Brazil Nuts: Brain Aging and Cognition

Table of Contents

1. Why the Brain Holds On to Selenium
2. What Selenium Does Inside Brain Cells
3. Animal Research: Selenomethionine in Mice
4. The Brazil Nut Trial in Older Adults with Mild Cognitive Impairment
5. Which Tests Changed and Which Did Not
6. How Much Weight One Pilot Trial Can Carry
7. Selenium Status and Cognition in Observational Research
8. Nuts in General and the Ageing Brain
9. Why Tablet and Animal Results Do Not Transfer
10. What Is Not Known
11. Who the Research Raises Cautions For
12. Connections
13. Key Research Papers
14. Featured Videos

Why the Brain Holds On to Selenium

Selenium is a trace mineral: the body needs only micrograms a day. Most of its work is done inside about 25 proteins, called selenoproteins, that carry selenium built into their structure as an unusual amino acid, selenocysteine (Lu and Holmgren 2009; Alexander 2007).

The body does not share selenium out evenly. A transport protein called selenoprotein P (often written SELENOP) is made in the liver and released into the blood. A 2022 review by Schomburg describes how selenoprotein P carries selenium preferentially to tissues judged essential for survival and reproduction, naming the endocrine glands and the brain. Inside these organs a recycling loop, which Schomburg calls the "SELENOP cycle", keeps their selenium supply "privileged" — meaning they stay supplied while other tissues run short.

The strongest evidence for how much this matters comes from mice. According to the same review, mice bred without selenoprotein P could survive when given extra selenium, but died when selenium was limited. In other words, the delivery system becomes essential precisely when selenium is scarce. These are animal findings; they explain why researchers think brain selenium is protected, but they do not by themselves show what happens in a human brain.

Selenoprotein P is also measured in blood as a marker of selenium status. Schomburg notes that its level rises with intake until the body is saturated and then plateaus, and climbs again with toxic intake (Schomburg 2022). That plateau is one reason extra selenium in a person who already has enough is not expected to keep adding benefit.

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What Selenium Does Inside Brain Cells

The selenoproteins best understood are enzymes that handle oxidation — the chemical damage caused when reactive forms of oxygen attack fats, proteins and DNA. The main families are the glutathione peroxidases (GPx), the thioredoxin reductases and the iodothyronine deiodinases that activate thyroid hormone (Lu and Holmgren 2009; Alexander 2007).

• Glutathione peroxidases use the body's own antioxidant glutathione to break down peroxides before they damage cells. Red-blood-cell GPx activity is one of the measures the Brazil nut trial below tracked. See glutathione peroxidase.
• Thioredoxin reductases keep other proteins in their working, reduced state and take part in redox signalling (Lu and Holmgren 2009).
• Deiodinases convert the storage thyroid hormone T4 into active T3. Thyroid hormone matters for brain function, which is one indirect route by which selenium could touch cognition. That link is covered on the selenium and thyroid page.

GPx4 and ferroptosis — a laboratory mechanism. One member of the glutathione peroxidase family, GPx4, is unusual in that it can repair oxidised fats inside cell membranes. Laboratory and animal research describes a form of cell death, named ferroptosis, in which iron-driven oxidation of membrane fats overwhelms a cell when GPx4 is missing or blocked. Brain cells are rich in the polyunsaturated fats that are most vulnerable to this kind of damage, which is why ferroptosis is studied as one possible contributor to neurodegeneration. This remains a mechanism described in cells and animals; none of the records reviewed for this page tested it in people eating Brazil nuts. The site's selenium antioxidant defence page covers the enzyme biology in more depth.

The Brazil nut trial's authors framed their study around this biology: oxidative stress is closely linked to cognitive impairment, and selenium acts as an antioxidant through its selenoproteins, so raising selenium status might, they reasoned, help preserve thinking in older adults (Cardoso 2016). That was the hypothesis, not a finding.

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Animal Research: Selenomethionine in Mice

Most of the selenium in Brazil nuts is in organic form (Lima 2019), chiefly selenomethionine — selenium attached to the amino acid methionine (Alcântara 2022). Selenomethionine can also slip into ordinary proteins in place of methionine, a non-specific route of storage (Alexander 2007).

Gao and colleagues gave purified selenomethionine to mice treated with D-galactose, a sugar used in laboratories to produce an artificial ageing-like decline. In these mice, selenomethionine improved performance on three memory and learning tasks (novel object recognition, a passive avoidance task and the Morris water maze), reversed the drop in the memory chemical acetylcholine in the hippocampus, lowered markers of oxidative stress, raised selenoprotein P in the hippocampus and shifted the gut bacteria (Gao 2022). The authors noted that selenomethionine had earlier shown effects in a mouse model of Alzheimer's disease.

Three limits apply. The animals were mice, not people. The decline was chemically induced, not natural ageing. And the substance tested was isolated selenomethionine, not Brazil nuts. The study shows a plausible pathway; it is not evidence that Brazil nuts improve memory in humans.

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The Brazil Nut Trial in Older Adults with Mild Cognitive Impairment

The only human trial of Brazil nuts and cognition in the research reviewed for this page was run at the University of São Paulo and published by Rita Cardoso and colleagues in 2016 (the paper went online in January 2015; a correction notice, or erratum, was published in 2021).

Who took part. Thirty-one older adults with mild cognitive impairment — problems with memory or thinking that are noticeable on testing but not severe enough to be called dementia. Their mean age was 77.7 years, and 70% were women (Cardoso 2016).

What they were given. Participants were randomly assigned to one of two groups. The treatment group ate one Brazil nut a day for 6 months, estimated to provide 288.75 micrograms (mcg) of selenium a day. The control group did not receive Brazil nuts; the abstract and the 2022 systematic review describe it simply as a control group, with no placebo food mentioned (Cardoso 2016; da Silva 2022).

Who finished. Eleven people in the Brazil nut group and nine in the control group completed the trial — 20 of the 31 enrolled, so roughly a third dropped out (Cardoso 2016).

What was measured. Blood selenium; the activity of the selenium enzyme glutathione peroxidase in red blood cells; oxygen radical absorbance capacity (ORAC, a laboratory measure of the blood's overall antioxidant capacity); malondialdehyde (MDA, a breakdown product of oxidised fats); and thinking skills, assessed with the CERAD neuropsychological battery. CERAD (the Consortium to Establish a Registry for Alzheimer's Disease) is a standard set of tests covering word-list memory, naming, verbal fluency and drawing (Cardoso 2016).

Selenium status at the start. The authors concluded that the nut "restores selenium deficiency", and the 2022 systematic review notes that most of the sample was selenium-deficient at baseline, a factor it considered likely to matter for any benefit (Cardoso 2016; da Silva 2022).

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Which Tests Changed and Which Did Not

Reading only the headline of this trial gives a misleading picture, because most of the measures did not differ between groups.

Measures that changed more in the Brazil nut group

• Blood selenium rose in the nut group; the control group showed no change (Cardoso 2016).
• Red-blood-cell glutathione peroxidase activity — the only antioxidant measure whose change differed significantly between groups (p = 0.006) (Cardoso 2016).
• Verbal fluency (naming as many words as possible from a category, such as animals, within a time limit) improved more in the nut group (p = 0.007) (Cardoso 2016).
• Constructional praxis (copying geometric drawings, a test of visual-spatial and planning skills) improved more in the nut group (p = 0.031) (Cardoso 2016).

Measures that did not differ between groups

The 2022 systematic review's table of randomised trials lists the following as unchanged between groups (da Silva 2022):

• ORAC (overall antioxidant capacity)
• Malondialdehyde (a marker of fat oxidation)
• The CERAD total score — the combined measure of overall cognitive performance
• The Boston naming test (naming pictured objects)
• The word-list learning test
• The word-list recall test (delayed memory)

So the overall cognitive score did not change, and the memory tests — the skills people usually mean when they worry about mild cognitive impairment — did not change either. Two of the sub-tests moved in the nut group's favour. The authors described their result as "preliminary evidence" that Brazil nut consumption "can have positive effects on some cognitive functions" (Cardoso 2016), and that wording is accurate: some, not overall.

The same research group also reported a secondary analysis of the nut group alone, looking at whether common gene variants in selenoprotein genes changed how blood selenium, GPx activity and gene expression responded to the nut. The systematic review summarises that these variants did appear to affect the responses (da Silva 2022). This analysis did not test cognition and had no comparison group.

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How Much Weight One Pilot Trial Can Carry

The trial was explicitly a pilot, and its size sets hard limits on what it can show.

• Twenty completers. With 11 people in one group and 9 in the other, a single participant having an unusually good or bad testing day can noticeably shift a group's average.
• Many outcomes, two positive. Several cognitive sub-tests and several blood measures were compared. When many comparisons are made in a small sample, some will reach statistical significance by chance alone. The overall CERAD score, which is less exposed to this problem, did not change.
• About a third dropped out. When 11 of 31 participants do not finish, the people left may differ from those who left in ways that affect the result.
• No placebo described. The control group is described only as receiving no Brazil nuts, with no placebo food, so participants may well have known which group they were in. Practice with the tests and expectation can both affect repeated cognitive testing.
• A selenium-deficient group. Most participants started out low in selenium (da Silva 2022). Any benefit seen may reflect correcting a deficiency, which says little about people whose selenium status is already adequate.
• No replication. The 2022 systematic review, which gathered 24 Brazil nut clinical trials, found this to be the only one in people with mild cognitive impairment, and stated that studies of Brazil nut intake in this group "are scarce" (da Silva 2022).

The trial did not measure progression to dementia, brain imaging or daily functioning, and it lasted six months. It cannot say whether Brazil nuts prevent, delay or slow dementia.

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Selenium Status and Cognition in Observational Research

Observational studies measure people's selenium levels and compare them with cognitive test scores, without giving anyone anything. They can show associations, not cause and effect.

The verified records for this page do not include the original observational studies themselves, so the summary below relies on how the 2022 systematic review describes them (da Silva 2022):

• Within the Brazil nut trial, higher selenium status was positively correlated with the CERAD score — a correlation inside a group of 20, not a comparison between groups.
• The review cites earlier work from the French EVA study as describing selenium deficiency as a possible contributor to poorer cognitive performance through loss of redox balance.
• The review also cites a cross-sectional study of 154 older Australian adults, 85% of whom had adequate selenium intake, in which plasma selenium was not associated with cognitive performance or with neurotrophic factors (proteins that support nerve cells).

Put together, the review's reading is that whatever benefit exists appears in people with low selenium, and that among older adults who already get enough, blood selenium levels did not track with thinking skills. Readers who want the original studies can use the PubMed topic searches at the end of this page.

For how selenium status is measured, see the selenium test page.

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Nuts in General and the Ageing Brain

Brazil nuts are also simply nuts, and there is a wider body of research on nut-containing diets and cognition. Almost none of it involves Brazil nuts specifically.

• A 2020 review of nine nuts. Arslan and colleagues reviewed almond, walnut, pistachio, Brazil nut, peanut, pecan, cashew, hazelnut and chestnut for memory. They stated that only walnuts, followed by almonds, hazelnuts and pistachios, had shown promising results in empirical studies of memory; for the others, including Brazil nuts, the review presented hypotheses rather than results (Arslan 2020).
• The MIND diet. In 4,066 middle-aged and older Chinese adults followed for a median of 3 years, closer adherence to the MIND diet pattern was associated with better global cognitive function, and nut consumption on its own showed an independent positive association. The accompanying meta-analysis of eight cohorts (26,103 participants) found an association with better cognitive scores and a smaller, borderline association with slower decline (Huang 2023). Nuts here were not separated by type.
• Replacing processed red meat. In two large US cohorts, replacing one daily serving of processed red meat with nuts and legumes (counted together) was associated with a 19% lower risk of dementia and 1.37 fewer years of estimated cognitive ageing (Li 2025). This is a substitution model in observational data, not a trial, and it does not isolate nuts.
• Healthy ageing in Spain. In the Seniors-ENRICA cohorts of older Spanish adults, closer adherence to the Planetary Health Diet's guidance on nuts was independently associated with improvement rather than worsening in "intrinsic capacity", a combined score of cognition, mood, vitality, hearing, vision and movement. The domain that changed significantly with the overall diet was hearing, not cognition (Gómez-Cao 2025).
• A 2016 overview. Grosso and Estruch described evidence on nut consumption and neurological or psychiatric disorders as "scarce", while noting studies suggesting protective associations with depression, mild cognitive disorders and Alzheimer's disease (Grosso and Estruch 2016).

The best-known randomised nut-and-cognition work — the walnut trials and the cognition analyses from the PREDIMED Mediterranean-diet trial, whose nut arm used walnuts, almonds and hazelnuts — is covered on the walnuts and cognitive aging page. Brazil nuts were not the intervention in those studies, and results from one nut do not automatically carry over to another: walnuts are rich in the plant omega-3 alpha-linolenic acid, while the distinctive feature of Brazil nuts is selenium. The cross-nut evidence is gathered on the nut benefits page.

No record reviewed for this page tested Brazil nuts against mood or depression.

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Why Tablet and Animal Results Do Not Transfer

Articles about Brazil nuts and the brain often borrow from studies that did not use Brazil nuts. Three gaps separate those studies from the food.

1. A nut is not a measured dose. The selenium in a single Brazil nut varies enormously. Lima and colleagues found up to eight-fold differences from one nut to the next within the same batch (Lima 2019). In a New Zealand trial, two nuts a day were expected to supply about 100 mcg of selenium but actually averaged 53 mcg, with a possible range of 20 to 84 mcg (Thomson 2008). The cognition trial estimated 288.75 mcg per nut (Cardoso 2016), and a trial in obese women estimated about 1,261 mcg in one nut (Duarte 2019). A selenium tablet delivers a fixed amount; a nut does not.
2. The food carries more than selenium. Brazil nuts also supply magnesium, copper, zinc, fats, protein and other bioactive compounds (Cardoso 2017). An effect of the nut could come from any of these, or from selenium alone, or from none of them.
3. Mice and purified compounds are a different exposure. The mouse study above used isolated selenomethionine in an artificial ageing model (Gao 2022). That can suggest a mechanism; it cannot show what eating Brazil nuts does to human memory.

Even so, Thomson and colleagues found two Brazil nuts a day raised plasma selenium and GPx activity as effectively as 100 mcg of selenomethionine over 12 weeks (Thomson 2008), so the selenium in the nut is clearly absorbed and used. The question that remains open is what that does, if anything, for the brain.

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What Is Not Known

• Whether the pilot result holds up. No larger or longer trial of Brazil nuts and cognition appears in the research reviewed for this page.
• Whether Brazil nuts affect dementia risk. No trial measured progression from mild cognitive impairment to dementia, and no cohort study in the reviewed records isolated Brazil nuts.
• Whether people with adequate selenium gain anything. The trial participants were mostly deficient (da Silva 2022), and the Australian study summarised by the review found no association in older adults with adequate intake.
• Whether GPx4 and ferroptosis matter in living human brains. That link comes from cell and animal research.
• Whether gene variants change who responds. The secondary analysis suggests selenoprotein gene variants alter the biochemical response (da Silva 2022), and a study of 130 healthy adults found that a selenoprotein P gene variant was associated with how fasting glucose responded to one nut a day for 8 weeks (Donadio 2018), but no study has linked these variants to cognitive outcomes from Brazil nuts.
• What amount, if any, is relevant. Because selenium per nut varies so widely, the trial's "one nut a day" is not a fixed selenium dose that can be repeated reliably.

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Who the Research Raises Cautions For

The selenium that makes Brazil nuts interesting is also what makes the amount matter.

• Upper limits are close to the amount in one nut. A 2007 review gives a tolerable upper intake level of 300 mcg a day (the European figure), with recommended intakes of 40 to 55 mcg (Alexander 2007). Lima and colleagues cite an allowable daily intake of 400 mcg and a toxicity threshold of 1,200 mcg, and calculate that a 30-gram serving of high-selenium nuts can exceed both (Lima 2019). The cognition trial's estimated 288.75 mcg from one nut a day is near the European limit on its own.
• Selenosis. Excess selenium can cause selenosis, which affects the liver, skin, nails and hair (Alexander 2007). Details are on the selenium overload and safety and selenium toxicity pages. In Amazon river communities whose traditional diet includes Brazil nuts, blood selenium was sometimes very high, yet researchers found no skin or breath signs of selenosis linked to it (Lemire 2012) — the authors called for selenium toxicity to be re-assessed with the chemical form of selenium, the route of exposure and co-exposures such as mercury taken into account. That finding comes from one population and does not apply automatically elsewhere.
• High-selenium nuts and inflammation. In 55 obese women who were not selenium-deficient, one very high-selenium Brazil nut a day for 2 months raised the gene expression of several inflammatory signals, although blood levels of inflammatory markers did not change (Duarte 2019). The authors read this as a possible pro-inflammatory stimulus. For the brain, it is a reminder that more selenium is not automatically better.
• People already taking selenium. Selenium from supplements and from Brazil nuts adds up. Questions about combining them are for a clinician.
• Tree nut allergy. Brazil nut is a tree nut allergen; see the main Brazil nuts page for the allergy findings.

Memory changes in older age have many causes, some of them treatable, such as thyroid problems, vitamin B12 deficiency and medication effects. The trial above does not address any of these, and the evaluation of memory symptoms belongs with a clinician.

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Connections

• Nuts — the nuts landing page
• Brazil Nuts — the main topic page
• Brazil Nuts — Benefits — the hub for these deep dives
• Selenium and Thyroid — the deiodinase link
• Selenium Status and Antioxidant Enzymes — what one nut does to blood selenium and GPx
• Cholesterol and Inflammation — the lipid and inflammation trials
• Selenium Overload and Safety — why the amount matters
• Brazil Nuts: Nutrient Profile — the composition table
• Walnuts and Cognitive Aging — the WAHA and PREDIMED cognition evidence
• Nut Benefits — the cross-nut evidence
• Selenium — the mineral itself
• Selenium: Antioxidant Defense — selenoprotein biology
• Glutathione Peroxidase — the enzyme the trial measured
• Selenium Test — how status is measured
• Dementia — the condition mild cognitive impairment can precede
• Alzheimer's Disease — the most common dementia
• Oxidative Stress — the damage selenoproteins counter

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Key Research Papers

1. 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
2. da Silva A, Silveira BKS, de Freitas BVM, et al. Effects of Regular Brazil Nut (Bertholletia excelsa H.B.K.) Consumption on Health: A Systematic Review of Clinical Trials. Foods (Basel, Switzerland). 2022;11(18):2925. PubMed PMID: 36141050
3. Schomburg L. Selenoprotein P - Selenium transport protein, enzyme and biomarker of selenium status. Free radical biology & medicine. 2022;191:150-163. PubMed PMID: 36067902
4. Lu J, Holmgren A. Selenoproteins. The Journal of biological chemistry. 2009;284(2):723-7. PubMed PMID: 18757362
5. Alexander J. Selenium. Novartis Foundation symposium. 2007;282:143-9; discussion 149-53, 212-8. PubMed PMID: 17913229
6. Gao Y, Xu Y, Yin J. Selenomethionine Ameliorates Cognitive Impairment, Decreases Hippocampal Oxidative Stress and Attenuates Dysbiosis in D-Galactose-Treated Mice. Antioxidants (Basel, Switzerland). 2022;11(1):111. PubMed PMID: 35052615
7. 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
8. 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
9. 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
10. 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
11. Duarte GBS, Reis BZ, Rogero MM, et al. Consumption of Brazil nuts with high selenium levels increased inflammation biomarkers in obese women: A randomized controlled trial. Nutrition (Burbank, Los Angeles County, Calif.). 2019;63-64:162-168. PubMed PMID: 31026738
12. Lemire M, Philibert A, Fillion M, et al. No evidence of selenosis from a selenium-rich diet in the Brazilian Amazon. Environment international. 2012;40:128-136. PubMed PMID: 21856002
13. Arslan J, Gilani AU, Jamshed H, et al. Edible Nuts for Memory. Current pharmaceutical design. 2020;26(37):4712-4720. PubMed PMID: 32767923
14. Huang L, Tao Y, Chen H, et al. Mediterranean-Dietary Approaches to Stop Hypertension Intervention for Neurodegenerative Delay (MIND) Diet and Cognitive Function and its Decline: A Prospective Study and Meta-analysis of Cohort Studies. The American journal of clinical nutrition. 2023;118(1):174-182. PubMed PMID: 37105521
15. Li Y, Li Y, Gu X, et al. Long-Term Intake of Red Meat in Relation to Dementia Risk and Cognitive Function in US Adults. Neurology. 2025;104(3):e210286. PubMed PMID: 39813632
16. Gómez-Cao M, Aznar de la Riera MDC, Ortolá R, et al. Association between a Planetary Health Diet and changes in intrinsic capacity in older adults: the Seniors-ENRICA cohorts. Age and ageing. 2025;54(6):afaf175. PubMed PMID: 40586122
17. Donadio JLS, Rogero MM, Guerra-Shinohara EM, et al. SEPP1 polymorphisms modulate serum glucose and lipid response to Brazil nut supplementation. European journal of nutrition. 2018;57(5):1873-1882. PubMed PMID: 28501922
18. Grosso G, Estruch R. Nut consumption and age-related disease. Maturitas. 2016;84:11-6. PubMed PMID: 26586104

PubMed Topic Searches

1. PubMed: Brazil nuts and cognition
2. PubMed: selenium status and cognitive performance in older adults
3. PubMed: selenoprotein P and brain selenium transport
4. PubMed: GPx4, ferroptosis and neurodegeneration
5. PubMed: nut consumption and cognitive decline

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