limit (EFSA NDA Panel 2023).
• People with or at risk of type 2 diabetes. The pooled supplement trials found more new diabetes cases with selenium, and one Brazil-nut study reported higher fasting glucose in people with diabetes (EFSA NDA Panel 2023; Macan 2022).
• People with a history of skin cancer. The one trial in this group linked 200 mcg a day of selenium to more squamous cell and total nonmelanoma skin cancer (Duffield-Lillico 2003).
• People allergic to tree nuts. Ber e 1 survives cooking and digestion in laboratory tests (van Boxtel 2008).
• People eating nut oils or poorly stored nuts. Aflatoxin was found in over a quarter of the Brazil nut oils tested in one survey (Kluczkovski 2022).

A selenium blood test is the measurement the trials and population studies used to track status. Decisions about selenium intake, especially for children, during pregnancy, or alongside supplements or diabetes treatment, belong with a clinician.

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Connections

• Nuts — the nuts landing page
• Brazil Nuts — the main topic page
• Brazil Nuts — Benefits Deep Dive — the hub for these articles
• Selenium and Thyroid Health — the thyroid trials
• Selenium Status and Antioxidant Enzymes — the blood-selenium trials
• Cholesterol and Inflammation
• Brain Aging and Cognition
• Selenium — the mineral in full
• Selenosis (Selenium Toxicity) — symptoms, causes and risks
• Selenosis: Hair and Nail Loss
• Selenosis: Garlic Breath
• Selenosis: Nerve Problems
• Selenosis: Stomach Upset
• Selenium Toxicity (Selenosis) — the Toxic Minerals view
• Selenium and Cancer Prevention — SELECT and the NPC trial
• Selenium Test
• Type 2 Diabetes
• Skin Cancer
• Hair Loss
• Barium
• Mould and Mycotoxins
• Peanuts: Aflatoxin and Safe Sourcing
• Food Allergy
• Anaphylaxis

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Key Research Papers

1. Institute of Medicine (US) Panel on Dietary Antioxidants and Related Compounds. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington (DC): National Academies Press (US). 2000. PubMed PMID: 25077263
2. Alexander J. Selenium. Novartis Foundation symposium. 2007;282:143-9; discussion 149-53, 212-8. PubMed PMID: 17913229
3. Lima LW, Stonehouse GC, Walters C, et al. Selenium Accumulation, Speciation and Localization in Brazil Nuts (Bertholletia excelsa H.B.K.). Plants (Basel, Switzerland). 2019;8(8):289. PubMed PMID: 31426292
4. Alcântara DB, Dionísio AP, Artur AG, et al. Selenium in Brazil nuts: An overview of agronomical aspects, recent trends in analytical chemistry, and health outcomes. Food chemistry. 2022;372:131207. PubMed PMID: 34634585
5. Dumont E, Vanhaecke F, Cornelis R. Selenium speciation from food source to metabolites: a critical review. Analytical and bioanalytical chemistry. 2006;385(7):1304-23. PubMed PMID: 16830114
6. Achouba A, Dumas P, Ouellet N, et al. Plasma levels of selenium-containing proteins in Inuit adults from Nunavik. Environment international. 2016;96:8-15. PubMed PMID: 27588697
7. Schomburg L. Selenoprotein P - Selenium transport protein, enzyme and biomarker of selenium status. Free radical biology & medicine. 2022;191:150-163. PubMed PMID: 36067902
8. Cardoso BR, Duarte GBS, Reis BZ, et al. Brazil nuts: Nutritional composition, health benefits and safety aspects. Food research international (Ottawa, Ont.). 2017;100(Pt 2):9-18. PubMed PMID: 28888463
9. Thomson CD, Chisholm A, McLachlan SK, et al. Brazil nuts: an effective way to improve selenium status. The American journal of clinical nutrition. 2008;87(2):379-84. PubMed PMID: 18258628
10. Rita Cardoso B, Apolinário D, da Silva Bandeira V, et al. Effects of Brazil nut consumption on selenium status and cognitive performance in older adults with mild cognitive impairment: a randomized controlled pilot trial. European journal of nutrition. 2016;55(1):107-16. PubMed PMID: 25567069
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Regulatory and agency sources: EFSA NDA Panel, Scientific opinion on the tolerable upper intake level for selenium, EFSA Journal 2023;21(1):e07704, doi:10.2903/j.efsa.2023.7704; Institute of Medicine 2000, chapter 7, National Academies Press; BfS, Natural radioactivity in Brazil nuts; ATSDR Barium ToxGuide; FDA CPG Sec. 570.200; Regulation (EU) 2023/915; Commission Decision 2003/493/EC.

PubMed Topic Searches

1. Selenosis case reports
2. Brazil nut and selenium toxicity
3. Selenium supplementation and type 2 diabetes
4. Selenium tolerable upper intake level
5. Radium in Brazil nuts
6. Aflatoxin in Brazil nuts
7. Brazil nut allergen Ber e 1

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